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CHILLED STORAGE | Quality and Economic Considerations
CHILLED STORAGE/Quality and Economic Considerations
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Predictions made using a mathematical model that calculated bacterial growth from temperature/time relationships indicated that increases of up to 1.8 generations in bacterial numbers (Table 3) can occur during this transport and domestic cooling phase. The model assumes that bacteria require a time to acclimatize to a change in temperature (the lag phase) and that no acclimatization had occurred during display. If this rather optimistic assumption was not made, then up to 4.2 generations of Pseudomonas and growth of both Salmonella and Listeria were predicted. Very small increases in bacterial numbers (< 0.4 generations: Table 3) were predicted when the insulated box was used, due to the lower product temperatures. Chilled foods spend a period between a few minutes and many days stored in domestic refrigerators. The average food temperature measured in a survey of refrigerators in 252 UK households was approximately 6 C (Figure 5a). However, in over 15% of the refrigerators the average food temperature was above 8 C. In a typical UK refrigerator, a chilled food would spend over 35% of its time in an air temperature above 7 C (Figure 5b). See also: Chilled Storage: Principles; Quality and Economic Considerations; Vegetables of Temperate Climates: Leaf Vegetables
Further Reading Anonymous (1972) Meat Chilling – Why and How? MRI Symposium no. 2. Bristol: Langford. Anonymous (1986) Proceedings of the International Institute of Refrigeration. Recent Advances and Developments in the Refrigeration of Meat by Chilling. Bristol. Anonymous (1997) Proceedings of Meat Refrigeration – Why and How? EU Concerted Action Programme CT94 1881. Bristol, Langford: FRPERC, University of Bristol. Anonymous (2001) Proceedings of the International Institute of Refrigeration. Rapid Cooling – Above and Below Zero. Paris: International Institute of Refrigeration. Bøgh-Sørensen L (1980) Product temperatures in chilled cabinets. In: 26th European Meeting of Meat Research Workers, Colorado, paper 22. Cutting CL (1972) Meat Chilling – Why and How? MRI Symposium No 2. Bristol: Langford, FRPERC, University of Bristol. Evans JA, Stanton JI, Russell SL and James SJ (1991) Consumer Handling of Chilled Foods: A Survey of Time and Temperature Conditions. London: Ministry of Agriculture, Fisheries and Food. Gigiel AJ, James SJ and Evans JA (1998) Controlling Temperature During Distribution and Retail. EU Conference on Better and Safer Food. Kalsrhue: Bundesforschungsanstalt fu¨ r Ema¨ hrang.
1175
James SJ and Evans JA (1990) Temperatures in the retail and domestic chilled chain. In: Processing and Quality of Foods, vol. 3. Chilled Foods: The Revolution in Freshness, pp. 3.273–3.278. London: Elsevier Applied Science. James SJ and James C (2002) Meat Refrigeration. Cambridge: Woodhead Publishing Limited. Olsson P (1990) Chill cabinet surveys. In: Processing and Quality of Foods. Volume 3. Chilled Foods: The Revolution in Freshness, pp. 3.279–3.288. London: Elsevier Applied Science. Ramila A, Valin C and Taylor AA (1988) Accelerated Processing of Meat. London: Elsevier Applied Science. Rose SA (1986) Microbiological and temperature observations on pre-packaged ready-to-eat meats retailed from chilled display cabinets. In: Recent advances and developments in the refrigeration of meat by chilling, pp. 463–470. Paris: International Institute of Refrigeration.
Quality and Economic Considerations S James, University of Bristol, Langford, North Somerset, UK Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Introduction Quality and economic considerations are often the most important factors that govern the choice of a chilling system and the subsequent chilled chain for a particular foodstuff. Since the microbiological aspects are covered elsewhere, quality considerations in this chapter relate to organoleptic and nutritional changes. Chemical and biochemical changes within foods cause modifications in the appearance, taste, or texture that can limit its high quality shelf life. These changes are not always detrimental to the eating quality of the food. For example, biochemical changes, referred to as ‘conditioning’ or ‘aging’, that occur in meat after slaughter lead to improvements in both its texture and taste. In general with meat- and fish-based foods, the longest high-quality shelf life is achieved by rapidly reducing the temperature of the food and then maintaining it at a temperature very close to its initial freezing point. However, this is not true of all foods. For example, tropical fruits such as bananas suffer discoloration at temperatures below 12 C, whereas salad vegetables such as cucumbers lose their textural properties at temperatures below 6 C. Different cultivars of tomato suffer damage, called watersoaking and softening at temperatures below 7 or 10 C, pitting, and russeting occur in beans below 7 C and brown core in apples below 2 C.
