16 Secondary chilling of meat and meat products Meat is chilled immediately after slaughter. Most of the subsequent opera- tions in the cold chain are designed to maintain the temperature of the meat. Cooking is a very common operation in the production of many meat products and operators appreciate the importance of rapidly cooling the cooked product. However, any handling such as cutting, mixing or tumbling will add heat to the meat and increase its temperature. A secondary cooling operation is always required with chilled meat and meat products to reduce their temperature to approaching 0 °C and maintain their storage life. The aim of any cooking process for meat/meat produce is to ensure the destruction of vegetative stages of any pathogenic microorganisms. However, there is always the possibility that the cooking process will not kill some microorganisms that produce spores or that the food can become recontaminated. Therefore, microbiologists recommend that the tempera- ture of the meat should be rapidly reduced, especially from approximately 60 and 5 °C, to prevent multiplication of existing or contaminating bacte- ria. Rapid cooling is also desirable with cooked products to maintain quality by eliminating the overcooking that occurs during slow cooling. There are specific cooling recommendations for cook–chill and cook–freeze catering systems. However, even with thin products these are difficult to achieve without surface freezing. Cooling large hams and other cooked meat joints is inherently a much slower process and studies have shown that companies often have very poor cooling systems. The methods available to cool meat joints, pies and other cooked prod- ucts have been described in detail by James (1990a). A review of the use of vacuum cooling in the food industry has been published by McDonald and Sun (2000). The majority of plants rely on air blast cooling systems for the chilling of pre-cooked meat products. In batch systems the products, packs or trays of cooked material are placed directly on racks in the chiller or on trolleys that can be wheeled into the chiller when fully loaded. Continuous systems range from trolleys pulled through tunnels to conveyorised spiral or tunnel air blast chillers. Some meals and products are chilled using cryogenic tunnels, however, care must be taken to avoid surface freezing. Imperviously packed prod- ucts can be chilled by immersion in cooled water or other suitable liquid. With some cooked products such as large hams in moulds and sausages, chlorinated water sprays can be used in the initial stages of cooling. Increas- ingly, pie fillings are pressure-cooked and vacuum cooled. With many prod- ucts an initial cooling stage using ambient air can often substantially reduce the cooling load in the cooling system. 16.1 Cooked meat 16.1.1 Legislation In the UK the Food Safety (Temperature Control) Regulations (1995) apply to any food that ‘is likely to support pathogenic micro-organisms or the formation of toxins’ and that must be kept at or below 8°C. Regulation 11 does not define a cooling time or rate, only that the food should be cooled as quickly as possible following the final heating stage. The guidance document to the above regulations produced by the Department of Health is even less specific. Under heading VIII, cooling of food, paragraph 47 it states: ‘The cooling period for any food would not be regarded as unacceptable merely because other equipment, not present at the business, could have cooled the food more quickly. The time taken to achieve cooling must be consistent with food safety. Cooling will often be a step which is critical to food safety’. The Meat Products (Hygiene) Regulations (1994) contain special condi- tions for meat-based prepared meals. They require that the meat product and the prepared meal shall be refrigerated to an internal temperature of +10 °C or less within a period of not more than 2 h after the end of cooking. However, they then go on to state that produce may be exempt from the 2 h period where a longer period is justified for reasons connected with the production technology employed. The wording is similar in the EC Meat Products Directive. In the USA the essential rules of the US Regulations (318.17 9 CFR CH III, 1.1.96 edition) on safe cooling