冷冻食品（英文第二版）：34990wp_11

11.1 Introduction Many different ingredients and raw materials are processed to make chilled foods. At harvest or slaughter these materials may have a wide range of microbes in or on them. Some of them carry the micro-organisms that cause their eventual spoilage (e.g. bacilli or Lactic acid bacteria) whilst others pick them up during harvesting or processing. Many food poisoning bacteria occur naturally with farm animals and agricultural produce (e.g. Salmonella, E.coli O157 and Campylobacter) and hence can contaminate meat and poultry, milk and vegetable products. The numbers and types present will vary from one ingredient to another and often product safety at the point of consumption will depend on manufacturing, consumer use and the presence or numbers of pathogens in the raw material and eventually in the manufactured product. In order to ensure safe products with a reliable shelf-life, the manufacturer must identify which food poisoning and spoilage bacteria are likely to be associated with particular raw materials and products (e.g. by microbiological surveys). Therefore it is essential to design food processing procedures according to principles that ensure that the hazard of food poisoning is controlled. This is especially important in the prepared foods and cook-chill sectors where safety relies on the control of many features of the manufacturing process (ICMSF 1988, Kennedy 1997). The appropriate means of control should be incorporated into the product and process design and implemented in the manufacturing operation. Often the means of control exist at several stages along the supply chain. For example Gill et al. (1997) have suggested that the overall hygienic quality of beef hamburger patties could be improved only if hygienic quality beef (i.e. lowest possible levels of contamination with pathogens) was used for 11 Microbiological hazards and safe process design M. H. Brown, Unilever Research, Sharnbrook manufacture and there was better management of retail outlets with regard to patty storage and cooking. Good manufacturing practice guides are available for many sectors of the chilled food industry (e.g. IFST Guide to Food and Drink Good Manufacturing Practice: IFST 1998, UK Chilled Food Association Guidelines: CFA 1997). These guides outline responsibilities in relation to the manufacture of safe products; adherence to their principles will ensure that the product remains wholesome and safe under the expected conditions of use. Product and process design will always be a compromise between the demands for safety and quality on the one hand, and cost and operational limitations on the other. Heat is the main means of ensuring product safety and the elimination of spoilage bacteria. The heating that can be applied may sometimes be limited by quality changes in the product. Usually, minimum cooking processes, either in-factory or in-home, will be designed to kill specific bacteria such as infectious pathogens or those causing spoilage. The skill of the product designer is to balance these competing demands for quality and safety and decide where an acceptable balance lies. Even so, usually more than one process step contributes to quality and safety, for example refrigerated storage is used to retard or prevent the growth of vegetative cells and spores that have survived factory heating. Hence the safety of chilled foods which have no inherent preservative properties, depends almost exclusively on suitable refrigeration temperatures being maintained throughout the supply chain including, for example, the defrosting of frozen ingredients and loading of refrigerated vehicles. Where preservation is used, for example, reduced pH/ increased acidity or vacuum packing, chilling will also contribute to the effectiveness of the preservation system and introduces the need for additional controls during processing. The techniques of risk assessment, either formal or more commonly informal, may be used to guide the manufacturer in achieving a predictable and acceptable balance between the sale of raw or undecontaminated components, cooking and the chances of pathogen survival. Successful process design must consider not only contaminants likely to be carried by the raw materials, but also the shelf- life of the food and its anticipated storage conditions with distributors, retailers or customers, CFDRA (1990). In this sense, the customer is an integral part of the safety chain and some additional level of risk attributable to consumer mishandling or mis-use is always accepted by a manufacturer when he designs products whose safety and high quality shelf-life relies on customer use (e.g. cooking or chilled storage). Brackett (1992) has pointed out that chilled foods contain few, or no, antimicrobial additives to prevent growth of pathogenic micro-organisms and are susceptible to the effects of inadequate refrigeration that may allow pathogen growth. He also highlights related issues such as over reliance on shelf-life as a measure of quality and the need to consider the needs of sensitive groups (such as immunocompromised consumers) in the product design. If the product design relies on the customer carrying out a killing step to free the product of pathogens, such as salmonellae, it is important that helpful, accurate and validated heating or cooking instructions are provided by the 288 Chilled foods manufacturer and that use of these instructions results in high product quality. Good control of heat processing and hygiene in the factory and the home or food service outlet are essential for product safety. The prevention of product re- contamination or cross-contamination after heating plays an even more critical role when products are sold as ready-to-eat. It is essential that foods relying on chilled storage for their safety are stored at or below the specified temperature(s) (from C01o to +8oC) during manufacture, distribution and storage. Storage at higher temperatures can allow the growth of any hazardous micro-organisms that may be present. Inappropriate processing in conjunction with temperature or time abuse during storage will certainly lead to the growth of spoilage micro-organisms and premature loss of quality. Labuza and Bin-Fu (1995) have proposed the use of time/temperature integrators (TTI) for monitoring the conditions and the extent of temperature abuse in the distribution chain. In conjunction with predictive microbial kinetics the impact of storage temperature on the safe shelf-life of meat and poultry products can be estimated. The risks associated with any particular products can be investigated either by practical trials (such as challenge testing) or by the use of mathematical modelling. The use of predictive models for microbial killing by heat (interchange of time and temperature to calculate process lethality based on D and z values) or the extent of microbial growth can improve supply chain management. In the UK, Food MicroModel (FMM: www.lfra.co.uk) and in the US, the Pathogen Modelling Program (www.arserrc.gov/mfs/regform.htm) are computer-based predictive microbiology databases applicable to chilled products. Panisello and Quantick, (1998) used FMM to make predictions on the growth of pathogens in response to variations in the pH and salt content of a product and specifically the effect of lowering the pH of pa?te′. Zwietering and Hasting (1997) have taken this concept a stage further and developed a modelling approach to predict the effects of processing on microbial growth during food production, storage and distribution. Their process models were based on mass and energy balances together with simple microbial growth and death kinetics and were evaluated using a meat product line and a burger processing line. Such models can predict the contribution of each individual process stage to the microbial level in a product. Zwietering et al. (1991) and Zwietering et al. (1994a, b) have modelled the impact of temperature and time and shifts in temperature during processing on the growth of Lactobacillus plantarum. Such predictive models can, in principle, be used for suggesting the conditions needed to control microbial growth or indicate the extent of the microbial ‘lag’ phase during processing and distribution where temperature fluctuations may be common and could allow growth. Impe et al. (1992) have also built similar models describing the behaviour of bacterial populations during processing in terms of both time and temperature, but have extended their models to cover inactivation at temperatures above the maximum temperature for growth. Adair and Briggs (1993) have proposed the development of expert systems, based on predictive models to assess the microbiological safety of chilled foods. Microbiological hazards and safe process design 289 Such systems could be used to interpret microbiological, processing, formula- tion and usage data to predict the microbiological safety of foods. However to be realistic, the models are only as good as the data input and at present there is both uncertainty and variability associated with the data available. Betts (1997) has also discussed the practical application of microbial growth models to the determination of shelf-life of chilled foods and points out the usefulness of models in speeding up product development and the importance of validating the output of models in real products. Modelling technology can offer advantages in terms of time and cost, but is still in its infancy (Pin and Baranyi 1998). Its usefulness is limited, as there is variation not only in the microbial types present in raw materials and products but also in their activities and interactions altering growth or survival rates or the production of metabolites recognised by customers as spoilage. There are, not surprisingly, major differences between manufacturers in the degree of time or temperature abuse they design their product to withstand and hence the risks they are prepared to accept on behalf of their customers. This can result in major differences in the processes, ingredients and packaging used and the shelf-lives given to apparently similar products. 11.2 Definitions Definitions are given below, firstly in order to avoid misunderstanding and secondly to introduce general comments and guidance for the design of processes which control microbiological risks adequately. They are discussed in the following groups: raw materials; Chilled foods; Safety and quality control; Processes. 11.2.1 Raw materials Undecontaminated materials These include any food components of the final product, that have not been decontaminated so that they are effectively free of bacteria prejudicing or reducing the microbiological safety or shelf-life of the finished product. Such starting materials should be handled in the factory so that numbers of contaminants are not increased and they cannot contaminate any other components that have already been decontaminated. For example, the layout of processing areas should be designed on the forward flow principle to prevent cross contamination; uncooked material should not be handled by personnel also handling finished product (except with the appropriate hygiene controls and separation), or allowed to enter high care areas (see below). If it is anticipated that these materials may contain pathogenic microbes, the severity of the risks should be assessed. Their handling, processing and usage should be controlled accordingly to prevent cross contamination or the manufacture of products which may be accidentally harmful to customers (see below). 290 Chilled foods Decontaminated materials These materials will have been treated, usually with heat, to reduce their microbial load. If they are intended for direct incorporation into ready-to-eat products then the heat treatment used in their preparation should be sufficient to ensure the safety of the product (i.e. predictable absence of pathogens) depending on whether it is of short or long shelf-life (see ‘Safe process design’ below). Suitable precautions must be taken to prevent their recontamination after treatment and during handling in the factory. Hence primary packaging should be removed from decontaminated materials only in high-hygiene areas. 11.2.2 Chilled foods This broad group covers all foods which rely on chilled storage (originally defined as from C01o to +8oC (Anon. 1982) but see below) as a component of their preservation system. It may therefore include foods made entirely from raw or uncooked ingredients. Some such foods may require cooking prior to consumption in order to make them edible, e.g. raw fish and meat products, and it is accepted that such foods may unavoidably contain pathogenic micro- organisms from time to time. Prepared chilled or ready-to-eat foods These chilled foods may contain raw or uncooked ingredients (Risk Classes 1 and 2, see ‘Risk classes’ below and Table 11.1), such as salad or cheese components. But their preparation by the manufacturer is such that the food is either obviously ready-to-eat or only requires re-heating, rather than full cooking, prior to use. The manufacturer should do his best to ensure that such foods are free of hazardous pathogens or hazardous levels of pathogens at the end of their shelf-life, and ingredients should be sourced with this objective in view. A scheme for the layout of process lines used in their manufacture is given in Figs 11.1 and 11.2. Cooked ready-to-eat foods Such foods (Risk Classes 3 and 4, see below, ‘Risk classes’ and Table 1) are made entirely from cooked ingredients and therefore should be freed of infectious pathogens during processing. Cooking procedures during production should be designed to ensure this and handling procedures after cooking, including cooling, should be designed to prevent recontamination of the product or its components, such as primary packaging materials. Often, the appearance of such foods makes it obvious to the customer that no heating, or mild re-heat, is all that are required before eating. Heating requirements should be made clear by any instructions. Typical process line layouts are shown in Figs 11.3 and 11.4. REPFEDS For the wider range of in-pack pasteurised foods, Mossel et al. (1987) and Notermans et al. (1990) have proposed the more informative name: ‘refrigerated pasteurised foods of extended durability’ (or ‘REPFED’), which includes sous- Microbiological hazards and safe process design 291 vide and other foods with preservation and pasteurisation combinations that ensure long shelf-lives under chill conditions. These products are processed to free them of spoilage bacteria and pathogens capable of growth at chill temperatures, and hence allow very long shelf-lives (42 days or so). Therefore processing, handling and packaging must specifically ensure that they are free of infectious pathogens and the spore-forming pathogens capable of growing under chilled conditions. There is still a lack of knowledge on realistic safety boundaries and the risks associated with these products, with respect to the most severe hazard non-proteolytic Clostridium botulinum (Peck 1997). The determinants of effectiveness of complex combination preservation systems that rely on mild heating and chilled storage are not fully understood. 