Part II Technologies and processes 4.1 Introduction Chilled foods are foods which are cooled to a temperature above their freezing point and which need to be maintained at that temperature to preserve quality. Generally such foods will lose value if frozen, and in many cases freezing will destroy them. From the refrigeration viewpoint, the range of foods regarded as chilled is very wide. In this chapter they are taken to include fresh fruits and vegetables, both temperate and tropical in origin, the whole range of meat, fish and dairy products, and prepared complete meals. Frequently a narrower definition covering only prepared foods is used (Anon. 1997). It is immediately obvious that refrigeration is essential for the production, storage and distribution of chilled foods. However, the range and variety of refrigeration equipment required is less readily apparent. Consider, for example, the operation of a cook-chill catering facility. Raw materials from around the world are cooled in distant pack houses and transported across the oceans in highly developed refrigerated transport systems. They then pass through refrigerated port stores and via refrigerated road transport to distribution depots from which, either directly or indirectly, they are despatched to the catering facility. This is shown diagrammatically in Fig. 4.1. Here, refrigerated stores maintain quality prior to use. Some raw materials may be frozen rather than chilled, and will require thawing equipment. Following the cooling operation, the food is chilled using blast chillers or, in some cases, immersion chillers, and will then be stored under refrigeration before distribution in insulated or refrigerated vehicles. It may then be held in refrigerated storage or display cabinets before re-heating. In addition, the waste produced during the food preparation may be stored under refrigeration. The 4 The refrigeration of chilled foods R. D. Heap, Cambridge Refrigeration Technology general lack of problems in refrigeration machinery, which is so essential, is a tribute to the reliability of the technology. Refrigeration is almost a forgotten part of the chilled food preparation process in the mind of the consumer; it is taken for granted. Food refrigeration is not new. Natural ice and evaporative cooling have been used for millennia, and the relatively recent use of mechanical refrigeration to Fig. 4.1 The chill chain. 80 Chilled foods store foodstuffs at chill temperatures in fact dates back to the US apple stores of the 1870s (The′venot 1979). Refrigerated transport of chilled (as distinct from frozen) meat started between the US and the UK around 1875, and longer- distance chilled transport from Australasia to Europe dates from 1895 (Critchell and Raymond, 1969). By 1901, the UK was importing over 160,000 tonnes of chilled beef annually. 4.2 Principles of refrigeration The basic principles of vapour compression refrigeration were established in the 19th century, and this form of refrigeration is almost universally adopted nowadays. At its simplest, such a refrigeration system has four interlinked components (Fig. 4.2). A refrigerant fluid in the vapour state is compressed to a higher pressure, and consequently a higher temperature. The high temperature gas is cooled and liquefied in a condenser. The cool liquid then passes through a restrictor to a lower pressure area, cooling further in the process. The cold liquid can then be used to extract heat from a storage space or cooling area, this heat vaporising the cold, low pressure liquid in an evaporator. The cold vapour is then fed back to the compressor to complete the cycle. The compressor, condenser, expansion restrictor, and evaporator form the basic components. Whilst heat is extracted from a process at the evaporator, the extracted heat plus the heat equivalent of the compression energy must be rejected at the condenser. This means that any refrigeration device must reject a quantity of heat, which is greater than the heat energy removed from the product or space being cooled. The energy used by a vapour compression refrigeration machine depends on its design, but generally the larger the temperature difference between evaporator and condenser, the greater the energy used in the compressor for a given amount of cooling duty. Also, the greater this temperature difference is, the smaller will be the refrigerating capacity of the system. Theoretical analysis of refrigeration cycles and full details of components may be found in numerous refrigeration textbooks (Gosney 1982, ASHRAE hand-books, Alders 1987) and would be out of place in the present publication. Nevertheless, a basic appreciation of the principles outlined above is useful for all users of refrigeration equipment. 