of cooked meats are: ? Chilling shall begin within 90 min after the cooking cycle is completed. ? All products should be chilled from 48.8 °C to 12.7 °C in no more than 6h. 322 Meat refrigeration ? Chilling shall continue and the product should not be packed for ship- ment until it has reached 4.4 °C. These US Federal Regulations have been widely adopted outside areas under the control of the USDA, including by European retailers. Further recommendations have been made by Gaze et al., 1998. Their main recom- mendations are that: ? ‘For a typical uncured cooked meat product, made from good quality raw material under hygienic conditions and with sound process controls, it is suggested that the following limitations (Table 16.1) for cooling time from completion of the cooking process should apply.’ ? ‘For products which are cured (defined as minimum 2.5% salt on water phase and 100 ppm nitrite in-going), these times may be extended (Table 16.2). As an approximation it is suggested this be by 25%.’ 16.1.2 Practical In many industrial cooking operations whole hams and large meat joints are often cooked and cooled in an intact form and then supplied to restau- rants or retail shops where they are sliced before sale. Surveys (Cook, 1985; James, 1990b and c; Gaze et al., 1998) have shown that industry uses a variety of methods for cooling whole hams (Table 16.3, Table 16.4 and Table 16.5). In these processes the earlier data showed that cooling times were as long Secondary chilling of meat and meat products 323 Table 16.1 Recommended good practice and maximum cooling times for uncured meat Cooling time (h) Good practice Maximum To 50 °C 1 2.5 From 50 °C to 12 °C 6 6 From 12 °C to 5 °C 1 1.5 Total time to 5 °C 8 10 Source: Gaze et al., 1998. Table 16.2 Recommended good practice and maximum cooling times for cured meat Cooling time (h) Good practice Maximum To 50 °C 1.25 3.25 From 50 °C to 12 °C 7.5 7.5 From 12 °C to 5 °C 1.25 1.75 Total time to 5 °C 10.00 12.50 Source: Gaze et al., 1998. as 21 h, and final temperatures high: 15–20 °C (Table 16.3,Table 16.4). In the later study cooling times were still as long as 16 h but final temperatures were no higher than 8 °C (Table 16.6). A similar picture is seen in data on the cooling of cooked pork with final temperatures as high as 12°C and cooling times of up to 20h (Table 16.4). Corresponding figures for the cooling of cooked beef were 15 °C and 22 h (Table 16.4). Data obtained from numerous sources within the UK cater- ing industry by Mottishaw (1986) indicate that the cooling procedures for bulk-cooked meats could also vary considerably (Table 16.6). For example, some meat products are said to be cooled to 1 °C in 2 h, whereas at the other extreme, products may take 72 h to cool to 4 °C. Commonly, a 4.5 kg product will take 12 h to cool below 5°C and larger products could take longer. In addition, there is often a delay before chilling begins, this may vary from 10 min up to 6 h. 16.1.3 Experimental studies The most relevant cooling data for cooling of cooked meat from laboratory investigations are shown in Table 16.7. A simple process for estimating the immersion cooling time of beef roasts has been produced by Nolan (1986). Generally, the results show that immersion cooling is almost twice as fast as air cooling at the same temperature.Vacuum cooling was an order of magnitude faster than immersion cooling but the weight loss was substantially (over twice) higher. Using a less severe vacuum treatment or a combination of the different methods is likely to provide an optimum solution. The James and Bailey (1982) study showed that in ham cooling, a 0.75 h initial cooling period in ambient air reduced the initial load on the refrig- eration by a factor of almost 2. If the ham was placed straight into air at 324 Meat refrigeration Table 16.3 Examples of commercial ham cooling in the UK Cooling method Weight Height Cooling Total Final (kg) of joint time to (h) temperature (°C) (cm) 20 °C (h) In metal mould in chill 6.4 19 12 1.4 15 room 17 °C to 6 °C, 0.2 ms -1 In bag in chill room 7.3 18 9.3 21 2 -3 °C to -10 °C, 0.3 ms -1 In bag water shower then 6.8 18 6.6 14 5 chill room at -1 °C then 3 °C to 5 °C In bag in ambient 6.8 20 – 13.5 24 15 °C to 7 °C Source: James, 1990c. Secondary chilling of meat and meat