11.2.3 Safety and quality control Good manufacturing practice Good manufacturing practice (GMP) covers the boundaries and fundamental principles, procedures and means needed to design an environment suitable for the production of food of acceptable quality. Good hygienic practice (GHP) describes the basic hygienic measures that establishments should meet and Table 11.1 Risk classes of chilled foods Risk Typical Critical Relative Required minimum Required manufacturing class a shelf-life hazard risk heat treatment class b MA HA HCA 1 1 week Infectious High Customer cook a19 (a19) pathogens (minimum 70oC, 2 min.) 21–2 weeks Infectious Low Pasteurization by a19a19a19 pathogens manufacturer (minimum 70oC, 2 min.) 3 C622 weeks Infectious Low Pasteurization by a19a19 pathogens manufacturer and spore- (minimum 90oC, formers 10 min.) 4 C622 weeks Spore- Low Pasteurization by a19a19 formers manufacturer (minimum 90oC, 10 min.) Notes a Class 1: Raw chill-stable foods, e.g. meat, fish etc.; Class 2: Products made from a mixture of cooked and low-risk raw components; Class 3: Products cooked or baked and assembled or primary packaged in a high-care area; Class 4: Products cooked in-pack. b MA: Manufacturing area; HA: Hygienic area; HCA: High-care area. 292 Chilled foods which are pre-requisites to other approaches, in particular HACCP. GMP codes and the hygiene requirements they contain are the relevant boundary conditions for the hygienic manufacture of foods and should always be applied. Governments (see Anon. 1984, 1986), the Codex Alimentarius Committee on Food Hygiene (FAO/WHO) and the food industry, often acting in collaboration with food inspection and control authorities and other groups have developed GMP/GHP requirements (Jouve et al. 1998). Generally GHP/GMP requirements cover the following: ? the hygienic design and construction of food manufacturing premises ? the hygienic design, construction and proper use of machinery MT82MT97MT119MT32MT77MT97MT116MT101MT114MT105MT97MT108 MT83MT116MT111MT114MT97MT103MT101MT32MT111MT110MT32MT82MT101MT99MT101MT105MT112MT116 MT82MT97MT119MT32MT77MT97MT116MT101MT114MT105MT97MT108 MT80MT114MT101MT112MT97MT114MT97MT116MT105MT111MT110 MT80MT114MT105MT109MT97MT114MT121MT32MT80MT97MT99MT107MT97MT103MT105MT110MT103 MT67MT104MT105MT108MT108MT105MT110MT103 MT83MT101MT99MT111MT110MT100MT97MT114MT121MT32MT80MT97MT99MT107MT97MT103MT105MT110MT103 MT82MT101MT116MT97MT105MT108MT105MT110MT103 MT68MT105MT115MT116MT114MT105MT98MT117MT116MT105MT111MT110 MT80MT82MT69MT70MT69MT82MT82MT69MT68 MT72MT89MT71MT73MT69MT78MT73MT67 MT65MT82MT69MT65 MT67MT104MT105MT108MT108MT105MT110MT103MT32MT80MT114MT101MT112MT97MT114MT101MT100 MT82MT97MT119MT32MT77MT97MT116MT101MT114MT105MT97MT108 MT67MT111MT109MT112MT111MT110MT101MT110MT116 MT65MT115MT115MT101MT109MT98MT108MT121 MT42 MT42MT32MT80MT104MT121MT115MT105MT99MT97MT108MT32MT97MT110MT100MT32MT115MT116MT97MT102MT102MT32MT115MT101MT112MT97MT114MT97MT116MT105MT111MT110MT32MT111MT98MT108MT105MT103MT97MT116MT111MT114MT121 Fig. 11.1 Typical flow diagram for the production of chilled foods prepared from only raw components. (Class 1) Microbiological hazards and safe process design 293 ? cleaning and disinfection procedures (including pest control) ? general hygienic and safety practices in food processing including – the microbiological quality of raw materials – the hygienic operation of each process step – the hygiene of personnel and their training in hygiene and the safety of food. MT82MT97MT119MT32MT77MT97MT116MT101MT114MT105MT97MT108 MT83MT116MT111MT114MT97MT103MT101MT32MT111MT110MT32MT82MT101MT99MT101MT105MT112MT116 MT82MT97MT119MT32MT77MT97MT116MT101MT114MT105MT97MT108 MT80MT114MT101MT112MT97MT114MT97MT116MT105MT111MT110 MT80MT114MT105MT109MT97MT114MT121MT32MT80MT97MT99MT107MT97MT103MT105MT110MT103 MT83MT101MT99MT111MT110MT100MT97MT114MT121MT32MT80MT97MT99MT107MT97MT103MT105MT110MT103 MT82MT101MT116MT97MT105MT108MT105MT110MT103 MT68MT105MT115MT116MT114MT105MT98MT117MT116MT105MT111MT110 MT72MT89MT71MT73MT69MT78MT73MT67 MT65MT82MT69MT65 MT67MT111MT109MT112MT111MT110MT101MT110MT116MT32MT65MT115MT115MT101MT109MT98MT108MT121 MT42 MT42MT32MT80MT104MT121MT115MT105MT99MT97MT108MT32MT97MT110MT100MT32MT115MT116MT97MT102MT102MT32MT115MT101MT112MT97MT114MT97MT116MT105MT111MT110MT32MT111MT98MT108MT105MT103MT97MT116MT111MT114MT121 MT67MT111MT109MT112MT111MT110MT101MT110MT116MT115MT32MT67MT104MT105MT108MT108MT101MT100 MT67MT111MT109MT112MT111MT110MT101MT110MT116MT115MT32MT67MT111MT111MT107MT101MT100 MT67MT111MT109MT112MT111MT110MT101MT110MT116MT115MT32MT67MT111MT111MT107MT101MT100 MT67MT111MT109MT112MT111MT110MT101MT110MT116MT32MT68MT105MT115MT105MT110MT102MT101MT99MT116MT105MT111MT110 MT42 Fig. 11.2 Typical flow diagram for the production of chilled foods prepared from both cooked and raw components. (Class 2) 294 Chilled foods HACCP The Hazard Analysis Critical Point Control System (HACCP) is a food safety management system using the approach of identifying hazards and controlling the critical points in food handling and processing to prevent food safety problems. It is a system or approach that can be used to assure food safety in all scales and types of food manufacture and is an important element in the overall management of food quality and safety. The widespread introduction of MT82MT97MT119MT32MT77MT97MT116MT101MT114MT105MT97MT108 MT80MT114MT101MT112MT97MT114MT97MT116MT105MT111MT110 MT80MT114MT105MT109MT97MT114MT121MT32MT80MT97MT99MT107MT97MT103MT105MT110MT103 MT83MT101MT99MT111MT110MT100MT97MT114MT121MT32MT80MT97MT99MT107MT97MT103MT105MT110MT103 MT82MT101MT116MT97MT105MT108MT105MT110MT103 MT68MT105MT115MT116MT114MT105MT98MT117MT116MT105MT111MT110 MT72MT89MT71MT73MT69MT78MT73MT67 MT65MT82MT69MT65 MT65MT115MT115MT101MT109MT98MT108MT121 MT42 MT42MT32MT80MT104MT121MT115MT105MT99MT97MT108MT32MT97MT110MT100MT32MT115MT116MT97MT102MT102MT32MT115MT101MT112MT97MT114MT97MT116MT105MT111MT110MT32MT111MT98MT108MT105MT103MT97MT116MT111MT114MT121 MT67MT104MT105MT108MT108MT105MT110MT103 MT67MT111MT111MT107MT105MT110MT103 MT68MT105MT115MT105MT110MT102MT101MT99MT116MT105MT111MT110 MT42 MT73MT110MT116MT101MT114MT109MT101MT100MT105MT97MT116MT101 MT83MT116MT111MT114MT97MT103MT101 MT72MT73MT71MT72MT32MT67MT65MT82MT69 MT65MT82MT69MT65 MT82MT97MT119MT32MT77MT97MT116MT101MT114MT105MT97MT108 MT83MT116MT111MT114MT97MT103MT101MT32MT111MT110 MT82MT101MT99MT101MT105MT112MT116 MT80MT114MT101MT99MT111MT111MT107MT101MT100MT32MT67MT111MT109MT112MT111MT110MT101MT110MT116MT115 MT105MT110MT32MT72MT101MT114MT109MT101MT116MT105MT99MT97MT108MT108MT121 MT83MT101MT97MT108MT101MT100MT32MT80MT97MT99MT107MT115 Fig. 11.3 Typical flow diagram for the production of pre-cooked chilled meals from cooked components. (Class 3) Microbiological hazards and safe process design 295 HACCP has promoted a shift in emphasis from end-product inspection and testing to preventive control of hazards at all stages of food production, but especially at the critical control points (CCPs). As such, it is a management technique ideally suited to the manufacture of chilled foods, where many MT82MT97MT119MT32MT77MT97MT116MT101MT114MT105MT97MT108 MT83MT116MT111MT114MT97MT103MT101MT32MT111MT110MT32MT82MT101MT99MT101MT105MT112MT116 MT82MT97MT119MT32MT77MT97MT116MT101MT114MT105MT97MT108 MT80MT114MT101MT112MT97MT114MT97MT116MT105MT111MT110 MT73MT110MT116MT101MT114MT109MT101MT100MT105MT97MT116MT101MT32MT83MT116MT111MT114MT97MT103MT101 MT65MT115MT115MT101MT109MT98MT108MT121 MT80MT114MT105MT109MT97MT114MT121MT32MT80MT97MT99MT107MT97MT103MT105MT110MT103 MT83MT101MT99MT111MT110MT100MT97MT114MT121MT32MT80MT97MT99MT107MT97MT103MT105MT110MT103 MT82MT101MT116MT97MT105MT108MT105MT110MT103 MT68MT105MT115MT116MT114MT105MT98MT117MT116MT105MT111MT110 MT72MT89MT71MT73MT69MT78MT73MT67 MT65MT82MT69MT65 MT67MT111MT111MT107MT105MT110MT103 MT67MT104MT105MT108MT108MT105MT110MT103 Fig 11.4 Typical flow diagram for the production of chilled foods cooked in their own packaging prior to distribution. (Class 4) 296 Chilled foods elements of the process contribute to safety and shelf-life, the shelf-life is restricted and any delay to await the results of microbiological testing uses-up shelf-life. HACCP involves: ? the identification of realistic (microbiological) hazards, such as pathogenic agents and the conditions leading to their presence, growth or survival (HACCP is also used for the control of chemical and physical hazards) ? the identification of specific requirements for the control of hazards and identification of process stages where this is achieved ? procedures and equipment to measure and document the efficacy of the controls that are an integral part of the HACCP system ? the documentation of limits and the actions required when these are exceeded. For steps in the manufacturing process that are not recognised as CCPs, the use of GMP/GHP provides assurance that suitable control and standards are being applied. The identification and analysis of hazards within the HACCP programme will provide information to interpret GMP/GHP requirements and direct training, calibration etc. for specific products or processes. The Microbiology and Food Safety Committee of the National Food Processors Association (NFPA 1993) has considered HACCP systems for chilled foods produced at a central location and distributed chilled to retail establishments. They used chicken salad as a model for proposing critical control points and give practical advice on HACCP planning; development of a production flow diagram, hazard identification, establishing critical limits, monitoring requirements; and verification procedures to ensure the HACCP system is working effectively. There are also USDA recommendations and outline HACCP flow diagrams for chilled food processes, cook-in-package and cook-then-package Snyder (1992). Risk analysis Ensuring the microbiological safety and wholesomeness of food requires the identification of realistic hazards and their means of control (risk assessment). The ability of a food producer to assess the impact of process, product and market changes on the level of risk and the type of hazard are important to the assurance of consistent standards of food safety. The effects of changes on risk and hazards need to be identified and can include the development of new products and processes, the use of different raw material sources, or the targeting of new customer groups, such as children. Food producers have always assessed these risks using either empirical or experiential approaches. As causal links have been established between food-borne illness and the presence, or activities (toxigenisis), of food-poisoning micro-organisms, so the control of specified microbial hazards has progressively become the means of ensuring food safety. These practical approaches have now developed into formal systems with well defined procedures and are known as Microbiological Risk Assessment (MRA). They are described in Microbiological Risk Assessment; an interim report (ACDP, 1996) or by the Codex scheme (Figure 1, Codex Alimentarius, 1996). Microbiological hazards and safe process design 297 The overall aim of risk analysis is to reduce risk by ? identifying realistic microbiological hazards and characterising them according to severity ? examining the impact of raw material contamination, processing and use on the level of risk ? and communicating clearly and consistently, via the output of the study, the level of risk to the consumer. When risk assessment is put together with risk communication (distribution of information on a risk and on the decisions taken to combat a risk) and used to promote sound risk management (actions to eliminate or minimise risk), a risk analysis is produced (ACDP 1996). Stages in a risk assessment Clear formulation of a problem is an essential prelude to risk assessment. A process or ingredient change, the emergence of a new pathogen or a change in public concern may trigger a risk assessment over a hazard and this can lead to the review of control, factory layout or sourcing options or the revision of cooking instructions. The first stage of the assessment is to identify the hazard (hazard identifica- tion). For example, concern may be over the presence of salmonella in a product, as ingesting products containing infective cells may cause salmonellosis. The chances of causing harm are governed by many factors specific to the hazard (its virulence, incidence and concentration) and its prevalence in raw materials. Exposure assessment describes the likely exposure of customers to the hazard, based on the size of the portion consumed and the impact of prior manufacturing etc. on the quantity of the infectious agent (i.e. Salmonellae) present (and infectious) at consumption. For a cooked product, exposure will depend on the numbers of salmonella entering the heating process, the heating characteristics of the product and the heating conditions used either in-factory or in-home, these combine to determine the number of pathogens surviving at consumption. If the heat sensitivity of salmonella and the product’s heat treatment are known, then numbers likely to survive can be estimated. For many chilled foods (e.g. burgers or flash-fried poultry products), microbiological safety is not necessarily guaranteed by manufacturing processes, but can be a joint responsibility with the customer (Notermans et al. 1996). This makes the consumer part of the process for ensuring that the end product is safe, and therefore an assessment of their effect an essential part of a risk assessment. Quantification of the risks of infection after product consumption is known as hazard characterisation. It links the sensitivity of consumers to infection (i.e. usually making use of expert opinion or knowledge of the dose-response within populations) with the concentration of the agent in the portion. The output of these three stages is a risk characterisation, which describes for a certain consumer the risks of (Salmonella) infection associated with the consumption of a particular product, sourced and manufactured under specified conditions. 298 Chilled foods To facilitate communication of decisions on risk and their basis, information must be accessible to the management, customers and staff. Where risk decisions or conclusions are communicated effectively, risk management prac- tices can be readily implemented, consistent standards applied and dangerous changes may be stopped. Implementation of an effective process for communication and understanding on a consistent, scientific, and yet practical basis is an unsolved problem. Risk assessment has been reviewed (Jaykus 1996) and applied to specific problems, listeriosis (Miller et al. 1997), the role of indicators (Rutherford et al. 1995) and links with HACCP (Elliot et al. 1995). Precautionary principle Generally, the actions taken to protect public health are based on sound science. However, from time to time decisions have to be taken in an area of scientific uncertainty, for example if the prevalence or severity of a new pathogen is unknown. Any decisions made using the precautionary principle should control the perceived health risks without resorting to excessively restrictive control measures and should be proportionate to the severity of the food safety problem. An example is the measures taken to control the presence of E.coli O157: H7 in vegetables (by pasteurisation) or salad crops (by disinfection during washing) or by the proposal of Good Agricultural Practices (De Roever 1998), when prevalence of the pathogen is unknown and the illness caused severe. The resulting actions need to have been derived in an understandable and justifiable way and any assumptions and uncertainties need to be clear. Most of all the reduction in risk achieved must be acceptable to all the parties involved. Decisions arising in this way should be regarded as temporary awaiting further information that will allow a more reliable risk assessment and lead to the appropriate control measures, as described above. 11.2.4 Processes Cooking Cooking indicates that a process step delivers or has delivered sufficient heat to cause all parts of a food to reach the required sensory quality and should have caused a significant reduction in the numbers of any infectious pathogens that may be present. A 6-log reduction in numbers of infectious pathogens, or 70oC for two minutes, is usually considered to be a diligent minimum target (see ‘Safe process design’ below). Some cooking stages may deliver considerably more heat than this, for example those involving prolonged periods of boiling. If the cooking stage is used as a pasteurization step, then recontamination must be prevented. It is important that cooking specifications distinguish between heat treatments (i.e. the conditions within a food) and the process conditions needed to provide the heat treatment. For example, for a given heat treatment, process conditions will vary according to the diffusion of heat through the product, its dimensions and the transfer of heat to or through its surface. Microbiological hazards and safe process design 299 Pasteurisation Pasteurisation is a processing stage involving heat. It is designed to bring about consistently a predictable reduction in the numbers of specified types of micro- organism in a food or ingredient. The safety and shelf-life requirements should determine the minimum severity of any pasteurisation stage, and the processes used may not necessarily heat ingredients sufficiently to give the required sensory quality or destroy all the micro-organisms present. The effectiveness of a particular heat treatment may be altered by various factors, including the preservation system employed in a particular food. For example, the heat resistance of bacteria and their spores is generally increased by low water activity, but decreased by low pH. Gaze and Betts (1992) have produced an overview of types of pasteurisation process and their microbiological targets. They also use the example of a pre-cooked chilled product to provide advice on process design and on manufacturing control points based on published heat resistance and growth data. Minimum pasteurisation processes should be targeted at foodborne pathogens, but in practice most are more severe, being targeted at the more heat-resistant bacteria causing spoilage in the product (see Gaze and Betts 1992 and CFDRA 1992). P-value Pasteurisation values (P-values) specify the effectiveness of a pasteurisation heat treatment. They are used to indicate the equivalent heat treatment corresponding to a specified heating time at a stated reference temperature. z-value z-value is an empirical value, quoted in temperature (C or F degrees), and used for calculating the increase or decrease in temperature that is needed to alter by a factor of 10 the rate of inactivation of a particular micro-organism. It assumes that the kinetics of microbial death at constant temperature is exponential (i.e. ‘log-linear’). Although z is fundamental to the calculation of sterilisation process equivalence, it should be used with extreme caution for pasteurisation processes, as the death kinetics of many types of micro-organisms are not log-linear; especially where vegetative micro-organisms are concerned and when heating rates are low. For instance, ‘heat adaptation’ may occur. This may even raise the resistance of the micro-organism during the heating process (Mackey and Derrick 1987). ‘Shoulders’ and ‘tails’ on survivor curves are more commonly seen (Gould 1989). In practice, the validity of the z concept is therefore particularly limited at low temperatures, such as those involved in pasteurisation. Consequently, where there are other factors such as preservatives interacting with the heat treatment, or if processes are designed to cause large log reductions (i.e. in excess of ten thousand-fold), then tailing of the survivor curves may become particularly important. Actual or challenge trials should be undertaken to establish confidently safe processes. Re-heating The customer usually does re-heating and it is a procedure intended by the manufacturer only to ensure the optimum culinary quality of a product. 