4.3 Safety and quality issues Food safety is concerned with freedom from pathogens and toxins – food should not make people ill, nor should it poison them. Food quality is the nutritional value and the perception of taste, texture and appearance of a foodstuff that is safe. Ideally, food safety is subject to legislative controls, whereas food quality is an issue best left to market forces. The refrigeration of chilled foods 81 For chilled foods, safety and quality may or may not be interlinked. For fresh fruits and vegetables, spoilage will make the food unpalatable but there is unlikely to be a health risk. For many prepared foods including cooked meats, growth of food poisoning pathogens will take place to an extent dependent on temperature and time, leading to injurious food which may look and taste satisfactory. For some dairy products, pathogen growth and off-flavours may develop together. In every case, the maintenance of safety and quality depends on maintaining as low a temperature as is possible without damaging the food. This has to be achieved throughout all stages of the chill chain shown in Fig. 4.1. 4.4 Refrigerant fluids and the environment Until the early 1990s, the choice of refrigerant fluids for use within the closed vapour compression refrigeration cycle was a matter of little concern to equipment users. Unfortunately, it is now realised that those fluids developed over the years for efficiency and for safety have unexpected environmental side- effects when they are released into the atmosphere. Ozone depletion and global warming are two quite separate environmental problems. The ozone layer, which protects the surface of the earth from excessive ultraviolet radiation, is damaged by the emission of stable chemicals containing chlorine or bromine. These chemicals include CFC (chlorofluoro- carbon) and HCFC (hydrochlorofluorocarbon) refrigerants, which contribute to ozone depletion in the stratosphere, and also to atmospheric global warming. Global warming is a natural phenomenon, in which heat from the sun is trapped Fig. 4.2 The basic vapour compression refrigeration circuit. 82 Chilled foods in the atmosphere by, particularly, carbon dioxide and water vapour. Fears of excessive global climate change are associated with high emissions of carbon dioxide (mainly from burning fossil fuels in power stations and elsewhere) and of other more powerful but much less abundant ‘greenhouse’ gases including HFCs (hydrofluorocarbons). Under the auspices of the Montreal Protocol (Anon. 1987), the developed world ceased the production of ozone-depleting CFCs during the 1990s. This was made possible by the substitution of less environmentally harmful HCFCs. The HCFCs themselves are expected to be phased out by 2010–20, if not earlier, and in some applications there are no known effective substitutes at present available. In Europe, the use of CFCs in existing equipment will be banned, and the supply of new equipment using HCFCs will be prohibited. At the time of writing, dates for these limitations are still uncertain. These matters have two direct impacts on users of chilling equipment. Firstly, every change of technology costs money, and may in some cases result in an increase in running costs as well as re-equipment costs. Secondly, it may be necessary in the future to move from locally safe but globally harmful CFCs and HCFCs to globally safe but potentially locally hazardous substances such as ammonia and propane. These latter substances can be used safely, but there are added costs and added needs for proper training of equipment users. HFCs (hydrofluoro- carbons) have been developed as alternatives. These do not deplete ozone and are widely available, but are being targeted by some environmentalists, as they are greenhouse gases within the Kyoto Protocol. The purchaser of equipment needs to be aware of these matters, as he may otherwise obtain machinery that will have to be modified or even replaced long before its expected economic life is over. There could also be financial implications at the time of machinery disposal. The reduction of CFC and HCFC use in insulating foams in storage cabinets and stores has been well publicised, but at the time of writing the implications for refrigeration machinery are insufficiently widely appreciated. A further related issue is the relation between global warming, energy use, and energy efficiency. New refrigerant fluids may be less efficient, but future environmental concerns may penalise excessive energy use. The equipment specifier is likely to face some difficult choices over the next few years. The user will face new responsibilities for minimising refrigerant leakage, ensuring efficient operation, and using only properly qualified maintenance staff. For a fuller discussion see (Heap 1998). 