products 325 Table 16.4 Previously unpublished survey data on cooling of cooked meat in industry and shops Meat Wt Diam mm Method Wrap Initial Time (h) 10 °C Final temp/time kg temp °C to 50 °C Ham 6.35 191 Chill room 17–6 °C 0.2 ms -1 Metal mould 71 3.6 – 15/14 70 3.8 – 15/14 7.26 178 Chill room -3/-1 °C 0.3 ms -1 Sealed bag 69 3.8 12.7 2/21 68 3.4 12.5 2/21 Chill room 17–6 °C 0.2 ms -1 Sealed bag – 4.8 20.0 9/21 – 4.7 20.0 9/21 6.80 203 Shop air 15–7 °C <0.2 ms -1 Sealed bag 69 3.7 – 24/13.5 75 4.2 – 21/13.5 75 4.7 – 23/13.5 75 4.3 – 22/13.5 6.80 178 Shower 20 min, chill 3/5 °C 4 h Sealed bag 72 2.6 9.3 5/14 Bacon 4.54 114 Factory air, 20–12 °C Bag 76 1.9 – 14/12.5 4.08 114 71 2.6 – 15/13.5 4.08 114 76 2.7 – 16/13.5 Pork 6.80 203 Shower 20 min, chill -1 °C 4 h, chill 3/5 °C Bag 77 2.2 6.8 5/12 6.35 203 79 2.2 7.2 5/12 5.44 165 Shop 30–20 °C 7 h, chill 18–6 °C Bag 82 4.5 19.5 12/20 2.04 76–101 Kitchen 22 °C 2 h, chill 2/5 °C Uncovered 100 2.0 5.9 4/7.5 2.04 76–101 100 1.9 5.5 4/9.0 Beef 5.90 165 Kitchen 23 °C 1.5 h, chill 13–5 °C Netted 93 2.4 13.0 9/15 6.80 152 Chill 20–0 °C Netted 70 2.6 11.0 0/16 5.90 165 Shop 30–20 °C 7 h, chill 18–6 °C Punctured 75 4.4 22.0 15/22 bag – 101 Blast chill -5/-3 °C 1.2 ms -1 Uncovered 90 – 3.4 7/3.9 3.86 127 Blast chill 4/5 °C 2 ms -1 Netted 73 1.7 5.3 5/7 3.40 127 Domestic fridge 2/7 °C static Netted 72 2.2 7.5 9/7 3.40 127 Chill 1/2 °C 0.3 ms -1 Netted 71 1.9 6.8 9/7 3.63 127 Blast freeze -30 °C 1 ms -1 Netted 73 1.6 4.6 9/4.8 -2 °C it released 220 Wh in the first hour compared with 118 Wh after 0.75 min in ambient air. In this case a period of ambient cooling would sub- stantially reduce the peak heat load on the refrigeration system. Alterna- tive methods of cooling are also available.A double cabinet cryogenic batch cooler has been used to cool cooked roast, smoked pork loin and smoked ham from 65 °C to below 10 °C with a weight loss of >0.5%. 326 Meat refrigeration Table 16.5 Examples of commercial ham cooling in the UK Cooling method Cooling time (h) Temperature (°C) Total To 10 °C Initial Final In casings and moulds in 16 7 65 4 batch air chillers at -1 °C, 0.93 ms -1 In casings and moulds in 8 70 3 forced air at 0 °C Dry cured hams in conveyorised 10 6 75 3 system using refrigerated brine Premium hams in conveyorised 14 6 75 3 system using refrigerated brine Rind-on hams in conveyorised 8 5.5 75 5 system using refrigerated brine Chill room with moderate air 5 3.5 73 3 movement In static air in a refrigerator 15 10.5 80 8 Revision of above. Immersion 8 5 78 0 in ambient water followed by moving air Source: CCFRA survey, 1995. Interpolated data from cooling plots. Table 16.6 Summary of previously unpublished survey data on cooling of cooked meat in industry and shops Joint size Time (h) Time (h) to Temp (°C) Time (h) to 60 °C 50–10 °C 5.44–7.26 kg, 178 mm Average 2.4 10.6 11.4 15.6 diameter Range 0.9/3.5 4.6/17.6 2/24 12/22 3.63–4.54 kg, 114 mm Average 1.4 3.8 10.5 8.6 diameter Range 1/1.7 2.3/5.3 5/15 4/13.5 1.81–2.27 kg, 76 mm Average 1.5 3.7 4 8.3 diameter Microbiologically acceptable: 5 h 50–10 °C, 12 h 10–1 °C. Secondary chilling of meat and meat products 327 Table 16.7 Cooling times (h) for meat joints from published sources Ref. Cooling regime 70–50 °C 50–12 °C 12–5 °C 70–5 °C 70–8 °C Good practice 1 6 1 8 Maximum 2.5 6 1.5 10 1 3.75 kg vacuum packed beef roasts 330 ￥ 160 ￥ 130 mm Vacuum 0.1 1.4 0.9 2.4 2.1 Immersion 1 ± 1 °C 1.5 4.2 2.8 8.5 6.9 Air 1 ± 1°C,2ms -1 1.2 3.5 2.9 7.6 6.1 Air 1 ± 1°C,1ms -1 1.5 4.2 4.6 10.3 8.1 2 7 kg hams in metal moulds Air -2°C,5ms -1 2.4 3.8 2.6 9.0 7.2 0.75 h at 15 °C then -2°C,5ms -1 3.0 4.0 2.9 9.9 7.9 3 5–5.5 kg (11–12 lb) ham logs 400 ￥ 120 ￥ 120 mm in metal moulds 30 m water spray at 18 °C 0.6 4.0 2.1 6.7 6.0 then air 0 °C, 3 ms -1 top rack 30 m water spray at 18 °C then 0.4 3.9 2.2 6.5 5.5 air 0 °C, 3 ms -1 bottom rack 4 0.94 kg beef slabs, 50 mm thick 50–10 °C 70–10 °C Air 0 °C, 1.2 ms -1 1.7 2.1 Water 0 °C 1.0 1.2 Vacuum 0.6 0.6 2.7 kg rolled beef forequarter, 110 mm dia. Air 0 °C, 1.2 ms -1 4.4 5.1 Water 0 °C 3.1 3.6 Vacuum 0.3 0.4 2.7 kg rolled beef silverside, 110 mm dia. Air 0 °C, 1.2 ms -1 4.0 4.9 Water 0 °C 2.6 3.1 Vacuum 0.6 0.7 6.4 kg boned out turkey Air 0 °C, 1.2 ms -1 5.4 6.6 Water 0 °C 4.6 5.6 Vacuum 0.2 0.3 7.1 kg boned out ham Air 0 °C, 1.2 ms -1 8.9 10.4 Water 0 °C 4.8 5.9 Vacuum 0.5 0.5 Sources: 1. McDonald et al. (2000); 2. James and Bailey (1982); 3. Anon (1987); 4. Burfoot et al. (1990). 