300 Chilled foods Depending on the product design, such a process may or may not provide adequate heating for safety; this is especially true where products are designed for microwave reheating. A re-heating process should, therefore, only be recommended to the customer if the food has effectively been freed of hazardous contaminants during processing and has remained so during any further processing, packaging, distribution and shelf-life. The chemical and physical characteristics of a ready meal, including saltiness, type of tray, and geometry and layout of components, affect microwave heating uniformity and rate. Ryynanen and Ohlsson (1996) heated four-component chilled ready meals in a domestic microwave oven and found that arrangement and geometry of components and type of tray mainly affected heating uniformity. Where microwaves are being used for cooking their effectiveness should be validated; a suitable method uses alginate beads containing micro-organisms with a known heat resistance (Holyoak et al. 1993). Cooling Cooling reduces the product temperature after factory cooking. Its aim should be to ensure that the product spends the minimum possible time in the temperature range allowing the growth of hazardous bacteria, i.e. between 55oC and 10oC. Cooling rates are often specified in legislation; for example, in the EEC Meat and Meat Products Directive 77/99, prepared meals must be cooled to below 10oC within two hours of cooking. Evans et al. (1996) have highlighted the importance of cooling and the mandatory requirements that exist in the UK for cook-chill products (Anon. 1982) These guidelines recommend that 80mm trays should be chilled to below 10oC in 2.5 hours, between 10 and 40mm they should be chilled to below 3oC in 1.5 hours. Assuming that surface freezing is to be avoided and a simple, single-stage operation used, only a 10-mm deep tray can be chilled within these time limits. Cooling of liquids or slurries may be done in line, using heat exchangers. Where batch cooling of solids and slurries is done in containers, the size of the container or the quantity of product should not be so large that rapid cooling is not possible. Product depths exceeding 10–15 cm should not be used because above this thickness conduction of heat to the surface, rather than removal of heat from the surface, will limit the rate of cooling of the bulk of the product and may allow microbial growth. When warm or hot material is loaded into containers for air cooling, the materials of construction of containers will exert a significant effect on cooling rates, thick-walled plastic containers cooling considerably more slowly than metal ones. The design of chillers, especially their air distribution pattern, air velocity and temperature and the way that product containers are packed into them, will control cooling rates. Racking or packing systems should allow the flows of cold air over the container surfaces so that cooling rates are maximised. Special attention should be paid to hygiene, and the control of condensation in chillers, as this is a major potential source of recontamination with Listeria if condensation is Microbiological hazards and safe process design 301 recirculated in aerosols over open containers, by the air flow. The importance of chilling after preparation, chilled storage and distribution have been discussed by Baird-Parker (1994) as critical control points in the manufacture of raw and cooked chilled foods. Farquhar and Symons (1992) have noted a US code of practice with recommendations for the preparation of safe chilled foods, it covers chilling, chilled storage, pre-distribution storage and handling and temperature management practices. Chilled storage Chilled storage should be designed to maintain the existing or specified temperature in a product. Product or ingredient containers should enter storage chills at their target temperature, because the performance of the air system (temperature and air velocity) and the way product is stacked are normally not designed to allow a significant reduction in temperature to take place. Manufacturing area (MA) A manufacturing area is a part of the factory that handles all types of ingredients. The process intermediates made in this area will be heat-treated before they are sold as products and will pass through hygienic or high-care areas during processing. Hygienic area (HA) A hygienic area is a defined processing area designed for the handling and preparation of low-risk raw materials and products containing a mixture of heat- treated and undecontaminated ingredients (Class 1). It should be designed and constructed for easy cleaning, so that high standards of hygiene can be achieved and especially to prevent bacteria, such as Listeria, becoming established in it and contaminating products. When it is used for the assembly of final products containing undecontaminated components such as cheese, it should not be used for the processing or preparation of any ingredients likely to carry pathogens and hence likely to increase the risks of products containing infectious pathogens. Areas conforming to this standard of hygiene should be used for the post-process handling of in-pack pasteurised products (Class 4). High-care area (HCA) This is a well-defined, physically separated part of a factory which is designed and operated specifically to prevent the recontamination of cooked ingredients and products after completion of the cooking process, during chilling, assembly and primary packaging. It is an integral part of the factory layout shown in Fig. 11.3 and is used for the preparation of products in Classes 2 and 3. Usually there are specific hygiene requirements covering layout, standards of construction and equipment, the training and hygiene of operatives, engineers and management and a distinct set of operational procedures (especially covering the intake and exit of food components and packaging material), all designed to limit the 302 Chilled foods chances of contamination. The usage of re-pack and re-work materials in high- care areas should be discouraged, and if this is not possible then very strict rules of segregation in time should be enforced. Air handling Air is a significant means of dispersing contaminants; therefore particular attention should be paid to the direction of flow of air within a manufacturing area or between areas. Within manufacturing areas the flow should be from ‘clean’ to ‘dirty’ to minimise the chances of contaminants being carried from raw to decontaminated product. The quality of the air should be related to the hygiene category of the area, see Manufacturing area (MA), Hygienic area (HA) and High-care area (HCA) requirements (CFDRA 1997). Cleaning Cleaning should remove food debris from process equipment, manufacturing and storage areas. Effective cleaning should remove all food debris from work surfaces, machines or an area, so that microbes cannot grow and subsequent production is not contaminated. Effective cleaning cannot be achieved unless equipment is hygienically designed and maintained. In practice, complete removal of food residues is rarely achieved by the techniques used for cleaning open plant (e.g. in slicers and dosing equipment). In factories manufacturing chilled foods, the residues after cleaning may provide growth substrates for the factory microflora, and experience has shown that many modern cleaning techniques and chemicals, when used in chilled areas, may actually select for Listeria. High-pressure (HPLV) cleaning if used in an uncontrolled fashion will generate aerosols that may contaminate products and equipment with food debris and bacteria. To minimise the risks of contamination, food and packaging materials should be removed from areas during cleaning. HACCP can make a valuable contribution in this area by identifying those process stages where hygiene is critical to the product quality and safety and also by checking from the process flow diagram that there is good access for cleaning within the factory layout. Disinfection Disinfection procedures should destroy any microbes left on cleaned surfaces and should be used at process stages where product re-contamination is a safety hazard. In practice, these procedures must often destroy or inhibit microbes remaining in the food residues invariably left after cleaning. Cleaning alone is able to achieve a satisfactory level of hygiene in hygienic or GMP areas, but high-care areas will require additional disinfection to provide extra confidence that viable bacteria are absent. Heat and a variety of chemicals are used as disinfectants. The effectiveness of disinfection will be reduced if the disinfectant is prevented from reaching the microbes by food debris; hence, thorough cleaning is always required to ensure effective disinfection. The assurance of good access of disinfectants is a primary objective of the hygienic design of Microbiological hazards and safe process design 303 equipment and the development of cleaning schedules. The systematic monitoring of specified sites in equipment or production areas either visually or microbiologically checks the effectiveness of cleaning. The effectiveness of disinfection can be checked by swabbing or chemical means. 11.3 The microbiological hazards The microbiological hazards of chilled foods can be roughly classified according to whether harmful microbes can infect the consumer or whether they multiply in the food and produce toxins that then cause disease soon after the food is eaten. The micro-organisms of greatest concern are listed in Table 11.2. All realistic microbiological hazards should be controlled by the product and process design. The real, not the specified, storage temperature and the length of the shelf-life must be assessed to determine whether a particular hazard is realistic in a particular product. Infectious pathogens can be hazardous at very low levels, whereas appreciable levels or growth of toxigenic micro-organisms are needed to cause a hazard. When designing a product or process it is very risky to assume that a particular microbiological hazard will be absent, e.g. because it has not been detected in a component. Processes should be designed to control all realistic hazards. The infectious pathogens (see Table 11.2 and Ch. 8) include Salmonella, E.coli O157:H7 and Listeria monocytogenes. They may be present in raw Table 11.2 Food-poisoning organisms of major concern and their heat resistance and growth temperature characteristics Minimum Heat resistance growth temperature Low Medium High Vegetative cells Spores Low Listeria monocytogenes (INF) a Clostridium botulinum type E, non-proteolytic B&F (TOX) Yersinia enterocolitica (INF) Vibrio parahaemolyticus (INF) Bacillus cereus (TOX) Medium Aeromonas hydrophilia (INF) Bacillus subtillis (TOX) Salmonella species (INF) Bacillus licheniformis (TOX) Clostridium perfringens (INF) High Escherichia coli O157 (INF) Clostridium botulinum type A & proteolytic B(TOX) Staphylococcus aureus (TOX) b Camplylobacter jejuni & coli (INF) Notes a INF, infectious; b TOX, toxigenic. 304 Chilled foods materials such as meat, vegetables and cheese made from unpasteurised milk. All may survive for long periods in chilled products (e.g. E.coli O157:H7 survives for 22 days at 8oC in crispy salad), if not eliminated by processing. All the infectious pathogens are heat sensitive and will be eliminated by the conditions used for pasteurisation (e.g. 70oC for 2 minutes or 72oC for 16.2 seconds). The growth of Salmonella and E.coli O157:H7 in products or in the factory environment may be controlled by refrigeration (i.e. by temperatures below about 10oC). E.coli O157:H7 has a low infectious dose and causes serious illness especially in the young and the elderly, as it attaches to the wall of the intestinal tract and causes acute, bloody, diarrhoea (hemorrhagic colitis) or haemolytic uraemic syndrome (a kidney disease). Several outbreaks of disease have been linked to chilled food and have usually had a bovine origin (e.g. undercooked ground-beef products), although unpasteurised cider and mayonnaise have been implicated. In the latter case it is thought that improper handling of bulk mayonnaise or cross-contamination with meat juices or meat products was the cause. It has been found that this E.coli is more tolerant of acid environments than other known strains, therefore it may survive in fermented dry sausage and yoghurt. Salmonella enteritidis is a potential hazard in products made from poultry and eggs, whereas the multi- drug resistant Salmonella typhimurium DT 104 can be found in a broad range of foods, outbreaks in the United Kingdom have been linked to poultry, meat and meat products and unpasteurised milk. Campylobacters may cause intestinal infections leading to fever, diarrhoea and sometimes vomiting. Sources include water, milk or meat. C. jejuni is regularly found on retail raw poultry and outbreaks have been associated with undercooked poultry and the cross-contamination of ready-to-eat materials via the hands of kitchen staff or work areas. It does not grow below 30oC and therefore conditions affecting survival are important, since sufficient cells must survive to form an infectious dose. Survival is better in chilled foods than at ambient or frozen temperatures. Vibrio cholerae may survive on refrigerated raw or cooked vegetables and cereals, if they have been sourced from tropical or warm areas where contamination is endemic. Particularly at risk are sea and other foods harvested from estuarine or inshore waters, waters subject to land- water run off or fields irrigated with sewage contaminated water. Contamination can also occur if the produce itself is cooled, washed or freshened with contaminated water. During preparation, food from this type of origin, which can include raw, pre-cooked and processed molluscs, crustaceans, fish and vegetables should be handled to minimise the chances of cross-contamination and pasteurised prior to sale. The psychrotrophic pathogens such as Listeria monocytogenes (Walker and Stringer 1987), can grow at refrigeration temperatures and may readily become established on badly designed or maintained equipment and in the factory environment. They are only detected in low numbers in environmental samples associated with the primary production of food (Fenlon et al. 1996) and are therefore likely to be contaminants arising from manufacturing conditions. Microbiological hazards and safe process design 305 Under otherwise optimal conditions, some strains of L. monocytogenes have been shown capable of slow growth at temperatures as low as C00.1oC, Yersinia enterocolitica at C00.9oC and Aeromonas hydrophila at C00.1oC (Walker 1990). For example, L. monocytogenes is able to grow well in components such as chill-stored, prepared vegetables