4.5 Chilled foods and refrigeration The benefit of chilled storage is the extension of life of the foodstuff in good condition, by slowing down the rate of deterioration. Chilling, it must be emphasised, cannot improve the quality of a poor product; neither can it stop the processes of spoilage – it can only slow them down (see Chapters 7, 9, 10). The refrigeration of chilled foods 83 For the international land transport of chilled (and frozen) foods, the UNECE Treaty Agreement on the International Carriage of Perishable Foodstuffs and on the Special Equipment to be used for such Carriage (ATP) lays down various provisions. Foods are classified and maximum temperatures are stated in the ATP agreement as follows (UNECE 1998): Red offal +3oC Butter +6oC Game +4oC Milk for immediate consumption +4oC Industrial milk +6oC Yoghurt, kefir, cream, fresh cheese +4oC Fish, molluscs, crustaceans in melting ice (0oC) Unstabilized meat products +6oC Meat (not offal) +7oC Poultry, rabbits +4oC This list excludes prepared vegetable foods with or without dressings and fresh fruit and vegetables. There are two quite distinct applications of refrigeration to chilled foods. These are the chilling operation itself, in which the foodstuff is cooled from either an ambient temperature of maybe 30oC or a cooking temperature of over 70oC, and the chilled storage, at a closely controlled temperature of between C01.5oC and +15.0oC depending on the foodstuff. Chilling equipment and chilled storage equipment are quite different in their requirements and their design, and although some chilling equipment may be used for chilled storage, storage equipment is not designed to cool products, only to maintain temperature. Transport refrigeration for chilled food distribution is a special case of storage, and transport equipment should not be expected to provide rapid cooling. 4.6 Chilling The rate at which heat can be extracted during chilling is dependent on many factors. The size and shape of the pack or container will affect the rate of heat transfer to the cooling air (or, in some cases, water). The temperature and speed of the air will also affect this. Within the pack, the weight, density, water content, specific heat capacity, thermal conductivity, latent heat content, and initial food temperature will each play a part. In the case of unpackaged foods, the factors leading to rapid cooling also lead to rapid loss of moisture, so it may seem that slow cooling is better. Generally, this is not the case, as the extended cooling time is also an extended drying-out time. More rapid chilling is possible with thinner packs, with higher airspeeds, and with lower air temperatures. All these lead to higher operating costs, so equipment design has to be a compromise to give the best overall operating system. This means that a range of equipment is available for different 84 Chilled foods applications, and an appropriate choice must be made, dependent on the planned operation. 4.7 Chilling equipment 4.7.1 Cooling systems For most prepared foods, air blast cooling chambers or tunnels are used. Water immersion (hydrocooling) is used for some vegetables; and for fresh, leafy produce, vacuum coolers may be appropriate. For some fresh produce which has a relatively long storage life, cooling may be achieved using storage chambers, but frequently cooling rates will be enhanced by the use of special air circulation arrangements. Each of these systems will be considered in turn below. 4.7.2 Blast chillers Blast chillers operate by passing cold air over foodstuffs at high speed. For cook-chill catering and similar operations, there are various guidelines, such as those issued by the DHSS in the UK. These recommend that equipment should be capable of chilling foods of up to 50 mm thickness from 70oC down to a core temperature of 3oC or below within 90 minutes. This requires an air speed of at least 4 metres per second and an air temperature of around C04oC. Small, ‘reach-in’ chillers taking batches of up to 30 kg are available, for ‘buffer’ supplies in catering and for teaching and research. Larger models with capacities of up to a quarter of a tonne of foodstuffs are designed to accommodate wheel-in trolleys of trays. A typical single trolley unit might have a nominal capacity of 45 kg, typically accommodated on a trolley taking 20 trays of food. The evaporator and fans are located to the side of the interior chamber, and the compressor and condenser may be located either in the top of the unit or remotely, depending on whether the heat and noise emitted can be accommodated locally or not. Controls will permit the unit to be used as a chilled store at 0–3oC, or will operate the chilling cycle using any combination of air temperature, product probe temperature, or simple timer. At the end of the chilling cycle, a defrosting cycle to remove ice and frost from the evaporator is operated. Total power draw for a 45 kg unit is about 7 kW. With a two-hour load/chill/defrost cycle, it is convenient to operate a four- batch shift, with the final batch being left in the cabinet as overnight storage. Optionally, temperature recorders may be fitted to monitor operation. For larger units, there may be doors at each side, so that chilled trolleys may be rolled through into a chilled food holding store at 0–3oC. It is also possible to obtain combination units with a frozen food storage cabinet alongside a chiller/chill storage unit. This allows a caterer to remove frozen food, cook and