16.2 Pastry products 16.2.1 Commercial operations Although it should be a far simpler and quicker operation to reduce the temperature of small individual items, such as meat pies, many manufac- turers allow an inadequate length of time for the cooling operation and the products are packaged at temperatures substantially above the storage value. After wrapping and boxing it is very difficult to remove the residual heat. Typically, pie manufacturers allow 1 h for their single-stage cooling operations and the core temperature of pies before packing can range from 17 to 37°C (Table 16.8). The surface temperature of cooked products is very high when they leave baking ovens and consequently the difference between the surface and the ambient is very large at this time. To reduce energy usage and costs a number of manufacturers operate two-stage cooling operations utilising ambient air followed by sub zero air in the second stage (Table 16.10). Com- paring the data from the first two examples in Table 16.9 and Table 16.10 it can be seen that similar final product temperatures are produced in the single- and two-stage cooling processes. The use of very low temperatures during single-stage cooling operations can produce quality problems due to freezing of the pastry at the surface of the products. In addition with some baked products manufacturers believe that the quality of the pastry suffers if taken below 10 °C. One minced beef pie manufacturer used air at 12 °C in the second stage of a two-stage cooling process to avoid this problem (Table 16.10). However, even when the cooling time was increased by 25%, the core temperature of the pie was 20°C at the packing stage. In two factories, large pies (4.5 kg) were also produced for catering use or to be sold after 328 Meat refrigeration Table 16.8 Examples of commercial single-stage cooling of pastry products in the UK Product Type of Air Cooling Temperature chiller Temp. Velocity time Initial Final (°C) (ms -1 ) (h) (°C) (°C) Steak and kidney Spiral -11/-17 0.5 1.0 90–95 17–20 pie (185 g) ≤ Spiral -3 3–5 1.0 90–95 17–20 ≤ Cold store -30 <0.2 1.0 90–95 30 ≤ Ambient 20 0.3 1.0 90–95 37 ≤ Ambient 20 3.5 1.0 90–95 32 Sausage rolls Spiral -11/-17 0.5 0.8 93–96 10 Pork pie (4.5 kg) Ambient 16–23 <0.2 8.0 68 25 Source: James, 1990c. slicing from delicatessen outlets. In a single-stage ambient cooling opera- tion the centre temperature was still 25°C (Table 16.8) after 8h; however in the two-stage process the centre of the pie had been reduced to 10 °C after 6.5h (Table 16.9). 16.2.2 Experimental studies Data from the most relevant experimental studies on pie cooling are shown in Table 16.10. The importance of achieving a minimum required air veloc- ity around small products was clearly demonstrated by data obtained from cooling pork pies (Fig. 16.1).To guarantee that all the crust remained above -2 °C on the unwrapped 400 g (70 mm high, 95 mm diameter) pies an air temperature of -1.5–±0.5 °C was used. At this temperature a small increase in air velocity from 0.5 to 1.0 ms -1 reduced the cooling time by 85 min (almost 30%). Even at very high velocities (>6.0 ms -1 ) appreciable reduc- tions in cooling time were still being achieved. In a high throughput baking line (>1000 items per hour) the 7% increase in throughput, which would be achieved by raising the air velocity from 6 to 10 ms -1 and consequently reducing the cooling time by 10 min, could justify the higher capital and running costs of larger fans. With larger pies cooling times of up to 6h have been measured (Fig. 16.2). Only one reference (McDonald and Sun, 2000) to the vacuum cooling of pork pies has been located. This quotes a cooling time for 0.5 kg pies from 80 to 10 °C of over 8 h. Secondary chilling of meat and meat products 329 Table 16.9 Examples of commercial two-stage cooling of pastry products in the UK Product Type of Air Cooling Temperature chiller Temp. Velocity time Initial Final (°C) (ms -1 ) (h) (°C) (°C) Steak and Ambient 20 3–5 0.33 93 – kidney pie (185 g) Spiral -3 2 0.67 – 17–20 ≤ Ambient 20 3–5 0.33 93 – Spiral -3 0.3–1.4 0.67 – 22 ≤ Ambient 20 0.3 0.33 93 – Spiral -3 2 0.67 – 24–27 Minced beef Ambient 20 <0.2 0.16 96 80 pie (140 g) Chill room 12 1 1.08 80 20 Gala pie Ambient 20 <0.2 1.75 96 76 (4.5 kg) Chill room 0 <0.4 6.5 76 10 Source: James, 1990c. 16.3 Solid/liquid mixtures Meat slurries, mixtures of solid meat and a gravy/sauce are commonly used as pie/pasty fillings and a growing range of ready meals. Surveys have shown that many companies have problems in cooling meat slurries (Table 16.11) and in the centre of large vats of pie fillings, for example, cooling rates can be as slow as 2 °C per hour. Subsequent laboratory studies showed that large 330 Meat refrigeration Table 16.10 Published data on the cooling of pork pies Reference Pie weight Conditions Time to (g) 5 °C (h) 1 & 2 75 20 °C still air for 60 min jelly then 7 °C 3.0 until centre at 10 °C then -20 °C, 0.5 ms -1 20 °C still air for 60 min jelly then 1.5 -10 °C, 5 ms -1 20 °C still air for 60 min jelly then -20 °C, 1.6 0.5 ms -1 20 °C still air for 60 min jelly then 1 °C, 2.3 0.5 ms -1 20 °C still air for 70 min jelly then -30 °C, 2.9 5ms -1 0°C,3ms -1 for 63 min jelly then -30 °C, 2.6 5ms -1 3 235 -1°C,3ms -1 1.5 450 -1°C,3ms -1 >2.5 900 -1°C,3ms -1 >6 4 450 17 °C, 2 ms -1 for 38 min then -1.5 °C, 3.2 4ms -1 17 °C, 2 ms -1 for 38 min then -1.5 °C, 2.9 10 ms -1 17 °C, 2 ms -1 for 38 min then -4 °C, 3.0 4ms -1 17 °C, 2 ms -1 for 38 min then -1.5 °C, 2.6 10 ms -1 17 °C, 2 ms -1 for 38 min then -10 °C, 2.5 4ms -1 17 °C, 2 ms -1 for 38 min then -10 °C, 2.2 10 ms -1 5 450 17 °C, 2 ms -1 for 38 min then -1.5 °C, 4.6 0.5 ms -1 17 °C, 2 ms -1 for 38 min then -1.5 °C, 3.6 1ms -1 17 °C, 2 ms -1 for 38 min then -1.5 °C, 2.9 6ms -1 17 °C, 2 ms -1 for 38 min then -1.5 °C, 2.6 10 ms -1 Sources: 1. Evans & Gigiel (1989a); 2. Evans & Gigiel (1989b); 3. James (1990b); 4. James et al. (1987); 5. James (1990c). Secondary chilling of meat and meat products 331 70 60 50 40 30 20 10 0 0 30 60 90 120 150 180 210 240 270 300 T emperature ( ° C) Time (min) Av. pie 10 Av. pie 6 Av. pie 0.5 Av. pie 1 Fig. 16.1. Temperature at slowest cooling point in 400 g pork pies in air at -1.5 °C and 10, 6, 1 and 0.5 ms -1 . 100 90 80 70 60 50 40 30 20 10 0 0123456 Time (h) T emperature ( ° C) Centre 0.9kg Meat surface 0.9kg Centre .45kg Crust .45kg Centre .235kg Crust .235kg Fig. 16.2. Cooling of 0.235, 0.45 and 0.9 kg pork pies in air at -1°C,3ms -1 (source: James, 1990c). 332 Meat refrigeration Table 16.11 Examples of commercial cooling of meat slurries and soups in the UK Cooling method Depth Cooling time Temperature (°C)of slurry To 20 °C Total Initial Final (cm) (h) (h) In ambient at 23 °C in 50 – 16 99 64 0.6 m 3 vats In 0.6 m 3 vats with 50 – 14 99 38 cold water jacket then ambient at 21 °C In 38 cm diameter pans 17 9.8 16 93 12 in chill room at 2 to 9 °C In 35 cm deep bucket at 30 17.5 18 55 18 20 °C for 7 h then 7 °C Source: James, 1990c. 1100 kg batches of meat sauce could be cooled from 85 to 10 °C in less than 30 min using a vacuum cooling system.When a conventional blast air system was used the cooling time achieved was related to the product depth and even when the depth had been reduced to 70 mm the cooling time was in excess of 6.5 h. 16.4 Process cooling Traditionally, ice has been added to meat mixtures during mixing and grind- ing to maintain their temperature. Liquid nitrogen (LN 2 ) can also be used to maintain the temperature of meat during mixing thus increasing the extraction of soluble muscle proteins. LN 2 or CO 2 can also be used to chill restructured meat during mixing and cutting to -3 °C in approximately 10 to 15 min. Jowls and bacon fat in 1400 kg batches can be mixed and cooled from 7.3 to 0 °C within 12 min. The system uses a cycle of 50 s LN 2 and mixing then 15 s mixing only to allow temperature equalisation. Cryogenic systems are also available to maintain temperatures during tumbling. In cooked ham manufacture the use of liquid nitrogen was claimed to reduce meat dust during tumbling, substantially shorten the process time and improve hygiene. 