and in many products lacking robust chemical preservations systems – such as chill-stored ready-meals and pa?te′. The most important of the toxigenic pathogens are the cold-growing non- proteolytic strains of Clostridium botulinum. Their growth in pasteurised foods is a particular risk if processing has eliminated the competing flora and their growth could precede spoilage. The proteolytic strains are less hazardous because they are able to grow only at higher temperatures and, unlike the non-proteolytic strains, they normally cause spoilage that renders the product inedible. At chill temperatures the growth rate of the non-proteolytic types is slow and so requires control only in products where the designed chilled shelf-lives exceed about 10– 14 days. Graham et al. (1996) suggest that non-proteolytic strains of Clostridium botulinum can grow at chill temperatures and therefore pose a potential hazard in minimally heat-processed chilled foods. They have developed models predicting growth and compared them with published data to demonstrate that they are suitable for use with fish, meat and poultry products. The models cover the combined effects of pH (5.0–7.3), salt concentration (0.1–5.0%) and temperature (4–30oC) and are based on the growth of non-proteolytic C. botulinum in laboratory media. Fortunately, the spores of the strains able to grow at chill temperatures are relatively heat-sensitive and therefore can be controlled by realistic cooking or pasteurisation processes (90oCC210 min. is the process recognised as suitable for these long-life chilled foods). Although cooking can eliminate the cold-growing types, storage temperature remains the most important control on the growth of clostridia. The United Kingdom, Advisory Committee on the Microbiological Safety of Food (1992) have discussed the potential hazards of chilled foods made using vacuum packaging and associated processes, such as sous-vide, and they particularly considered the risks associated with botulism. They highlighted methods to prevent and/or control the risks of botulism, including adequate heating based on the heat resistance of the spores and the restriction of shelf-life. See Smith et al. (1990) for details of the use of HACCP to address this problem. At temperatures above 12–15oC the mesophilic types (producing more heat- resistant spores) are able to grow, and the processes used in the manufacture of chilled foods certainly do not inactivate their spores. Bacillus cereus is sometimes mentioned as a potential hazard in chilled foods, though evidence of its ability to produce harmful toxins in these foods (with the possible exception of dairy products) is equivocal. Staphylococcus aureus is the other toxin producer causing concern in chilled foods, though it is of importance only when the food does not contain a competing microflora and has been substantially temperature abused. Nevertheless, it is important that high-care areas and the operational procedures used in them are designed to prevent heat-processed foods becoming contaminated with S. aureus. 306 Chilled foods 11.4 Risk classes Customers may well consume chilled ready-to-eat foods without sufficient heating to free them of infectious pathogens. Hence the risk to the customer depends on the number and type of microbes in the foods after manufacture and any growth during distribution and storage. Therefore, the processing and hygienic principles employed in the manufacture, distribution and sale of prepared chill-stable foods should be primarily designed to control the risks of them containing infectious or toxigenic pathogens. The control of spoilage microbes should be a secondary consideration in process design, although it may often require the application of more severe heating processes or more stringent conditions of hygiene or preservation, than the control of safety. Sometimes the microbiology cannot be controlled without prejudicing the sensory quality of a food product, and there must then be a commercial decision made on the acceptable balance between a controlled loss of quality and spoilage in the marketplace. In process and product design, however, microbiological safety standards should never be compromised to improve sensory quality. If the required processing conditions cannot ensure safety against the background of realistic consumer usage, then the product should not reach the market-place (see Gould 1992, Walker and Stringer 1990). Chilled foods may be categorised into well-defined risk classes (Table 11.1). Some prepared chilled foods (Class 1) are made entirely from raw ingredients and will obviously require cooking by the customer. Others, containing mixtures of raw and cooked components processed or packaged to ensure a satisfactory shelf-life (Class 2), may not so obviously require cooking and may contain infectious pathogens which may (e.g. L. monocytogenes) or may not (e.g. Salmonella) be able to grow during chilled storage. The manufacturer is able to control the safety of products only in the latter category (Class 2) by minimising the levels and incidence of pathogens on incoming materials (e.g. by careful choice of suppliers). To control risk, storage and processing procedures should not introduce additional contaminants or allow numbers to increase. The shelf- life and storage temperatures of such foods should be designed to ensure that only ‘safe’ numbers of infectious pathogens could be present if foods are stored for the full indicated shelf-life. At present there is no generally accepted estimate of the infectious dose of Listeria and it remains up to individual manufacturers or Trade Associations to decide on acceptable risks. Salmonella should be absent from these foods as the infectious dose is very low. Because Listeria is able to grow at chilled temperatures (for example, during storage, distribution and domestic storage), only its complete absence at the point of manufacture will ensure that ready-to-eat foods are safe, without qualification. Where it is present after manufacture, then the producer is accepting the risk associated with sensitivity of his customers to any L. monocytogenes that may be found at the point of consumption. Other chilled foods may contain only cooked or otherwise decontaminated components (Class 3), or may be cooked by the manufacturer within their Microbiological hazards and safe process design 307 primary packaging (Class 4). If manufactured under well-controlled conditions such foods will be free of infectious pathogens (such as Listeria and Salmonella) and spoilage microbes, hence they will have a substantially longer shelf-life (up to 42 days or more) than those containing raw components, as they will not be subject to microbial spoilage. This substantial extension of spoilage-free shelf- life has important consequences as to which of the potential changes in their microbiology should be recognised as limiting safety during storage (see below), and hence which controls and especially pasteurisation conditions are appropriate during manufacturing. 11.5 Safe process design 1: equipment and processes The manufacture of chilled foods is a complex process. From a microbiological point of view, processes should be designed to control the presence, growth and activity of defined types of bacteria. Some of the unit operations making up a typical process provide the opportunity to eliminate or reduce numbers of bacteria, others provide opportunities for re-contamination or growth. Product design and shelf-life (see Table 11.2) and possibly factory hygiene and layout, will determine the target bacteria for each stage of the process. At the very beginning of the supply chain, agricultural produce, farm animals and their products can act as reservoirs of food-poisoning bacteria (e.g. Salmonella, Campylobacter and E.coli O157). Therefore it is important that handling and processing takes account of this and reliable means of eliminating them or preventing contamination of products are put in place by the process design. The extent of precautions needed to provide effective control of a particular hazard will be proportional to the length, complexity and scale of the supply chain. For short-shelf-life (less than 10–14 days) prepared chilled foods, the main safety risk is the presence of infectious pathogens, and processes should be designed to cause a predictable reduction in their numbers. These types of bacteria may be a hazard if they survive processing or if recontamination occurs after a process step designed to remove them. There are two main routes for re- contamination via food-handlers or cross-contamination from other foods. In designing process flows and handling procedures it must be assumed that raw foods contain low numbers of food-poisoning bacteria, therefore effective separation of material flows is necessary. Risks are further increased if the product is ready to eat and the shelf-life and storage temperatures allow growth. Prolonging the chilled shelf-life introduces an additional hazard, arising from the growth of toxigenic bacteria, and processes need to be designed to eliminate them. This is because under normal chill conditions (i.e. below 10oC), over two weeks or so, cold-growing strains of Clostridium botulinum can grow to levels where toxin production is possible. Their spores may survive in foods pasteurised using the mild processes designed to destroy infectious pathogens. More severe heat processes need to be used in the manufacture of long-life foods 308 Chilled foods and these should be designed to cause a predictable reduction in the numbers of heat sensitive spores. If the preservation system of the food can prevent the outgrowth of these spores, then the heat process need be designed to control only spoilage micro-organisms. Many chilled foods do have such effective intrinsic preservation systems and are therefore safe, although having received very low heat processes. There is still no general agreement among manufacturers or regulatory authorities on the seriousness of the risks of botulism from unpreserved chill- stored foods. However, there is ample evidence that, in spore-inoculated model systems based on ready meals, growth occurs and toxin is produced at temperatures representing those known to be found under commercial conditions (Notermans et al. 1990). It is essential that if heating has not been done in the primary packaging, that unwrapped, heat-treated components intended for long-life foods are chilled, handled and assembled in a high-care-area to prevent recontamination with spores and infectious pathogens. Even if the risks of recontamination are controlled, there is a remaining risk from the survival in the components of heat- resistant bacterial spores able to grow at chilled storage temperatures. These are mostly Bacillus species that can eventually cause a musty form of spoilage. 11.5.1 Equipment Many of the critical quality and safety attributes of chilled foods are controlled by the technical performance of processing plant and equipment. The lethality of these heating processes used will exert a critical effect on the chances of microbial survival and presence in the product; therefore the correct design and reliable operation of heating stages is most important. Lag periods and growth rates of any contaminants will be influenced by a variety of process factors, including cooling rates and the accuracy with which storage temperatures are held. The uniformity with which preservatives, such as curing salts and acidulants, are dosed and mixed and the effectiveness of packaging machines at producing gas-tight packs (e.g. containing an inhibitory gas mixture such as CO 2 and N 2 ) are also critical factors. Food residues that have remained in a machine or in the processing area after cleaning or in a processing area for some time, are the dominant cause of microbial contamination and unless these residues are regularly removed they pose a hazard in finished products. Contamination is usually influenced to a much lesser extent by either airborne contaminants or contamination by personnel. Many codes of practice state that ‘food processing equipment should be designed to be cleanable and capable of being disinfected’. However, these statements hide the real issues in designing, operating and maintaining food manufacturing equipment, which often centre on finding an acceptable balance between cost, manufacturing efficiency and hygienic design (see Chapter 15). Microbiological hazards and safe process design 309 11.5.2 Heat processes – the use of heat to decontaminate products Heating methods In the manufacture of chilled foods, heat is the agent most commonly used to inactivate micro-organisms and cause beneficial textural and colour changes in products. Depending on the type of product, different types of equipment are used for heating; some examples are given below. ? Culinary steam or hot water in open vessels may directly heat mixes or particles suspended in sauces. ? Discrete pieces of meat, fish or vegetables may be cooked in trays, moulds or sous-vide packs heated in atmospheric ovens. Where the packs are sealed these products may be heated in water baths. Open vessels and ovens may be heated indirectly using a jacket or by direct injection, or circulation, of steam or air, or a mixture of both. Where this comes directly in contact with product, it should be of culinary quality. ? Liquid or pumpable ingredients may also be heated indirectly via jacketed equipment or using heat exchangers in pumped circuits. Normal production equipment can be used for product temperatures up to 100oC; where temperatures above this are used, or anticipated, any closed equipment should be specified and used as a pressure vessel. ? Solids such as pieces of meat or vegetable may be heated by contact heating or deep frying, for product surface temperatures exceeding 100oC, and lower centre temperatures. These depend on the initial temperature and heat transfer and heat penetration characteristics of the product. Packaged products or ingredients may be heated in retorts or other pressurised vessels using heating media with temperatures above 100oC; quite commonly, they are also heated in water baths. It is important for packs treated in this way either to be evacuated to ensure good heat transfer or that heating processes are set to take account of the insulating effect and expansion of any headspace during heating. Control of heating The heating process must be designed to deliver certain minimum heat treatments as specified above. Because of the importance of this, critical control parameters and tolerances should be defined in a specification – which must be available to the operatives in charge of the process. The training of these operatives should enable them to carry out the process reliably and to monitor and record its progress. This is most effectively done by following the time/ temperature conditions in the vessel or chamber or sometimes in the product itself. For each batch of product, it must be verified that the specified process and hence the target lethality have been achieved. To make reliable use of heat for killing micro-organisms, there are some essential points that must be considered by food processors and equipment manufacturers. 