portion it, then chill and finally store the completed portions. Other forms of blast chillers have been developed for the chilling of fresh poultry, which use a carbon dioxide tunnel in which CO 2 snow is used to provide The refrigeration of chilled foods 85 cooling. Although this can achieve good results, there is considerable risk of surface freezing which would be unacceptable for many products. Liquid nitrogen is another ‘total-loss’ refrigerant that may be used to cool cabinets. As the temperature of liquid nitrogen at atmospheric pressure is C0196oC, careful control is necessary. An alternative may be synthetic liquid air (SLA) (Waldron and Pearce 1998), which overcomes the danger of asphyxiation that exists with other cryogens. All ‘total-loss’ systems depend on the availability of compressed, liquefied gases, and it should be noted that the total energy use of such systems (including that needed for liquefaction) is much greater than that of equivalent mechanical refrigeration systems, so running costs may be high. In some applications, either reduced capital costs or increased chilling speed may make such systems attractive. 4.7.3 Hydrocoolers The use of chilled water, either sprayed down through a chamber or in an immersion tank, provides very rapid cooling with no risk of freezing. It is normally only applicable to fresh fruits and vegetables that can withstand water immersion, and so is a little outside the scope of the general range of chilled foods, though it may be applied to vacuum packs of prepared foodstuffs. Water is normally recirculated in such systems, so great care is necessary to ensure continued cleanliness by regular flushing out, addition of fungicides, or whatever may be necessary for the particular product. It is of course possible to combine a degree of hydrocooling with normal cleaning operations for items such as root vegetables. 4.7.4 Vacuum coolers Vacuum coolers are highly specialised and expensive pieces of equipment, well suited to the rapid cooling of pre-packaged leafy vegetables. They operate at low pressure with wet produce in a sealed chamber, under which conditions the cooling is mostly achieved by low temperature evaporation of moisture. The process is in batches, with cooling times of about 15–30 minutes, and typical equipment can accommodate several tonnes of produce, normally on pallets or trolleys. 4.7.5 Store cooling For large volumes of live produce, particularly fresh fruits and vegetables, cooling may be achieved by placing cartoned or binned produce in a cool store and allowing the circulation of air in the store to provide all the cooling that is necessary. This is a slow process, taking several days and dependent on the store air circulation and the stacking of the produce. In many fruit stores, a combination of store extract fans, curtains, and planned stacking as shown in 86 Chilled foods Fig. 4.3 is used to provide a simple form of something approaching a blast chiller. Air is extracted through a uniform thickness of cartons, with air entry other than through the cartons blocked by the curtain. If necessary, pallet bases are closed to air movement by the insertion of plastic foam or other convenient material. 4.7.6 Summary of equipment These types of equipment may be summarised as follows: Blast chillers: preferred equipment for most applications, design must be matched to production requirements. Hydrocoolers: an excellent alternative in those cases where they may be used, especially fresh fruits and vegetables. Vacuum coolers: specialist equipment for limited application. Store cooling: with suitable stacking and curtains, a good method with wide application. 4.8 Chilled storage Chilled storage equipment may be seen around the world in a wide range of sizes, each suited to the particular operation for which it is designed. At its smallest, it may be an absorption cycle refrigerator in a caravan or boat. There Fig. 4.3 Cooling tunnel arrangement within a store. The refrigeration of chilled foods 87 are larger domestic and commercial refrigerated storage cabinets, then small walk-in stores, and finally stores large enough to be served by forklift trucks handling pallets or bins, some of which can accommodate thousands of tonnes of produce. Some refrigerated fruit stores have the addition of atmosphere control in which low levels of oxygen and high levels of carbon dioxide can be maintained, which further enhances storage time of respiring fresh produce. This technology is not new, but is gaining wider acceptance in the search for better quality maintenance (Bishop 1996). Storage cabinets designed primarily for display are discussed in a separate section below, and the design of domestic refrigerators and slightly larger commercial storage cabinets is outside the scope of this book. For most chilled food preparation and short-term storage areas, walk-in stores are appropriate. These can be constructed and designed as part of a total building, but more often are likely to be modular units sited within