16.5 Cook–chill The term ‘cook–chill’ usually refers to a catering system where food is pre- pared, cooked and cooled in a central facility before being distributed to the place where it will be reheated and consumed. The term is equally applicable to the system of producing chilled ready meals for retail sale. Sales of chilled ready meals reached ￡973m in the UK in 2001 (www.chilledfood.org) and continue to rise. Within this market growth there is a strong move towards greater variety, with ethnic meals showing the fastest increase. This has meant that more meals than ever before are being produced by manufacturing facilities operating cook–chill systems, using a wide range of production methods and equipment. 16.5.1 Cook–chill guidelines Cook–chill systems are normally used to supply food in institutional (hospitals, schools, canteens, etc.) catering operations. Normally the food is cooked and cooled under near industrial conditions. It is stored and trans- ported to the institution under refrigeration and reheated (not cooked) before serving. One of the key elements to a successful cook–chill operation is the strict monitoring and control of temperature throughout. Cooking rarely elimi- nates all food poisoning organisms and a number survive as spores that will germinate and grow if cooling rates are slow. In the UK the Department of Health Cook–chill Guidelines published in 1989 recommend maximum cooling regimes and the use of special equipment to rapidly reduce product temperatures after cooking. Many other countries in Europe have similar guidelines or recommendations for the cooling of cooked products (Table 16.12). The UK Guidelines recommend that joints of meat or packs of food should not exceed 2.5 kg or 100 mm in thickness or height. It is also advised that containers have lids to help prevent contamination and to minimise dehydration during cooling. The Guidelines also state that the actual chill- ing process should commence as soon as possible after completion of cooking and certainly within 30 min of leaving the cooking process (this is to allow for portioning of meals). Smaller portions (less than 50 mm deep) should be chilled to between 0 and 3 °C within 90 min and larger portions Secondary chilling of meat and meat products 333 Table 16.12 Chilling time requirements for cooked foods in different countries Country Chilling times Chilling rate Storage (°C/min) temperature (°C) Denmark from 65 °C to 10 °C in 3 hours 0.31 <5°C France from 70 °C to 10 °C in 2 hours 0.50 0–3 °C Germany from 80 °C to 15 °C in 2 hours 0.54 2 °C (from 15 °C to 2 °C in 24 hours) Sweden from 80 °C to 8 °C in 4 hours 0.30 3 °C UK from 70 °C to 3 °C in 1.5 hours 0.74 3 °C to 10 °C within 2.5 h after removal from the cooking process. Rapid cooling is also often desirable with cooked products to maintain quality by elimi- nating the overcooking that occurs during slow cooling. The equipment used to chill products should have performance specifi- cations such that it is capable of reducing the temperature of a 50 mm thick layer of food from 70 to 3°C (or less) in a period not exceeding 90min when fully loaded. Air blast chillers are commonly used; to prevent freezing of the product, cooling air temperatures cannot be much below 0 °C. Air temperatures from around -4 °C to -2 °C at speeds from 4 to 6.5 ms -1 are commonly employed (Heap, 2000; Trott, 1989). However, there is little, if any, published data on the conditions required to achieve these times when cooling different cooked products. 16.5.2 Practical cooling time data Investigations have been carried out into air blast cooling of bolognese meat sauce in metal trays of different depths but having the same lateral dimensions (Evans et al., 1996). These trials showed that, assuming surface freezing was to be avoided and a simple single-stage operation used, only 10 mm depth of product could be chilled within these limits (Table 16.13). A computer model was also developed which showed that other foods 334 Meat refrigeration Table 16.13 Cooling time to 10 and 3 °C in trays with (without) lids under different cooling systems Tray depth Flow regime Initial Cooling times Cooling times (mm) temperature to 10 °C (h) to 3 °C (h) (°C) Experimental Vertical Experimental Vertical model model 80 Air 0.5 ms -1 70 11.9 (9.4) 19.3 (15.2) Air 3.0 ms -1 70 6.4 (5.4) 10.5 (8.9) Brine low 65 4.60 3.24 7.31 5.07 80 – 3.53 – 5.37 Brine high 65 – 3.21 – 5.02 80 4.93 3.51 7.83 5.32 40 Air 0.5 ms -1 70 6.0 (4.3) 9.8 (6.9) Air 3.0 ms -1 70 2.9 (2.9) 4.7 (3.3) Brine low 65 1.51 0.92 2.57 1.43 80 – 1.00 – 1.52 Brine