310 Chilled foods ? The delivery of heat to the product surface must be accurately and reliably controlled by the equipment and the way it is used and maintained, so that predictable heat transfer rates occur. Therefore a retort must always be loaded with packs positioned in the same way or the contents of a jacketed vessel must be stirred, to ensure that each product or pack is uniformly exposed to the heating medium. ? The rate of heat penetration into the product must also be known and controlled, so that target time/temperature integrals at the coldest or slowest heating points are reliably achieved. This is controlled by product formulation (for example the size of particles, the viscosity and other physical characteristics of the product), the pack size and shape and the thermal conductivity of the packaging material. ? The overall design of equipment, control systems and the provision of services, such as steam and cooling water or air, must be capable of ensuring that similar amounts of heat are delivered whenever the apparatus is run. Equipment suitability is not the only factor dictating how successfully or reliably the designed product temperatures are achieved. Management of the process may affect the characteristics of in-process material. For example, the temperature of a component at the start of processing – whether it is frozen, thawed or warm – will dictate heating rates, and such variables should be covered by the process specification. For safe products it is essential that whatever heating technique is used can ensure that a certain minimum amount of heat is delivered to all parts of the product or pack. Equipment performance Many pieces of commercial heating equipment, such as ovens, are not supplied with information indicating the distribution of heat within them. In addition the heating of a product will depend on the delivery of heat to the surface of the product, product unit or pack (heat transfer) and the penetration of heat within the pack. Investigation of these features is an essential part of process or product development. For example, the uniformity of heating in a jacketed vessel will depend on mixing – which may be done by an added stirrer or may be in the hands of the operator. In an oven, or retort, heat distribution can depend on the packing density or positioning of product, creating or blocking channels between product units upsetting uniform circulation of the heating medium. At a simple level, measurement of product centre temperatures and at a more complex level, process management involves estimation of heat flow within the product in response to conditions in the equipment (Fraile and Burg, 1998a, b). It is there- fore up to the user to produce his own ‘map’ of the distribution of heat under his particular conditions of usage, so that processes can be set using product units situated at the coldest part to ensure the consistent delivery of a minimum process. Stoforos and Taoukis (1998) have proposed a procedure for process optimisation that relies on the use of a two- or three-component time-temperature integrator for thermal process evaluation. Their proposed procedure can take Microbiological hazards and safe process design 311 account of the different z values associated with microbial killing and quality loss to assess the impact of particular combinations of time and temperature. The more variable or non-uniform the delivery of heat, the higher the target heat process needs to be fixed in order to ensure that the required minimum is achieved. An essential part of process development is investigation of the range of heat treatments delivered by equipment during the predicted range of operating conditions. Lethality criteria for process design are given elsewhere in this chapter and are related to the types and number of microbes that the process is designed to kill. Hence it is important that the input level of these microbes is also controlled; if the input numbers are higher than those intended by the process designer, then survivors will be found (possibly by the customer). Cooling Although heat is effective at killing micro-organisms, the effectiveness of cooling processes (USDA 1988) and the hygiene of cooling equipment must be specified equally carefully (James and Bailey 1990). Even in equipment designed to achieve rapid cooling rates, the risk of product recontamination remains, either from micro-organisms endemic in the chiller or taken in and then spread by the forced air circulation, often associated with rapid cooling. The rate of cooling is also critical as it will determine the extent of dormancy of any spores surviving in the product – this will affect their readiness to germinate and grow during product storage. The extent of dormancy is particularly important in shelf-life determination when heat has been used in conjunction with chemical preservation systems, such as salt and nitrite. 11.5.3 Microbiology of heat processing The heat process for packed products is usually delivered by hot water, by steam, by use of an autoclave, or, less commonly, by microwaves or ohmic heating. The extent of heating is selected to match the intended distribution and shelf-life of the food and the target micro-organisms for the type of raw materials and product (see minimum processes suggested for short and long shelf-life products, below). Target bacteria include the pathogenic non-spore- forming organisms, Salmonella, entero-pathogenic E.coli, Campylobacter Listeria monocytogenes and Yersinia enterocolitica and the spore-forming non-proteolytic strains of Clostridium botulinum, type E, some type B and F. Possibly some strains of Bacillus cereus that can grow slowly at temperatures as low as 4oC (van Netten et al. 1990) may be hazardous, but there is little epidemiological evidence. As raw materials are sourced globally, other pathogens may be introduced and the heat processing element of the HACCP plan should be reviewed to ensure that adequate heat processes are used. Whilst a heat treatment of 70oC for two minutes at the coldest part of a pack will ensure at least a 10 6 -fold reduction of L. monocytogenes, the most heat- resistant of the vegetative micro-organisms mentioned above such a treatment will have no effect on the spores of the psychrotrophic strains of Cl. botulinum. 312 Chilled foods Consequently, in the UK the low heat treatments of 70oC for two minutes are recommended only for short-shelf-life products or those, e.g. in catering operations, for which 3oC storage is certain to be maintained. (Glew 1990). In the Netherlands, a heat treatment of 72oC for two minutes has similarly been recommended, aiming at ensuring in excess of a 10 8 -fold inactivation of L. monocytogenes (Mossel and Struijk 1991). Although long slow mild heating, as in the original sous-vide processes, may sometimes be desirable for organoleptic reasons, it is important to remember that slow heating may trigger the so-called ‘heat-shock’ response, during which the resistance of vegetative micro-organisms to subsequent heating increases (Mackey and Bratchell 1989). For reasons of microbiological safety, therefore, warm-up or warm-holding times during processing should be short, or increased heat resistance may be found. Although psychrotrophic strains of Cl. botulinum cannot grow at or below 3oC, the possibility of their slow growth in long-life products at temperatures just above this, demands a more severe thermal process. This process should be designed to reduce substantially, i.e. by more than 10 6 -fold, the chance of survival of spores, but there is still debate about the minimum heat required. For instance, Notermans et al. (1990) concluded that there still remained insufficient data on the heat resistance of spores of non-proteolytic Cl. botulinum to ensure adequate lethality during conventional sous-vide processing (see below). They found that surviving spores could germinate, grow and form toxin within about three weeks at 8oC. Pre-incubation at 3oC shortened the subsequent time to toxin at 8oC. They concluded that if storage below 3.3oC cannot be guaranteed (as is often the case during retailing, and storage in the home), then storage time must be limited. However, it must be said that these products have been on the market for many years, with no recorded microbiological safety problems to date. The comments above refer to pasteurised vacuum-packed foods that do not rely on any preservation factor other than heating, vacuum-packing and temperature control during distribution for their shelf stability and safety. Such products are normally high water activity, near-neutral pH and preservative-free. Many other pasteurised vacuum-packed products have additional intrinsic preservation designed into them that additionally enhance keepability and safety (CFDRA 1992). For example, salt- and nitrite-containing products such as hams and other cured meat products, acidified pasteurised meat sausages and a wide range of a w -reduced traditional products, some of which are chill-stable and some even ambient-stable, e.g. the so-called SSPs (‘shelf-stable products’)of Leistner (1985). The processing, safety and stability requirements of these products should not be confused with those of conventional pasteurised chilled foods. 11.5.4 Pasteurisation for short shelf-life (Classes 1 and 2) Short-shelf-life products are designed to have a shelf-life of up to 10–14 days. The heat processes they receive during manufacture should cause at least a 6-log Microbiological hazards and safe process design 313 reduction in the numbers of infectious pathogens (Salmonella and Listeria) and their handling after heating and packaging should prevent recontamination. In neutral pH, high water activity products that do not contain antimicrobial preservatives, combinations of temperatures and times equivalent to 70oC for two minutes are more than adequate for this reduction. But practical experience has shown that longer times in this temperature range are needed for the effective control of some of the non-sporing spoilage bacteria, such as lactic acid bacteria. This combination of time and temperature is also suitable for customers to use to free the products from infectious pathogens. 11.5.5 Pasteurisation for long shelf-life (Classes 3 and 4) Long-shelf-life products have chilled shelf-lives that are sufficiently long to allow the outgrowth of any psychrotrophic spores surviving in them. Therefore, to ensure their safety and freedom from spoilage during their shelf-life, it is essential that the heat processes used eliminate any spores capable of growth. Therefore, processes designed to ensure safety should be designed to give at least a 6-log reduction in the numbers of cold-growing strains of Clostridium botulinum;90oC for ten minutes, or an equivalent process, is generally accepted as being sufficient for safety. But a process of this severity is not sufficient to eliminate similarly the spores of all psychrotrophic Bacillus species. In unpreserved products some types are able to grow to levels causing spoilage within three weeks or so, at 7–10oC – temperatures which are known to occur in the chill distribution chains in many countries (Bogh-Sorensen and Olsson 1990). These cold-growing spores often have D values at 90oCofupto11 minutes (Michels 1979). 11.5.6 Microwaves Microwave cooking or re-heating of foods, particularly in the home, has expanded greatly in recent years. At the same time, an increasing range of chill, ambient, and frozen-stored foods designed for microwave re-heating have been developed and marketed that meet consumers’ desires for improved conve- nience. Furthermore, it is likely that the use of microwaves to cook or pasteurise foods during processing will continue to grow in the future also. Key issues are the design and preparation of foods that have predictable microwave absorption, it is known that heating is determined by the dielectric of the food and its positioning and thickness within a product container (van Remmen et al. 1996). The practical problem is dosing sufficiently accurately on a commercial scale to ensure uniform and predictable heating from microwave absorption. Whether microwave or other forms of energy generate heat there is no fundamental difference in the lethal effect on micro-organisms. There is no well- documented ‘non-thermal’ additional microbicidal effect from commercial or domestic microwave equipment. However, there has nevertheless been some concern about the microbiological safety of foods re-heated in domestic 314 Chilled foods microwave ovens; see Sage and Ingham (1998), Tassinari and Landgraf (1997) and Heddleson et al. (1996) for discussions of heating variability and its consequences for microbial survival. Particularly if these foods have not been fully pre-cooked, they may have been contaminated after cooking or may contain raw ingredients. The concern has mainly been expressed following the demonstration of the presence of Listeria monocytogenes in a wide range of retailed foods, including some suitable for microwave re-heating. For example, a UK Public Health Laboratory Service survey found the organism to be detectable in 25 g samples of 18% of commercially available chill meals tested (Gilbert et al. 1989). This concern has led the Ministry of Agriculture, Fisheries and Food in the UK to review the issues thoroughly with oven manufacturers, the food industries, retailers and consumers, and to promote recommendations concerning the proper employment of microwave ovens for reheating – so that effective pasteurisation is achieved. Although it has been suggested that the unexpected survival of micro- organisms in food heated by microwaves may result from enhanced heat resistance of the micro-organisms (e.g. Listeria monocytogenes; Kerr and Lacey 1989). It is now generally accepted that when survival has occurred, it has resulted solely from non-uniform heating, leading to ‘cold spots’ in particular parts of the product (Lund et al. 1989, Coote et al. 1991). This is a consequence of heating using microwave energy and the fact that its absorption (and hence the rate of heating) depends, far more than with conventional means of heating, on the composition and quantities of the ingredients, the geometry of the product and its pack. Measurement of the heat-induced inactivation rates of Listeria monocytogenes in a variety of food substrates has shown that a 10-fold reduction in numbers is achieved by heating at 70oC for 0.14–0.27 minutes (D 70 = 0.14– 0.27 minutes: Gaze et al. 1989). Consequently, the recommendation that Listeria-sensitive products should receive a minimum microwave-delivered heating throughout of 70oC for 2 minutes, as in the UK Department of Health guidelines for cook-chill foods, is intended to result in a reduction of this organism of more than 10 6 -fold (Anon. 1989). Likewise, if microwaves are used during manufacturing for cooking short-shelf-life chill foods, they should be able to reliably deliver minimum amounts of heat to all parts of the product or ingredient. This is necessary to ensure a satisfactory reduction in numbers of Listeria and other, less heat-resistant, vegetative food-poisoning bacteria, i.e. a treatment of 70oC for two minutes, or some other time/temperature combination that delivers equivalent lethality, based on the D-values at 70oC of 0.14– 0.27 minutes and a z-value of between about 6 and 7.4oC (Gaze et al. 1989). 11.5.7 Products cooked in their primary original packaging (sous-vide products and REPFEDs) Although the term ‘sous-vide’ strictly refers to vacuum packing, without any indication of thermal processing, it has nevertheless become an accepted name for pasteurised ingredients or foods that are vacuum-packed prior to heat processing, Microbiological hazards and safe process design 315 often in their primary packaging (Risk Class 4). Sous-vide processed foods include meals and meal components, soups and sauces – all of which have extended chill shelf-lives for use in catering or manufacture and, more recently, retail sale. Sous- vide products are processed at relatively low temperatures, 55+oC. The heat treatment has to be sufficient for them to remain safe and microbiologically stable at storage temperatures below 3oC (the minimum theoretical growth temperature of the cold-growing types of Clostridium botulinum). Depending on the severity of the heat process and the microflora of their ingredients, they may have shelf-lives of up to about six weeks, Church and Parsons (1993). An outline for a HACCP study has been published (Adams 1991) The most comprehensive early research and evaluation of sous-vide processing were for catering, at the Nacka Hospital in Stockholm. Prepared foods were vacuum-packed, rapidly chilled, then stored under well-controlled refrigeration for periods of one or two months prior to reheating for consumption (Livingston 1985). This was followed by extension to a variety of more-or-less centralised catering activities in a number of European countries before the more recent extension, mostly in France, to retailed foods. Concerns about possible microbiological safety problems have been expressed, not because of doubts about the principles underlying the sous-vide process, but because of the difficulty in confidently ensuring the maintenance of the required low temperature (max. 3oC) throughout long-distance distribution chains and, especially, in the home (see Betts and Gaze (1995), Juneja and Marmer (1996), Peck (1997) for a discussion of the risks of botulism and Hansen and Knochel (1996) and Ben-Embarek and Huss (1993) for the effectiveness of sous-vide processes against Listeria monocytogenes). Turner et al. (1996) for effectiveness against Bacillus cereus and spoilage bacteria in chicken breast). A proposal for a Code of Practice is given by Betts (1996). 11.5.8 Alternative process routes Industrial sous-vide and REPFED processing procedures mostly follow the original concept, and the conventional canning route, of filling food into packs, sealing, then heating. The alternative – heating, followed by filling and sealing – unless undertaken truly aseptically, risks the introduction of micro-organisms after the heat process and prior to sealing, and is therefore principally used for short- shelf-life products. If products are hot-filled (C6280oC), extended shelf-lives may be achieved when storage is at 3oC. For this type of process, the design of the filling equipment and the type of packaging and sealing machinery are very specialised. 