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1176 CHILLED STORAGE/Quality and Economic Considerations
Primary and Secondary Chilling Quality Considerations 0003
0004
0005
tbl0001
In the majority of foods, fast cooling is beneficial, but there are instances where excessively rapid chilling rates cause quality problems in particular foods. For example, a serious defect known as ‘woolly texture’ can be produced in rapidly cooled peaches. Biochemical constraints lead to toughening if the lean tissue of beef or lamb is reduced to 10 C or below within 10 h of slaughter. Due to differences in the biochemistry of pork muscle, the same temperature has to be achieved within 3 h to cause toughening. (See Meat: Preservation.) Different foodstuffs exhibit particular quality advantages because of rapid chilling. In meat, the pH starts to fall immediately after slaughter, and protein denaturation begins. The result of this denaturation is a pink, proteinaceous fluid, commonly called ‘drip’, often seen in pre-packaged joints. The rate of denaturation is directly related to temperature, and it follows therefore that the faster the chilling rate, the less the drip. With both pork (Table 1) and beef, the use of rapid rates of chilling can halve the amount of drip loss. Fish passing through rigor mortis above 17 C are largely unusable because the fillets shrink and become tough. A relatively short delay of an hour or two before chilling can demonstrably reduce shelf life. In a different commodity area, freshly harvested sweetcorn loses 5, 20, or 60% of its sugar content after 24 h in air at 0, 10, and 30 C, respectively. Prompt cooling is clearly required if this vegetable is to retain its desirable sweetness. Similarly, the ripening of fruit can be controlled by rapid cooling, the rate of ripening declining as temperature is reduced and ceasing below about 4 C. (See Fish: Processing; Ripening of Fruit.) Rapid cooling is also often desirable with cooked products to maintain quality by eliminating the overcooking that occurs during slow cooling. For vitamin retention, the time taken to reduce the center temperature from 80 to 15 C is a critical factor. The vitamin C content in nonpasteurized meals is reduced Table 1 Drip loss after 2 days’ storage at 0 C, from leg joints from different breeds of pig cooled at different rates Breed
Landrace Large White Wessex Large White Pietrain
Drip loss (percent by weight) Slow
Quick
0.47 0.73 0.97
0.24 0.42 0.61
1.14
0.62
by 1 to 12% if cooling is carried out in 0.5 h, by 2 to 17% if the time is increased to 2 h, and by 10 to 38% when cooling takes 5 h. (See Ascorbic Acid: Properties and Determination.) Vacuum-cooling is a very effective method of cooling cooked beef joints and hams. However, the process results in high weight losses and some reduction in eating qualities of the meat. Intermittent spraying to replace the water loss is practiced in vacuum-cooling of vegetables. It could have an application with meat.