the overall structure. If pre-cooked chilled foods are to be stored, they should not be mixed with any other products requiring chilled storage. UK Electricity Association recommendations (Anon. 1989), suggested the following points should be taken into consideration in specifying a store for cook-chill products: ? first decide on the container type and handling method to be used, e.g. disposable containers in baskets to be stored on roll pallets, or whatever; ? consider frequency and size of consignments to be taken from store to calculate storage time required and size of store; ? thermal insulation should be adequate to maintain temperature at satisfactory running costs (with allowance for insulation deterioration with time and use); ? surface finishes (interior and exterior) must be durable and easily changed; ? twin compressor systems should be considered to give security in the case of breakdown; ? air curtains or secondary ‘flap’ doors should be specified for larger stores with frequent movements in and out; ? refrigeration evaporator coils should have capacity sufficient to allow for reduced efficiencies due to frosting or fouling; ? defrosting should be efficient, with adequate facilities for taking away defrost water; ? an alarm or safety system should be in place to prevent staff being accidentally locked in the store; ? continuous temperature monitoring and/or recording equipment should be installed, with some form of ‘out-of-hours’ monitoring in case of equipment failure. Modular stores are available from small, self-contained units of about 2 m 3 volume up to 30 m 3 or more. The refrigeration condenser units may be mounted above or alongside the store, or may be remotely sited if there is insufficient ventilation to take away the heat rejected. If required, banks of multi- compartment stores can accommodate chilled, frozen or fresh produce. One particular arrangement for a modular store constructed within a building with a 88 Chilled foods low ceiling height is illustrated in Fig. 4.4. Many other arrangements are possible, dependent on local requirements. For larger stores, a basic requirement is the consideration of product movements and handling methods. This will dictate store height, whether or not a fixed racking system for pallet or carton storage is required, and how large the store should be. It is very common for large stores to be designed for specific applications, and for them to be used for something quite different after a few years, so flexibility of possible use should be designed in. Large stores should have provision for subdivision without major structural alteration. Refrigeration evaporators, from which the cooled air is circulated, may be mounted at one end of the store, or at high level on one wall, or at high level within a central ridge. Whatever the arrangement, it is essential to consider the pattern of air movement for every likely store loading pattern, and ensure there is sufficient fan power to provide even temperatures. Most of the general considerations listed above for cook-chill stores also apply to larger stores. Many stores have been constructed in which a steel structure is used to support an internal insulation barrier, but increasingly this design is being superseded by pre-formed panel constructions in which the interlocking panels are self-supporting. In suitable applications, the latter system offers appreciable cost savings. In designing any large store, it is essential to consider protection of vehicles and products during loading and unloading operations. Protection against sun and rain is essential, and in more critical operations, temperature-controlled loading bays may be necessary. Fig. 4.4 Modular cold room for low ceiling height accommodation. The refrigeration of chilled foods 89 4.9 Refrigerated transport 4.9.1 General requirements Refrigerated transport of chilled foods must be seen as a total operation involving the movement of chilled goods from one fixed storage area to another. The operation involves a ‘chain’ of events, of which the actual movement of goods in a road vehicle, intermodal freight container, rail wagon, ship or aircraft is only a part. Temperature maintenance throughout the chain is essential for success, and the finest transport equipment cannot compensate for poor handling at loading, wrong packaging and stowage, or inadequate product cooling (Frith 1991). The term ‘refrigerated transport’ may itself be misleading, in that frequently it should be ‘temperature-controlled transport’. In cold winter conditions, it may be necessary to heat chilled foods in order to prevent freezing damage, and for many fresh tropical fruits quite moderate temperatures can produce irreversible chilling damage. For example, bananas should not be allowed to cool below about 13oC. In areas of the world having severe winter conditions, heating requirements can be considerable. The distinction between ‘refrigeration’ and ‘temperature control’ is important for equipment users, who may not