11.6 Safe process design 2: manufacturing areas Manufacturing areas and production lines should be laid out on the principle of forward flow so that the chances of cross-contamination, or of products missing a process stage, are minimised. 316 Chilled foods 11.6.1 Raw material and packaging delivery areas Most factories will have designated areas for deliveries; they may be divided into different areas for the various commodities to be processed or according to their storage requirements, e.g. frozen, chilled or ambient-stable. Separation may also be governed by legislative requirements. The delivery area should allow the efficient and rapid unloading of vehicles and it should have facilities for the inspection and batch coding or maintaining batch integrity of incoming raw materials. Its organisation should allow the direct removal of these materials to storage areas; thereby allowing maintenance of storage conditions (e.g. frozen or chilled temperatures). Incoming materials should be protected from contamination and there should be facilities for the removal and disposal of secondary packaging such as cardboard boxes. If product is unpacked, clean containers may be required for product handling and storage prior to use. Reception areas should be operated to minimise the opportunities for cross- contamination, especially when high-risk materials, which may contaminate other ingredients, are brought in or when materials for direct use in finished products (such as packaging) are handled. Areas must be capable of being effectively cleaned and should not be used for the storage of any packaging that is removed after delivery. It may be necessary to disinfect the outer packaging of materials prior to their admission to hygienic or high-hygiene areas. Ideally, materials that will be used in HCAs should be delivered to separated areas handling only low-risk ingredients and not to areas handling raw materials that are likely to contaminate them with pathogens or with excessive numbers of spoilage bacteria. 11.6.2 Storage areas Chilled foods are made from a variety of ingredients or components made within the factory, which should all be stored so that contamination and premature spoilage are prevented. These materials differ in the storage conditions they require prior to use and also in the stage of the process at which they are made, added or combined. Therefore a chilled products factory will require a number of designated storage areas. All these areas should be controlled, most commonly for time and temperatures in the chilled (0C03oC) or frozen (below C012oC) ranges, and the rotation of stock should also be controlled. A low humidity store is required for dry ingredients and packaging materials. All temperature controlled areas should be fitted with reliable control devices, monitoring systems (to generate a record of conditions in the store) and an alarm system indicating loss of control or the failure of services. Batches of stored materials should be labelled so that their use by dates and approvals for use are clear and particular deliveries or production batches can be identified. Layout of the store should allow access to the stored items and effective stock rotation. The operation of storage areas and the quantity of product they contain, their services and control of the means of access, such as doors, should ensure that the designed conditions (such as 2–4oC in chillers) are maintained during the working day. The services, especially supply of refrigerant, should have Microbiological hazards and safe process design 317 sufficient capacity to allow maintenance of product temperatures during high outside temperatures or peak demand for cooling. All storage areas should be easily cleaned, using either wet or dry methods, as appropriate, and racking should not be made of wood. The layout of racking and access to floors, walls and drains should allow cleaning to take place. 11.6.3 Raw material preparation and cooking areas These areas receive ingredients from the storage areas and are used to convert them into components or ingredients by a variety of techniques, such as cutting, mixing, or cooking. Preparation and cooking may take place in a single area where the materials are to be used in cook-in-pack (Class 4) products, but when a long shelf- life and prevention of contamination are important (for Class 2 or 3 products) cooking may be done in a separate area to prevent cross-contamination. Where the cooked product is to be taken to a hygienic area for cooling and packaging to make Class 2 or 3 ready-to-eat products, it is essential that the layout and operation of the area, and access from the preparation area, are designed to prevent recontamination after cooking. If space permits, cooking operations should be carried out in a separate area from preparation, to minimise the chances of contamination by airborne particles, dust, aerosols or personnel. Where physical separation cannot be achieved, cooked product should not be handled by personnel or with equipment that has been in contact with uncooked material. In rooms or areas where cooking is done, the cooking vessels or ovens may be used to form the barrier between ‘dirty’ areas, i.e. those handling uncooked material, and ‘clean’ areas. Air flow should be from ‘clean’ to ‘dirty’ areas, and the supply of air to extraction hoods should be designed to ensure that it does not cause contamination of cooked product. Where single-door ovens are used there is an increased risk of cross-contamination, as it is not possible to segregate raw and cooked material effectively. The entry/exit areas from these areas need to be kept clean, loading and unloading done by separate staff, and the opportunities for product contact minimised by the organisation of the area. For short-shelf-life (Class 1) product and products containing components which have not been reliably decontaminated (Class 2), the main risk is recontamination with infectious pathogens. When the cooking stage is used to provide the 90oC/10 minute heat treatment required for long-life products (Class 3), then stringent precautions must be taken to prevent re-contamination of the cooked product with clostridial spores. These include a forward process layout, physical separation of process stages and control of airflow away from de- contaminated product. Typical process routes for short- and long-shelf-life products are shown in Figs 11.1–4. The control of condensation by the extraction of steam from cooking areas is very important and the effective ducting away of steam is essential if the recontamination of cooked product by water droplets is to be avoided. Preparation rooms or areas should be chilled, but it is often impractical to chill cooking areas, and indeed this may increase problems associated with condensation. 318 Chilled foods 11.6.4 Thawing of product Prior to use, it may be necessary to thaw frozen ingredients such as meat and fish blocks. This should be done under conditions that minimise the potential for the growth of pathogens, i.e. the maximum surface temperature of the ingredient should not be within their growth range (i.e. it should remain below 10–12oC). If this is not possible, then thawing times should be minimised to prevent growth. Special thawing equipment such as microwave tempering units, running-water thawing baths or air thawing units may be used for microbiologically safe thawing. Such equipment should be operated according to a technically justifiable specification. Thawing of perishable products at ambient tempera- tures is not desirable as it can lead to the uncontrolled and unrecognised growth of hazardous micro-organisms. Where frozen ingredients are to be directly heat processed, it is important that the particles or pieces entering the cooking stage of the process are of a uniform size and controlled minimum temperature, so that cold-spots, which will be insufficiently heated, are not accidentally created. 11.6.5 Hygienic areas Hygienic, not high-care, areas should be used for the assembly or processing of prepared foods made entirely of raw (Class 1), or of mixtures of raw and cooked (Class 2) components. Such areas should be designed and operated to prevent infectious pathogens becoming established and growing in them, by attention to a number of key requirements shown below. Temperature Hygienic areas should ideally be chilled to 10–12oC or below, to limit the potential for growth of Salmonella. However, chilling alone will not limit the growth of Listeria, as these organisms can grow at chill temperatures. A very effective, if expensive, solution is to operate production areas and chillers dry, with an environmental humidity below 55–65%, which is well below the minimum water activity for the growth of Listeria and also low enough to ensure relatively rapid drying of surfaces. A relative humidity in this range is both comfortable for operatives and effective at drying floors, walls, ceilings and equipment. However if control of humidity is not possible, e.g. because an existing area is being used or has been upgraded, then hygienic control should be concentrated in areas known to harbour Listeria and these should be effectively cleaned, disinfected and dried on at least a daily basis. To do this effectively is very demanding of time, resources, and training of hygiene operatives and management. Construction The standards of construction of hygiene areas for storage and manufacturing should enable hygiene to be managed effectively. Floors, walls and ceilings should be constructed of materials that are impervious to water, are easy to clean, and allow routine access to services from outside the area. Floors should Microbiological hazards and safe process design 319 be sloped so that pools of water do not collect and remain on them. Drains, especially, should be designed so that food waste is not retained in them for long periods. The direction of flow should ensure that material from preparation and raw material handling areas does not flow into or through hygienic areas and any drain traps used should be capable of being cleaned to the same standards as food processing equipment. All these precautions are designed to minimise the chances of product contamination during processing and to reduce the chances of infectious pathogens being present. Stock control Minimum amounts of material undergoing processing should be kept in hygienic areas or in any associated chillers. Stocks of these components or materials known to contain infectious pathogens or to support the rapid growth of Listeria, under chilled conditions, such as prepared vegetables, should be minimised so that storage periods are as short as possible. In summary: operational procedures should be designed to minimise three things: 1. the numbers of Listeria or other infectious pathogens brought into the area 2. the opportunities for their growth in the area 3. the number of environments where they can survive. It is worth remembering that during the periods between cleaning and production, when the area may not be chilled, Listeria and maybe other pathogens and spoilage bacteria that may be present, will continue to grow in any dilute food residues left on imperfectly cleaned equipment or in drains and chillers. The layout of the area and the operating procedures should be designed to minimise the opportunities for contamination of the product by equipment or personnel. In addition, the transfer of contaminants from one batch of material to another should be minimised by effective working practices, including a clean- as-you-go policy and a suitable product flow and a batching system providing hygiene breaks at suitable intervals. This is critical if allergenic materials are handled. Mixed raw and cooked components Where ready-to-eat products are to be made from mixtures of raw and cooked components, it is most important that any components known to have a high risk of containing pathogens or high numbers of spoilage bacteria are excluded from the area (and the product!) unless they have been effectively decontaminated. In practice, this often means ‘cooked’. With some products, such as hard cheeses, prior processing means that they can be used safely in hygienic areas provided that their levels of surface contamination are controlled – to stop cross- contamination leading to spoilage or to a safety risk. It is important to remember that pathogens such as L. monocytogenes have been demonstrated to sometimes survive even the processing of spray-dried milk powder, and in cheddar, cottage and camembert cheese (see listings by Doyle, 1988). Examples of higher-risk materials, which should be excluded from such areas, are prawns and other 320 Chilled foods shellfish from warm waters and untreated herbs and spices, which may carry Salmonella. However, it must be accepted that from time to time these products and their components will contain pathogens, and hence storage conditions and hygiene practices in manufacturing areas should be designed with this in mind. 11.6.6 High-care areas General High-care areas are designed for the post-cook handling, cooling and assembly of ready-to-eat products made entirely of cooked (or otherwise decontaminated) components (Class 3). They should therefore be designed and operated to prevent recontamination with food-poisoning bacteria and minimise re- contamination with spoilage bacteria. High-care areas are used for handling cooked components which will be chilled, assembled and packaged or may be further processed (e.g. sliced, cut or portioned cooked meat products, such as pate), and then packaged. The shelf-life and safety of this type of ready-to-eat product relies very largely on the prevention of recontamination, though a few of them do have, in themselves, preservation systems capable of halting the growth of Listeria. High-care areas are also used for the handling of long-shelf-life (Class 3) products made from components which have been freed of hazardous bacteria (such as cold-growing strains of Clostridium botulinum) by the cooking process. All operations downstream from the heating process, including chilling, assembly and packaging prior to the further mild in-pack, heating that frees them of vegetative spoilage microbes must be done in a high care area. For these processes to yield safe product with a reliable long (up to 42 days) shelf-life, effective control of recontamination after cooking must be achieved. There are a number of specific requirements for high-care areas. Physical separation Sometimes such areas are designed and operated to a higher standard of hygiene than strictly necessary to guard against contamination by pathogens, in order to limit product recontamination with spoilage micro-organisms, such as yeasts, moulds and lactic acid bacteria. These precautions will minimise the incidence of spoilage during the products’ shelf-life. Only materials, including both foodstuffs and packaging, which have been reliably decontaminated and handled to prevent recontamination, should be admitted to high-care areas. Such areas should be physically separated from all other production areas handling components that may still be contaminated and this separation should extend to the staff operating in those areas. Surfaces All the surfaces of the high-care area should be sealed and impervious to water and capable of being easily cleaned, disinfected and kept dry. After cleaning and Microbiological hazards and safe process design 321 disinfection, surfaces should have fewer than 10 micro-organisms per 9 cm 2 and Enterobacteriaceae should not be recoverable. The drains in high-care areas should not connect directly with areas processing, storing or handling raw materials or used for cooking; their direction of flow should be away from the high-care area. Chillers and cooling An integral part of the high-care area will be chillers, either designed to cool components (blast chills; James et al. 1987) or to maintain chill temperatures in previously cooled components. Cooling of cooked, hot products should be started as soon as practicable after cooking. As suggested before, cooling rates should be designed to prevent the growth of any surviving spore-forming bacteria. Chillers will often receive naked product and therefore both the quality of the air used for cooling and the environmental hygiene are critical (see below). As warm product will be placed in the chillers, condensation will occur and it is important that this is ducted to drain in such a way that product contamination and the retention of condensate in the area are avoided. The design, hygiene and operating temperature of the cooling elements in fan-driven evaporator units in chillers is critical in limiting the potential for product recontamination by aerosols. If the temperature of the heat exchanger coils is too high, water will condense on them, without freezing, and then be blown onto the products by the draught of the fan. Many evaporator units are designed so that the condense tray does not drain and is inaccessible for cleaning; such units can harbour Listeria and cause contamination in high-care areas. Where water is used as the direct cooling medium – either as a spray or shower or in a bath for products for hermetically sealed containers – chlorination or some other disinfection procedure should be used to ensure that the product will not be recontaminated by the cooling water. Stringent cleaning and disinfection systems should be used to ensure the hygiene of circulation or recirculation systems, including heat exchangers. Packs should be dried as soon as possible after cooling and manual handling of wet packs minimised. Air supply The air supply to these areas should be filtered to remove particles in the 0.5–50 micron size range and the system should provide control of the air flow, from clean to dirty, by means of a slight overpressure, which prevents the ingress of untreated air. Air supply and heating and ventilation systems should be designed for easy access for inspection and cleaning. 