0006
Economic Considerations
It is clear from restrictions already detailed that attempts to increase chilling rates are complicated by many factors, but there are a number of clear advantages in production economics if faster cooling can be achieved. Most foods are of high value, and any increase in rate of product throughput will improve cash flow and utilize expensive plants more efficiently. For example, the cooling time of 400-g pies can be reduced by 10 min if the air velocity is raised from 6 to 10 m s1. In high-throughput baking lines (> 1000 items per hour), the 7% increase in throughput could justify the higher capital and running costs of larger fans. The power required by the fans to move the air within a chill room increases with the cube of the velocity. A fourfold increase in air velocity from 0.5 to 2 m s1 results in a 4.4-h (18%) reduction in chilling time for a 140-kg beef side, but requires a 64-fold increase in fan power. Increasing air velocity to 3 m s1 only achieves an extra 6% reduction in chilling time. In most practical situations, where large items (e.g., meat carcasses, tuna, bins of vegetables) are being cooled, it is doubtful whether an air velocity greater than 1 m s1 can be justified. Efficient chilling produces a reduction in weight loss, which results in a higher yield of saleable material. Most foods have a high water content, and the rate of evaporation depends on the vapor pressure at the surface. Vapor pressure increases with temperature, and thus any reduction in the surface temperature will reduce the rate of evaporation. The effect of air temperature and velocity on evaporative weight loss during chilling is dependent upon the endpoint of the chilling process (Figure 1a and b). When chilling for a set time, weight loss increases as the temperature decreases and velocity increases. The opposite effect is found when chilling to a set temperature. If specified cooling schedules are to be attained, refrigeration machinery must be designed to meet the required heat extraction rate at all times during the chilling cycle. Heat enters a chill room via open doors, from personnel, through the insulation, from lights and cooling fans, and from the cooling
0007
0008
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CHILLED STORAGE/Quality and Economic Considerations (a) Velocity
5
4 hours 22 hours
Weight loss (%)
4 13 ⬚C
3
7 ⬚C 4 ⬚C
2 1
0 0
1
2
3
4
Air velocity (m/s)
(b) Temperature Thin 18 h Thin 10 ⬚C Fat 18 h
Weight loss (%)
3.0
2.5
2.0
2
4
6
8
10
Air temperature (⬚C) Figure 1 Relationship between weight loss and (a) air velocity for lamb carcasses and (b) air temperature for beef sides when chilling either for a set time or to a final temperature.
Most packaging systems substantially reduce evaporative weight loss from the surface of the product, and many retard the rate of chemical degradation. The use of modified atmospheres or opaque packaging can limit color changes during the chilled storage of meat, fish, and fruits. Losses in vitamin content tend to be reduced at lower storage and display temperatures. Vitamin C losses from lettuce average 4.8, 5.6, and 6.6% per day at temperatures of 1, 5, and 10 C, respectively.
Table 2 The weight loss, saving (£) over conventionally chilled controls, total work done at 24 h post mortem, cooling time to 10 C for each treatment, drip loss from loin chops and annual saving for each treatment compared with the conventional chill used as a control for an abattoir slaughtering an average of 1000 pigs per week (74 kg dead weight at £0.90 kg1) Chilling treatment
(1) Ultrarapid Side Whole carcass (2) Ultrarapid two-stage (3) Immersion (4) High humidity (5) Delayþhigh humidity (6) Delayþspray (7) Rapidþhigh humidity (8) Rapidþconventional a
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0011
Packaged Food
0
tbl0002
products. The product load is the major component of the total heat to be extracted from a fully loaded chill room. The rate of heat release from a food varies with time. It is at a peak immediately after loading and then falls rapidly. The peak value is primarily a function of the environmental conditions during chilling. In commercial systems, the peak load imposed on the refrigeration plant is also a function of the rate at which hot food is introduced into the chill room. Increasing the air velocity, decreasing the air temperature, or shortening the loading time increases the peak heat load. In beefchilling, for example, there is a fourfold difference in peak load between a chill room operating at 8 C, 0.5 m s1 loaded over 8 h and the same room operating at 0 C, 3 m s1 and loaded over 2 h. Alternative chilling treatments can produce large increases in throughput and reductions in weight loss over conventional treatments (Table 2). The cash savings that result from the reduced weight losses can substantially increase the overall profits of many slaughtering operations (Table 2).
Commercial Storage, Retail Display, and Domestic Storage
Fat 10 ⬚C
1.5
fig0001
1177
Weight loss at 24 h (%)
1.13 (0.85) 1.10 (1.01) 2.00 (0.29) 0.32 (2.08) 1.96 (0.21) 1.93 (0.24) 0.95 (1.22) 1.53 (0.64) 1.67 (0.50)
Measured at intersection of lean and fat.