appreciate that a wrong temperature-setting on transport equipment may lead to foodstuffs being heated, whereas in many static stores it would only lead to lack of refrigeration. In general, transport equipment is designed to maintain temperature, and not to provide cooling. Whilst foodstuffs can be cooled to some extent during transport, this is a slow and non-uniform method of attempting to cool, and it should not be depended upon. Pre-cooled foodstuffs should be loaded under temperature-controlled conditions wherever possible. In some cases, packaging designed for horizontal airflow coolers may not allow further cooling in transport, where vertical airflow is usual. The range of transport refrigeration equipment is wide, as are the needs for transport. At its simplest, it could be an insulated box containing water ice. At its most complex, it might be an intermodal freight container with integral refrigeration machinery. This equipment is capable of maintaining either frozen or chilled goods at any selected temperature between C025oC and C13530oCin ambient temperatures from C020oCtoC13550oC. Most frequently it will be a road vehicle designed either for local deliveries or for long distance or bulk distribution (Fig. 4.5). The temperature control requirements for chilled foods are more difficult to achieve than those for frozen foods. Typically, it may be necessary to maintain cook-chill products between 0oC and 5oC, and for many products closer tolerances are required, whereas with frozen foods there will be an upper limit temperature, perhaps C018oC, but no lower limit. To ensure temperature uniformity in a load of chilled foodstuffs, relatively high rates of continuous air circulation and high levels of temperature control are necessary, and careful stowage within the vehicle may be needed to achieve this. 90 Chilled foods Fig. 4.5 Refrigerated semi-trailer, delivery van and container. Courtesy of Cambridge Refrigeration Technology. The refrigeration of chilled foods 91 4.9.2 Road vehicles Refrigerated road vehicles fall into two basic categories. Firstly, there are large semi-trailers, with refrigeration units that can be run independently of the tractor unit. Secondly, there are rigid-bodied vehicles of various sizes, which may have independent refrigeration units, or may have units driven from the vehicle engine or axles, or may depend on eutectic storage media. Semi-trailers are used for long-distance or bulk movements, generally with only one or a few destinations. Journey times may vary from two hours for supermarket distribution to several days for fresh produce transportation. A typical arrangement is shown in Fig. 4.6. Whilst most such vehicles use diesel engine drives with optional electric alternatives, some use total-loss refrigerant tanks (liquid nitrogen or carbon dioxide) to reduce both capital cost and noise levels in sensitive areas. In most developed countries, semi-trailers are designed for use in ambient temperatures of 30oC or above, with thermal insulation with overall value of 0.7 W/m 2 K or better. If frozen goods may also be carried, insulation of less than 0.4 W/m 2 K would be used. Increasingly, multi-purpose multi-compartment vehicles are being produced, capable of carrying frozen, chilled and fresh produce simultaneously in different compartments. Rigid-bodied vehicles vary from large vehicles very similar in use to the semi-trailers to small delivery vehicles for multiple deliveries of chilled foods to corner shops. Refrigeration units may be driven by diesel or electric motors, or by hydraulic drive from the vehicle chassis, or may be based on either total-loss or eutectic systems. The latter two are more often used for frozen food transport, being relatively difficult to control at chilled temperatures. Delivery vehicles may require walk-in access for order selection from fixed shelving, and may have to operate with large numbers of daily door openings. Legislation such as the UK food safety regulations (Anon. 1995a) has provided the impetus for much development of vehicle design to meet increasingly stringent temperature requirements. Fig. 4.6 Schematic arrangement of refrigerated semi-trailer. 92 Chilled foods Commercial requirements have led to the development of multi-compartment vehicles with independent temperature control in each compartment. These vehicles are suited to distribution from stores to retailers, as they can move frozen, chilled and ambient temperature goods simultaneously. Such vehicles may have separate cooling coils in each compartment, or may depend on fans to transfer a limited amount of cold air from the coldest to a warmer compartment. Generally, refrigerated vehicles control the temperature of the air supplied to the cargo space, and monitor the temperature of the air returning to the refrigeration units with an external gauge or display or, increasingly, a display within the vehicle cab. Some older vehicles primarily designed for frozen foods may only control the temperature of air returning