11.6.7 Waste disposal The efficient removal and disposal of waste from manufacturing areas is essential to maintaining the hygiene level. Suitable storage facilities and containers should be provided and the design and operation of waste disposal systems should prevent product contamination. Any equipment or utensils used 322 Chilled foods for the handling of waste should not be used for the manufacture or storage of products. Such equipment must be maintained and cleaned to the same standards as the area in which it is used. If waste is not removed frequently (4–8 hourly) then separated refrigerated storage facilities should be provided. 11.7 Safe process design 3: Unit operations for decontaminated products 11.7.1 Working surfaces for manual operations Many of the operations involved in preparation or product assembly will be carried out on tables or other flat surfaces. It is important that these surfaces are both hygienic and technically efficient, i.e. stainless steel is not suitable as a surface for cutting on, although hygienically it is excellent and easy to keep clean. For cutting, surfaces made of nylon, polypropylene or occasionally Teflon can be used. Working surfaces should enable most operations to be done effectively and also should be easy to remove for cleaning or be completely cleanable without removal. It should be possible to restore the integrity of the surface by machining or some other process, as cut or scratched surfaces are impossible to keep in a hygienic condition and are a potent source of contamination. The use of Triclosan impregnation of surfaces has been recommended to provide a degree of antimicrobial protection on surfaces and polyurethane or fluoropolymer conveyor belts, but Cowey (1997) recommends that it is effective only as an additional line of defence, not as a replacement for existing hygiene procedures. 11.7.2 Cutting and slicing Many products designed for chilled sale, such as meats and pa?te′s are prepared or cooked as blocks. Automated slicing or cutting after cooking is an integral part of their conversion into consumer packs. Slicers can be potent sources of contamination because they are usually mechanically complex, providing many inaccessible and uncleanable sites that can harbour bacteria. These bacteria live in the debris continually produced by the cutting operation, which is not effectively removed by cleaning procedures used at the end of production and hence constitutes a microbiological hazard. The effectiveness of cleaning procedures is further reduced if cleaning operatives try to avoid wetting sensitive parts of machines, such as electronic controls, motors and sensors, which may be rendered inoperative by water penetration during cleaning – especially when high-pressure cleaning is used. An integral part of the hygienic design of such machines is good access for cleaning and inspection, and effective water-proofing. An insight into the routes for microbial recontamination of product can be gained by auditing the machines for product and debris flow during operation, so that the sources and the risks of recontamination can be identified. After production, when the equipment has been cleaned, machines should be re- Microbiological hazards and safe process design 323 examined to detect areas where product is still retained, so that difficult-to-clean parts are identified, and effective cleaning schedules specially referring to them developed, implemented and monitored. The difficulties of controlling the hygiene of such machines typify the much more widespread problem of recontamination of product by process equipment. Slicing and other forming machines handling cooked product generally operate in chilled areas; the intention of this practice is both to improve their technical performance and to slow or halt the growth of microbial contaminants by minimising temperatures in retained product debris. Temperature auditing of this type of machine will show that there are, however, many sources of heat, which are not effectively controlled by environmental cooling, e.g. motors and gearboxes. In a well-designed machine this heat is conducted away into the machine without significant temperature rise, but in other cases there will be localised hot spots in contact with food or debris where microbial growth will occur. It is not uncommon for some parts of these machines to operate at temperatures well above the design temperature of the area. For example, in the area of the main cutter shaft bearing and motor of a slicer, temperatures of 25oC+ can be found when material at below 4oC is being processed in an area operating at 7–10oC, providing an ideal growth temperature for many bacteria. During operation, such warm spaces in machines may fill with product debris, which is retained, warmed or incubated and then released back into or onto product. Ideally, such hot spots should have been eliminated during the design of the machine, but in practice in many existing machines, these risks can only be minimised. For safe, hygienic operation, the critical areas of machines, i.e. warm ones retaining product, must be identified, and then controlled by suitable cleaning or cooling or other preservation techniques to ensure that microbial growth and product contamination are minimised. In some cases, simple engineering modifications may be used to improve the hygiene of machines by reducing the quantities of product retained and its retention time. 11.7.3 Transport and transfers From the time that a product is cooked, it can undergo many transfer stages from one production area to another before it is finally put into its primary packaging and contamination is excluded. Containers In the most simple process lines, products that have been cooked in open vessels will be unloaded into trays or other containers for cooling. Whatever sort of containers are used they should not contaminate the product, and their shape, size and loading should ensure that rapid cooling is possible. Generally, stainless steel or aluminium trays are more hygienic than plastic ones, because the latter type become more difficult to clean as their surfaces are progressively scratched in use. Plastic also has slower rates of heat conduction than either stainless steel or aluminium, and therefore metal trays are to be preferred. 324 Chilled foods Belts In most complex production lines, transfer or conveyor belts may be used for transporting both unwrapped product or product in intermediate wrappings from one process stage to another. Belts may also be an integral part of tunnel or spiral equipment, such as cookers, ovens and coolers. Although many types of belting material are used, they can be broken down into two broad groups: fabric or solid, and mesh or link. For hygienic operation, fabric conveyors should be made of a material that does not absorb water and has a smooth surface finish, so that it is easy to clean and disinfect. Fabric belts are generally used only for transport under chill or ambient conditions, as they are not often heat-resistant. Solid belts used for heating or cooling (by conduction) are usually made of stainless steel, which is hygienic and can be heated or cooled indirectly. Fabric and other solid belts can be cleaned in-place by in-line spray cleaners, which can provide both cleaning and rinsing and may incorporate a drying stage using an air knife or vacuum system. The hygienic problems of conveyor belts are usually associated with either unhygienic design of drive axles and the beds or frames supporting them or poor maintenance. Mesh or plastic link belts are used for a wide variety of transport and other functions. Their main advantages are that they can carry heavy loads and form corners or curves. Metal mesh belts are widely used in ovens and chillers where circulation of hot or cold air is a part of the process, for example in spiral coolers or cookers. Belts that are regularly heated, or pasteurised, do not present a hygiene problem, as any material retained between the links will be freed of microbes by the heating. When such belts are used in coolers, or for the transport of unwrapped product at ambient temperatures, special attention must be paid to their potential to recontaminate product with micro-organisms growing in retained debris. Cleaning systems should be devised which remove the debris from between the links (for example, by high pressure spray cleaning on the return leg of the belt). After cleaning, the belt should be disinfected or preserved, for example by chilling, so that microbial growth does not occur during the processing period. With all forms of belt, hygienic performance becomes more difficult to achieve if routine engineering maintenance is not carried out correctly and the belt becomes damaged or frayed during use. Specialised belts are often used in product delivery, packaging and collation systems. In such equipment, incorrect setting, or use with fragile products, will increase the quantity of product waste generated, so that even a properly designed and operated cleaning system will not keep the system in a hygienic condition. Product characteristics, such as stickiness and crumbliness, should therefore be considered when transfer systems are designed, so that the generation of debris and, in turn, hygiene problems are minimised. 11.7.4 Dosing and pumping Most chilled products are sold in weight-controlled packs, often with individual ingredients in a fixed ratio to one another, for example meat and sauce. Where Microbiological hazards and safe process design 325 the ingredients are liquid or include small (ca. 5 mm) particles suspended in a liquid, they may be dosed using filling heads or pump systems. If this type of equipment is used for dosing decontaminated materials, then its freedom from bacteria and its hygienic design and operation is critical to product safety and shelf-life. Dosing and filling systems may be operated at cold (below 8–10oC), intermediate (20–45oC) or hot (above 60oC) temperatures. The most hazardous operations are those run at intermediate temperatures, which allow the growth of food-poisoning bacteria, and unless the food producer is completely confident of the hygiene of his equipment and is prepared for frequent cleaning/disinfection breaks, such temperatures should not be used. Where hot filling is the preferred option, control of the minimum filling head and residual product temperatures is critical to safety. Often, ingredient target temperatures are set well above the growth maximum of food-borne pathogens (ca. 55–65oC) to allow for cooling during dosing, and especially for breaks in production when the flow of product is halted. If product is supplied to the filling heads by pipework, it is necessary to ensure that an unacceptable temperature drop (leading to temperatures in the growth range) does not occur at the boundaries of the pipes or during breaks and stoppages. For this purpose a recirculatory loop returning to a heated tank or vessel may be used. Such dosing equipment is often cleaned by CIP (clean-in- place) systems and it is essential that the pumps, valves and couplings used are suitable for this form of cleaning as well as for their production function. Weight control Where in-pack pasteurisation systems are used, the reliability of dosing systems in delivering a consistent amount of product plays a major role in ensuring that packs with uniform heating characteristics and headspaces are controlled. Therefore, dosing accuracy should be carefully controlled. 11.7.5 Packaging Primary packaging Most chilled products are sold in a packaged form, the most common technical functions of packaging being to prevent contamination and retain the product. Packing materials may be chosen for a variety of technical reasons, such as machinability and heat resistance; but from a microbiological point of view, their most important attributes are the ability to exclude bacteria, physical strength and gas barrier properties. This last property is most important if packaging is part of the product’s preservation system; for example, if it contains an inhibitory modified atmosphere or a vacuum. Hence packaging equipment can perform an important function in ensuring product safety, and it must be able to consistently produce strong, gas- and bacteria-tight (hermetic) packs. 326 Chilled foods Modified atmosphere packs Many products designed for chilled storage rely on control of the composition of the gas surrounding the product as part of the preservation system. Exclusion of oxygen from the headspace by vacuum packing, or its replacement with either carbon dioxide or nitrogen or a blend of both gases during the packaging operation, gives a considerable extension of shelf-life at chilled temperatures, for example with chilled meats. The efficiency of flushing and replacement of oxygen with the gas mixture, control of vacuum level, and the frequency with which leaking packs are produced are key operational parameters for this type of packaging. Colour indicators have been advocated to indicate leaking packs (Ahvenainen et al. 1997). There are many reasons for the production of leaking packs (leakers) which have a shortened shelf-life and also present an increased risk of contamination. Apart from the reasons (such as incorrect choice of packaging film, or equipment faults such as mis-setting of the sealing head temperature, its pressure, alignment or dwell time), soiling of the seal area of the pack with product during filling is the major cause of pack failure. If product or product residues remain in the seal area during the sealing operation, then the two layers of plastic of the lid and base cannot be welded together by the sealing head. Some sealing heads are profiled to move soiling out of the seal area during the sealing cycle. But with many combinations of sealing head profile and products (i.e. fills) this is not an effective solution to the problem, as food still remains in the seal area, preventing formation of a continuous weld or causing a bridge to be formed across it. Sealing problems are encountered especially when flat or unprofiled sealing heads are used, as these trap material under them during sealing. Problem fills are fat, water (which turns to steam on heating) and cellulosic fibres. Where the pack is vacuumised, a faulty seal will be evident as the pack will not be gas tight; this will be seen, or can be felt, soon after manufacture. Where the pack has a headspace, a faulty one may feel soft, if squeezed. If packs with the incorrect headspace volume or with weakened seals are processed in systems such as retorts or ovens that generate a pressure differential between the pack and its surroundings, bursting or pack weakening may result. Other types of pack Short-shelf-life products may be an exception to the general preference for bacteria-tight packs. Some of these products are sold in packs where a crimped seal is formed between the lid and the base container (which may be made of aluminium, for example). In this type of pack there is a risk of product contamination, unless it is overwrapped in a sealed pouch. The function of such tray packs is to provide a container for oven cooking or reheating by the customer. Additionally, it is important that the packaging films or trays coming into direct contact with the products do not contaminate them either chemically or microbiologically. Overwrapping of the primary or food contact packaging and Microbiological hazards and safe process design 327 its handling within the storage and production areas should be designed to minimise the chances of contamination. Hence handling and disinfection or overwrapping procedures used in the factory should take full account of the destination of the product and the hazards that must be controlled. Secondary packaging Once the product is in its primary packaging, it is either completely protected from recontamination (in a hermetically sealed pack) or well protected in a sealed, film wrap or pack with a crimped seal. Primary packs sometimes have additional, secondary packaging. The function of this packaging may merely be decorative, but in some cases it may be to protect the pack from damage or stress during handling or transport. For this latter function, control of the secondary pack characteristics should be a part of the factory QA system. When chilled products are handled or marketed either in boxes or closely packed on pallets, there is only a limited opportunity for changing their temperature – as the surface area to volume ratio is unfavourable to rapid temperature changes, and rates of heat penetration through product and packaging are low. Therefore it is essential that product in its primary packaging is at the target temperature prior to secondary packaging and palletisation. 