high 65 – 0.90 – 1.41 80 1.59 0.99 2.63 1.50 10 Air 0.5 ms -1 70 1.5 (1.0) 2.2 (1.7) Air 3.0 ms -1 70 0.8 (0.5) 1.2 (0.7) Brine low 65 0.22 0.10 0.35 0.16 80 – 0.11 – 0.17 Brine high 65 – 0.09 – 0.15 80 0.21 0.10 0.34 0.16 Source: Evans et al., 1996; Ketteringham & James, 1999. such as beef curry and chicken Italian will have a similar cooling response to those produced for bolognese sauce. It can therefore be concluded that the cooling response of most meat-based convenience meal mixtures will be similar to that of bolognese sauce. Although the Department of Health guidelines allow 30min before chill- ing should commence, results showed that even allowing for this ambient cooling period it would still not be possible to cool a 40 mm thick product within the specified time (Fig. 16.3). Further reductions in cooling times could be achieved if air temperatures were reduced or air velocities increased (Fig. 16.4). However, as already stated, if air temperature were reduced product freezing would be likely to occur. Initial freezing points of the products examined were between -1.2 and -2.1 °C and therefore air temperatures of much below -2 °C could result in some degree of freezing. Alternatively, air velocities could be increased, a two-stage cooling system used, or an alternative cooling method with a higher rate of heat transfer could be considered, e.g. vacuum, cryogenic or immersion. Further work has provided cooling times for cooked meat sauce using immersion cooling (Ketteringham and James, 1999). This cooling method has great potential for use in the food industry as it provides a more effec- tive source of temperature reduction than air blast chilling which is in wide use at the moment. The rate of heat transfer is much greater when using a fluid to remove the thermal energy, than when using air. Typical values for surface heat transfer coefficients for air blast chilling of food products are less than 50 W/m 2 °C as compared to values greater than 500 W/m 2 °C for agitated water. Experimental data gathered from this work can be used to compare results to the previous work carried out using air blast chilling to Secondary chilling of meat and meat products 335 T emperature at slowest cooling position Air temperature (°C) –30 –20 –10 –5 –2+20 60 50 40 30 20 10 60 50 40 30 20 5 3 –1 10 60 50 40 30 20 5 3 –1 10 60 50 40 30 20 5 3 –1 10 60 50 40 30 20 10 5 3 –1 60 50 40 30 20 10 5 3 60 50 40 30 20 10 5 3 –1 10mm tray 40mm tray 80mm tray 10mm tray with lid 40mm tray with lid 80mm tray with lid 0.0 –0.0 –1.0 –1.5 –2.0 –2.5 –3.0 –3.5 –4.0 –4.5 302826242220181614121086420 Time (hours) In (T -T a/T i-T a) Fig. 16.3. Time required to cool centre of bolognese sauce in an air velocity of 0.5 ms -1 (source: Evans et al., 1996). determine if it is possible to meet the Department of Health cook–chill guidelines. Table 16.13 shows that there is a significant reduction in the cooling time of meat sauce from 70 to 3 °C using immersion compared to air blast chilling. However, these preliminary results indicate that even though there is an improvement, it is still not possible to cool 40 mm thick meat sauce (in sealed lidded trays) from 70 to 3 °C in less than 2.25 h under these conditions.Therefore, to meet UK guidelines using immersion cooling using water at -1 °C (to avoid surface freezing) product thickness still needs to be less than 40 mm. It is estimated that a maximum product thickness of between 25 and 30 mm could be cooled within the 1.5 h UK guideline, but experimental trials have yet to verify this. Further reductions in cooling time with a single-stage immersion system could be achieved by using a packaging method that eliminates the insulating effect of the air trapped between the lid and the product. Vacuum packed product in hermetically sealed bags or pouches (as used in Sous-vide) could be used to ensure improved heat transfer and cooling rates, as long as product thickness is kept to a minimum. 