to the refrigeration unit, with the risk of freezing chilled foods that are loaded too warm. 4.9.3 Intermodal freight containers Intermodal freight containers (‘ISO’ containers) with integral refrigeration machinery are widely used for the long-distance transport of fresh fruit and vegetables and chilled meat. Because of journey times of up to six weeks, they have highly developed refrigeration and control systems, and they are capable of operating over a wide range of conditions. They are normally used only for point-to-point international transport involving a substantial sea journey (Frith 1991, Heap 1989, Anon. 1995b), though there are occasions on which the lease of such a container can be a very convenient way of providing a temporary chilled storage facility. Standard sizes are either 20 foot or 40 foot length with capacities of about 28 or 60 m 3 , and refrigeration units are electrically driven from three-phase supplies from either mains or a diesel generator. They differ from most road vehicles in that air is supplied to the load space from a ‘T’- section floor grating (Fig. 4.7). Fig. 4.7 Schematic arrangement of refrigerated container. The refrigeration of chilled foods 93 4.9.4 Summary of equipment Note that the requirement is for temperature control, which may include heating. Road transport: large semi-trailers for international and national distribu- tion, large rigid bodied vehicles mainly for distribution to retailers, smaller vehicles for local distribution. Containers: ISO integral refrigerated containers for long-distance movements primarily by sea. 4.10 Refrigerated display cabinets The refrigerated display cabinets used in retail premises fall into two distinct groups. Most are vertical multi-deck cabinets for the display and self-service retailing of packaged chilled foods, fresh meat and poultry. Examples of these are shown in Fig. 4.8. There are also the delicatessen or ‘serve over’ cabinets for foods which are normally not packaged but cut and served. Multi-deck cabinets have a refrigeration evaporator in the base, and this may be supplied either from a self-contained condensing unit or, in larger installations, be piped to a central store cabinet refrigeration system. The evaporator coil is mounted in the lower part of the cabinet behind or under the display area, and fans blow cooled air both from behind the shelves in a forward direction and also downwards in an air ‘curtain’ from the top front of the cabinet. Warmed air is returned through a grille at the base of the cabinet. Modern multi-deck cabinets may be designed to maintain food temperatures at 5oC or below, but some older cabinets will frequently have difficulty achieving temperatures below 10oC. Food temperatures are not just a function of cabinet design: they also depend on method of use. Very tight or untidy cabinet loading can prevent proper air circulation, as can indiscriminate placing of large price or advertising tickets. High store temperatures or excessive radiant heating from lights can lead to warm foodstuffs. Good housekeeping allied to the use of some type of night covers when the store is closed will give the best results. Cabinets are designed to maintain temperatures, and should not be loaded with foodstuffs which are warm. In some countries, cabinets with doors have largely superseded the use of open-fronted multi-deck cabinets, to provide more positive refrigeration at all times. Such cabinets have severe disadvantages to the retailer, both in loading time and in customer resistance. Some open-fronted cabinets also incorporate shelves for display of non-chilled goods related to the chilled products on display. Serve-over display cabinets have food displayed on a base over which cold air flows, and normally have a glass front from behind which the food is served. Air from a rear evaporator may be gravity-fed or fan-assisted, but, as much of the food in these cabinets is not wrapped, excessive air speeds must be avoided to prevent dehydration and weight loss. For the same reason, these cabinets are usually used for display only whilst sales are in progress, and other storage 94 Chilled foods cabinets are used to store food overnight. A diagrammatic representation of a serve-over cabinet is shown in Fig. 4.9. A variation on the serve-over cabinet is the chilled ingredient display and store used in some catering establishments. This type of cabinet stores refrigerated ingredients below the counter section, and cold air blown across the underside of the display pans keeps these cool, Fig. 4.8 Multi-deck display cabinets. (a) Freestanding module. (b) Extended display area. Courtesy of George Barker and Company (Leeds) Ltd. The refrigeration of chilled foods 95 aided by a curtain of cool air blown across the surface of the pans. This is just one example of the way in which display equipment can be designed to meet specific requirements. In selecting any cabinet, the method of use, standard of temperature control and cost will be major factors. Ease of