11.8 Control systems 11.8.1 Instrumentation and calibration Wherever control limits are specified in the HACCP plan, it is essential that reliable instrumentation or measurement procedures are present and correctly located and calibrated. Their output can either be used for the control of process conditions (such as during pasteurisation, chilling or storage) or for monitoring compliance with specifications. Sensors and their associated instruments may be in-line (e.g. oven, heat exchanger or fridge thermometers), at the side of the line (e.g. drained weight apparatus, salt or pH meters) or in the laboratory (e.g. colour measurement or nitrogen determination). Wherever the instrument is situated, it needs to be maintained, with its sensor kept free of product debris – as this may produce erroneous signals, it needs to be calibrated and the operative needs to have a measurement or recording procedure. 11.8.2 Process monitoring, validation and verification Manufacturing, storage and distribution operations within the supply chain should be controlled and monitored to ensure that the whole chain performs within the agreed limits. Wherever possible the data from control systems should be recorded and used to produce management and operative information and trend analyses. Thermocouples can be used to measure product temperatures. The sensors need to be prepared, installed and monitored, to 328 Chilled foods prevent errors in temperature readings (Sharp, 1989). Often responsibilities for the safety and quality a range of products will be shared between several suppliers and producers. Concentration of businesses on their core activities means that vertical integration within the supply chain is uncommon, therefore product safety and quality systems rely on effectively managed and specified customer–supplier relationships. Even if safe process and product design principles originating from a HACCP study have been used and cover the realistic hazards, unsafe products can result if the conditions and procedures noted in the plan are not carried out correctly or are not working effectively. Verification is an auditing activity that systematically analyses the working and implementation of the HACCP plan, by examining process and product- related data. These data may also be compared with specifications and other technical agreements which are not part of the HACCP but none the less form the customers’ requirements. In-house QA departments, regulatory authorities, auditors and those who are inspecting suppliers on behalf of customers, are usually responsible for verification. They act as systems experts working to establish compliance with systems, procedures and the other outputs of the HACCP plan, or in some cases the ISO 9000 series documentation. As a minimum, verification should focus on data showing the producer’s performance at each CCP and use his existing procedures, systems and records, supplemented if necessary with sample analysis, record inspection and auditing to form an opinion on the consistency of product quality or how well the process is controlled. To establish what data should be included in verification, the series of steps in the HACCP plan, hazard analysis, identification of critical control points (CCPs), establishment of control criteria and critical limit values and monitoring of the CCPs, should be considered. Any process stages where the risks of microbial contamination, survival or growth are significant should be covered. The aim is to show how well the workforce and management is complying with the requirements of HACCP plan. Operational risks investigated by verification include poor training and management procedures, poor hygiene and segregation of raw and cooked material, inadequate management of heating, cooling or packaging and occurrence of defective products. It is done routinely after implementation and is part of the review of the scientific and technical content of a HACCP plan. Reliable verification is based on validation. Validation examines the scientific basis of the HACCP plan and the range of hazards covered. It should be done prior to its implementation of the plan and regularly during production, when it becomes part of the HACCP review procedure. The function of validation is to determine whether realistic hazards have been identified, and suitable process control, hygiene and monitoring measures implemented, along with safe remedial actions for use when processes go outside their control limits. Because many chilled foods are sold as ready to eat and are unpreserved, except by chilled storage, the specification of correct heat processing, prevention of re- contamination and assurance of storage temperatures and times are essential activities and should be directed at specific hazards. Unsuitable process features Microbiological hazards and safe process design 329 which should be uncovered by validation include the specification or operation of unhygienic equipment, the recommendation of working practices, targets, controls or layouts, which may lead to product contamination or the design of unstable preservation or distribution systems. 11.8.3 Process and sample data As chill foods have relatively short shelf-life and are often distributed immediately after manufacture, microbiological results cannot be used for assurance of safety, if they are to have the maximum time available for sale. This is for two reasons. ? Firstly, sufficient numbers of portions cannot be sampled to provide any confidence that processing and ingredient quality were under control for the duration of processing. ? Secondly, the time for a microbiological result will be longer than the pre- despatch time in the factory, even when rapid methods are used. Microbiological results should be used only for supplier monitoring, trend analysis of process control and hygiene and for ‘due diligence’ purposes. Data showing the control of CCPs should be taken at a defined frequency and kept for a period equal to at least the shelf-life plus the period of use of the product to verify the performance of the supply chain. Process control records may be generated and retained according to the framework proposed in the ISO 9000 documents on Quality Management. The provision of conformance samples, to track long-term changes in quality is a problem because of the short shelf-life of the products. Some manufacturers retain frozen samples and accept the quality change caused by freezing. Others take end-of-shelf-life samples and score their sensory attributes against fixed scales or parameters, or use physical measurements of colour or ingredient size. 11.8.4 Lot tracking Packs of chilled product should be coded to allow production lots or batches of ingredients to be identified. This is a requirement of the EU General Hygiene Directive (93/43). Documentation and coding should allow any batch of finished product to be correlated with deliveries or batches of the raw materials used in its manufacture and with corresponding process data and laboratory records. In practice, the better defined the lot tracking system, the better the chances of identifying and minimising the impact of a manufacturing or ingredient fault. Consideration should be given to allocating each delivery or batch of ingredients a reference code to identify it in processing and storage. Deliveries of raw materials and packaging should be stored so that their identity does not become lost. If there is a fault, re-call or good coding and tracking procedures will facilitate responding to a complaint. 330 Chilled foods 11.8.5 Training, operatives, supervisors and managers The staff involved in the manufacture of chilled products should be adequately trained because they make an important contribution to the control of product quality and safety. The best way of achieving this is by a period of formal or standardised training at induction, which will enable them to make their contribution to assuring the safe manufacture of high quality products (Mortimore and Smith 1998, Engel 1998). After training they should at least understand the critical aspects of hygiene for food handlers, product composition, presentation and control and the prevention of re-contamination. Many manufacturing operations will be done under conditions of ‘High Care’ and will involve team work, therefore all staff must be trained in, and understand, the reasons for specified hygiene standards and procedures. Because the products are often ready to eat, or will only receive minimal heat processing by the customer, staff and supervisors should have defined responsibilities to ensure that quantities of low quality or unsafe product produced by the manufacturing process are minimised. The role of external HACCP consultants has been identified (White 1998) as being especially useful to small food companies in helping them put together effective HACCP programmes and training of staff. Health screening may be required to ensure that personnel have the standard of health and personal cleanliness required for the job. 11.8.6 Process auditing Auditing is the collection of data or information about a process or factory by a visit to the premises involved. Inspection and auditing may be used to determine whether the HACCP plan is correctly established, effectively implemented, and suitable to achieve its objectives, this is especially important where safety relies on many aspects of the process (van Schothorst 1998, Sperber 1998). An integral part of an audit is to see the line operating; effective auditing does more than inspect records. Topics covered should include an assessment of the company structure typified by company policy on quality and safety, the resources and management of the Development department and in-take procedures, storage and handling of raw and packaging materials. Design, control and operation of the manufacturing should be assessed under the headings that include the safe design of products and processes, including matching the products to consumer use and the facilities available. Lastly, the operational aspects of manufacturing should be examined – production, hygiene and housekeeping. Because of the importance of chilled distribution, logistics should be given special attention. How the Quality Assurance Department and the Laboratory contribute to the management of the operation and training should be assessed. Internal or external auditors may do the auditing, a checklist may be used and often a scoring system or noting of non-compliances may be applied if there is a customer–supplier relationship. A more recent development is supplier self- auditing. This involved the development of the supplier-customer relationship on a partnership basis. It starts with a clear statement of the trading objectives Microbiological hazards and safe process design 331 and the resources and materials involved. Next an evidence package covering specifications and records is agreed between the supplier and customer. Because of the variability of quality parameters and process control, a mechanism for challenge must also be agreed, to prevent false rejection of product and the realistic management of any risks or non-compliances. Successful operation of such a system relies on the identification and allocation of responsibilities and ownership of technical factors within both organisations, so that the person best placed to know can always be identified. This type of audit system has a better cost/benefit than the traditional audit visit and produces information for decision-making on a continuing basis. In some cases it is carried out by externally accredited auditors; their ability to examine a process can be limited by their access to commercially sensitive data on processing. The basis of product safety in such systems is a continuing supply of data, and if properly handled this provides a better level of customer protection than end-product sampling and testing – as by the time microbiological results are available, the product is likely to have been consumed. Procedures for managing process breakdowns and other deviations must be developed and audited to ensure that risks to customers are minimised. 11.9 Conclusions A major expansion in the sale of prepared chilled foods continues, particularly in Europe, to meet many of the constantly changing requirements of the consumer for convenience, a variety of flavours and tastes and less severe processing and preservation than hitherto. Such foods are processed using less heat, and they contain less preserving agents such as salt, sugar and acid. In addition they are less reliant on additives (e.g. less use of antimicrobial preservatives such as sorbate and benzoate). Most of these requirements are not immediately compatible with an improvement in microbial stability or safety. Indeed most of them lead to a lessening of the intrinsic preservation or stability of foods. At the same time, assurance of the safety of such foods is an essential consumer requirement, and as such is paramount for the producer. It is here that modern chill food processing and distribution techniques backed up by HACCP procedures (Mayes 1992), properly applied, can more than compensate for the stability and safety that otherwise could be lost. Although complex in detail, as indicated above, the basic elements of effective and safe processing of chilled foods are few. They include: ? the reliable identification of hazardous micro-organisms for products, so that product designs will control them, plus the controlled and predictable inactivation of micro-organisms by processes, storage conditions and product formulations ? the avoidance of recontamination and cross-contamination after decontami- nation 332 Chilled foods ? control of the survival or growth of any micro-organisms that remain in the food, for example by refrigeration ? the provision of clear consumer use instructions, that are compatible with customer expectations. Safe effective processing, and distribution, of chilled foods depends on confident and robust control of these four elements, by the means detailed in the sections above. Although these present means are effective if properly carried out, it is likely that effectiveness could be further improved in the future. This may come mainly from improvements in the control and management of process stages critical to the control of microbes in the product. To a smaller extent, improvements could come through the availability of new processing techniques, such as high hydrostatic pressure to ‘pressure-pasteurise’ foods. This is likely to be applicable only to products in which (pressure tolerant) bacterial spores are not a problem, e.g. low pH jams and fruit juices (Hoover et al. 1989, Smelt 1998). Irradiation (Haard 1992, Grant and Patterson 1992, McAteer et al. 1995) also has the potential to inactivate micro-organisms in chilled products, but the changes in sensory characteristics caused may limit the potential of irradiation to extend shelf-life and improve safety. 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