16.5.3 Refrigeration problems in practice Surveys of ready meal plants performing industrial scale cook–chill opera- tions have highlighted a number of factors that impede the effective chill- ing of cooked products. Any delay and extension to the optimum cooling time, after cooking to the critical control temperature, will lead to shorter 336 Meat refrigeration 60 50 40 30 20 10 5 3 –1 60 50 40 30 20 10 5 3 –1 60 50 40 30 20 10 5 3 –1 60 50 40 30 20 10 5 3 –1 60 50 40 30 20 10 5 3 –1 60 50 40 30 20 10 5 3 60 50 40 30 20 10 0246810121416 –4.5 –4.0 –3.5 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 0.0 In (T -T a / T i-T a)10mm tray 10mm tray with lid 40mm tray 40mm tray with lid 80mm tray 80mm tray with lid Time (hours) T emperature at slowest cooling position –30 –20 –10 –5 –20+2 Air temperature (°C) Fig. 16.4. Time required to cool centre of bolognese sauce in an air velocity of 3ms -1 (source: Evans et al., 1996). shelf life of the product and greater risk of bacterial growth to unaccept- able levels. A key factor when considering the performance of a blast chiller is its ability to reduce the air temperature and maintain it at the control point. The longer the room takes to pull-down to the pre-set air temperature, the greater the delay to maximum cooling rate and hence the longer the product cooling time. Figure 16.5 shows clearly the effect of loading warm product into a blast chiller with insufficient refrigeration capacity.The temperature of the air returning to the cooling coil reached 8.5 °C after the first batch of hot sauce was loaded and peaked at over 11°C after the final load. Seven and a half hours after loading the air temperature had still not recovered to its set point. During the survey of one facility, in approximately 40% of the chilling runs monitored, the blast chiller never managed to reduce the air temperature to its control point, indicating overloading/ undersizing of refrigeration load capacity. A practice that must be avoided is the removal of product from the blast chillers before the specified minimum temperatures have been reached. Loading of the blast chiller with hot product whilst partially chilled prod- ucts are already being chilled has also been identified as causing problems. Monitored temperatures clearly show a rise in the air temperature as the hot product is introduced, which in turn causes a rise in product tempera- tures. Failure to ensure that evaporator coils are effectively defrosted and all fan units are operational before chilling can drastically reduce the effectiveness of blast chillers. These problems are associated with incorrect Secondary chilling of meat and meat products 337 –2 0 2 4 6 8 10 12 23 further trays loaded 27 more trays loaded 40 trays loaded 0 50 100 150 200 250 300 350 400 450 Time (h) D air on food out D air off food out Fig. 16.5. Air on and air off coil temperatures during cooling in blast chiller. operation of blast chillers and are not caused by incorrect design of the refrigeration system. These may all be obvious but have been observed on several occasions in more than one factory. Other problems include: incor- rect thermostat settings resulting in too warm or even frozen product; insuf- ficient airflow around all (or any) loaded products limiting effective heat transfer; deterioration in refrigeration performance due to lack of mainte- nance; leaving products too long in blast chillers designed to use sub-zero temperatures, resulting in partial product freezing. 16.6 Conclusions 1 There is a large and growing demand for data on methods and cooling times for a range of cooked products. 2 The majority of commercial systems use air as the cooling medium but vacuum and immersion offer viable alternatives. 3 With small products, increasing air velocities can substantially reduce the cooling time. 4 Results from practical trials using pilot scale facilities, indicate that achieving rapid cooling of cooked meat sauce type material in trays, to meet UK cook–chill guidelines, is only feasible with depths of less than 30 mm. 5 Experience from surveys of commercial chilled ready meal manufac- turing facilities has highlighted that additional operational and design problems can add to the difficulty of meeting the rapid cooling requirements. 6 The importance is emphasised of putting in place strict scheduling of product and temperature monitoring and control throughout the pro- duction process, taking into account the additional complications of fluctuating demand and ‘just in time’ manufacturing. 16.7 References anon (1987), Rapid cooling of ham. 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