maintenance and cost of running are also important. Choice of refrigerant must be considered in the light of consumers’ views of the environmental issues discussed above. Cabinets using HFCs are favoured in many countries at the time of writing, with CFCs and HCFCs regarded as obsolete for new equipment. Some other countries favour indirect systems in which cabinets are cooled by cold liquids such as brine or glycol solutions, cooled by central plant which in large stores could operate on ammonia. Purchasers need to be aware of both current and pending legislation in their country of operation, and future trends are not always clear to see. Summary. Manufacturers produce a wide range of cabinets to meet all retail requirements, including combinations of goods with different temperature needs. 4.11 Regulations and legislation There are various regulations and laws relating to the refrigeration of chilled foods, and this topic is covered in more detail in Chapter 16. In some countries, there are general food safety requirements, and sometimes there are, as in the UK, specific requirements relating to temperatures of certain Fig. 4.9 Serve-over cabinet. 96 Chilled foods foodstuffs (Anon. 1995a). It must be appreciated that such requirements are dependent both on having suitable refrigeration equipment and on using it appropriately, and this has considerable implications for staff training for all those involved in preparation, storage, distribution and retailing of chilled foods. For international transport, the ‘ATP’ regulations (UNECE 1998) apply for journeys between countries which are party to that particular UN agreement. These regulations specify insulation and refrigeration requirements for vehicles, together with previously listed temperature requirements for various particular classes of foods. Some countries, e.g. France, Spain, Portugal and Italy, have national regulations related to the ATP requirements, but these internal regulations are not part of ATP itself. The ways in which the EC’s single market development will lead to either harmonisation or mutual recognition of the various requirements have yet to be resolved. In some cases, refrigeration of chilled and of frozen foods are closely linked, and it is necessary to distinguish between the quite distinct legislative requirements for the two groups. Requirements for common handling equipment may impose additional regulations on chilled foodstuffs refrigeration systems if they are to be suitable for use with frozen goods, for example in the case of multi-temperature cold rooms or distribution vehicles. 4.12 Sources of further information The Institute of Refrigeration, Kelvin House, 76 Mill Lane, Carshalton, Surrey SM5 2JR, UK. Web site: www.ior.org.uk British Refrigeration Association, FETA, Henley Road, Medmenham, Marlow, Bucks SL7 2ER, UK. Web site: www.feta.co.uk International Institute of Refrigeration, 177 bd Malesherbes, F 75017, Paris, France. Web site: www.iifiir.org Cambridge Refrigeration Technology, 140 Newmarket Road, Cambridge CB5 8HE, UK. Web site: www.crtech.co.uk 4.13 References ALDERS A W C, (1987) Marine refrigeration manual, RMCA, The Hague. ANON, (1987) UNEP, Montreal protocol on substances that deplete the ozone layer. ANON, (1989) Planning for cook-chill, The Electricity Council, London, 5th edn. ANON, (1995a) Food safety (general hygiene) regulations, HMSO S.I. 1763. ANON, (1995b) Guide to refrigerated transport, International Institute of Refrigeration, Paris. ANON, (1997) Guidelines for good hygienic practice in the manufacture of chilled foods, Chilled Food Association. The refrigeration of chilled foods 97 ASHRAE HANDBOOKS, ASHRAE, Atlanta, updated annually on a 4-year cycle for Fundamentals, Refrigeration, Systems, Equipment. BISHOP D, (1996) Controlled atmosphere storage. A practical guide. David Bishop Design Consultants, Heathfield. CRITCHELL J T and RAYMOND J, (1969) A history of the frozen meat trade, Dawsons, London, (original edition 1912). FRITH J, (1991) The transport of perishable foodstuffs, SRCRA Cambridge. GOSNEY W B, (1982) Principles of refrigeration, Cambridge University Press. HEAP R D, (1989) Design and performance of insulated and refrigerated ISO intermodal containers, Int. J. Refrig., Vol. 12 (May), 137–45. HEAP R D, (1998) Global warming – considerations for the air-conditioning and refrigeration industry – Dreosti Memorial Lecture. SAIRAC, Cape Town. THE ′ VENOT R, (trans. Fidler, J. C.) (1979) A history of refrigeration throughout the world, International Institute of Refrigeration, Paris. UNECE, (1998) Consolidated text of the agreement on the international carriage of perishable foodstuffs and on the special equipment to be used for such carriage (ATP), UNECE Inland Transport Committee, Geneva. WALDRON S N and PEARCE I A, (1998) The application of synthetic liquid cryogens in the distribution, storage and production of food. IIR Conference, Refrigerated transport, storage and retail display, Cambridge, 1998. IIR, Paris. 98 Chilled foods