Cooling time to10 C Deep leg (h)
Surface leg (h)
>7.0 >8.0 7.0 5.4 12.9 14.8 14.1 11.6 11.5
5.0a 4.0a 3.6 8.7 9.1 9.2 1.3 1.6
Drip (%)
Texture work (J)
Saving (£)
2.3 0.9 0.2 1.5 0.8 1.0 1.0 0.9 1.0
0.18 0.21 0.23 0.30 0.20 0.20 0.20 0.21 0.24
28 300 33 600 9 600 69 200 7 000 8 000 40 600 21 300 16 600
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1178 CHILLED STORAGE/Quality and Economic Considerations
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Larger effects of temperature are found in French beans with equivalent losses of 1.9, 3.0, and 5.1% per day. No losses were reported in carrots at the three temperatures. Oranges and pineapples are examples of foods that have an optimum temperature for vitamin C retention. In oranges, a 10% loss per month at 10 C increases to 12% at 5 C and 26% at 0 C. Raising the storage temperature above 10 C also increases the rate of loss to 13% at 15 C and 22% at 30 C. A similar optimum at 10 C is found in pineapples. Vitamin B6 retention is also a function of both storage temperature and species. Loss from lettuce and French beans increases twofold and threefold, respectively, as the temperature is increased from 1 to 10 C. No change was reported in parsley or carrots. (See Vitamin B6: Properties and Determination.) Unwrapped Food
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Changes in appearance are normally the criteria that limit storage and display of unwrapped foods, commercial buyers or domestic consumers selecting fresh or newly loaded product in preference to that displayed or stored for some time. Deterioration in appearance has been related to degree of dehydration in red meat (Table 3), and similar changes are likely to occur in other foods. Apart from any relationship to appearance, weight loss is of considerable importance in its own right. The direct cost of evaporative loss from unwrapped foods in chilled display cabinets in the UK is in excess of £5 million per annum. In the equation that governs weight loss, already described, the mass-transfer coefficient, m, is a function of air velocity. Typical values range from approximately 2 108 kg m2 s1 Pa1 at 0.25 m s1
to 14 108 kg m2 s1 Pa1 at 1.5 m s1. The vapor pressure difference (PsawPm) is a function of surface temperature and dryness, and the temperature and relative humidity of the air. In storage rooms at temperatures in the range of 0–2 C, 76–90% RH, the average weight loss from cauliflower, French beans, and green peas is in the range of 0.1–0.4% per day. Under conditions typical of domestic refrigeration, 4–8 C and 70–90% RH, weight losses from the same products range from 0.3 to 3.0% per day. In a nonrefrigerated ambient, 16–24 C and 50–70% RH, losses are even higher in the range of 1.0–4.0% per day. The relative effects of air temperature, velocity, and humidity on weight loss in retail display are demonstrated in Figure 2. Changes in relative humidity have a substantial effect with a reduction from 95 to 40%, increasing weight loss over a 6-h display period by a factor of between 14 and 18. The effect of air velocity on weight loss is confounded by that of relative humidity. Raising the air velocity from 0.1 to 0.5 m s1 has little effect on weight loss at 95% RH but
Table 3 Relationship between change in appearance and evaporative weight loss (g cm2) Evaporative loss
Change in appearance
Up to 0.01
Red, attractive, and still wet; may lose some brightness Surface becoming drier; still attractive but darker Distinct obvious darkening; becoming dry and leathery Dry, blackening Black
0.015–0.025 0.025–0.035 0.05 0.05–0.10
2 ⬚C, 0.1 m/s
2 ⬚C, 0.3 m/s
2 ⬚C, 0.5 m/s
6 ⬚C, 0.1 m/s
6 ⬚C, 0.3 m/s
6 ⬚C, 0.5 m/s
0.09
Weight loss (g/sq.cm)
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 40% RH fig0002
60% RH
Figure 2 Weight loss from samples of beef steak under simulated display conditions.
95% RH
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CHILLED STORAGE/Quality and Economic Considerations
40% RH, 100 W spots + 50 W son
65% RH, color 83 + 100 W son
65% RH, 100 W spots + 50 W son
85% RH, 100 W spots + 50 W son
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0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0
100
200
300
400
500
Time on display (m) fig0003
0020
0021
0022
Figure 3 Weight loss from delicatessen products under different humidity and lighting conditions.
increases the loss by a factor of between 2 and 2.4 at 60% RH. A temperature change from 2 to 6 C has a far smaller effect on weight loss than changes in either relative humidity or velocity. In further work, a model developed to predict the rate of weight loss from unwrapped meat under the range of environmental conditions found in chilled retail displays showed that it was governed by the mean value of the conditions. Fluctuations in temperature or relative humidity had little effect on weight loss, and any apparent effect was caused by changes in the mean conditions. There is a conflict between the need to make the display attractive and convenient to increase sales appeal and the optimum display conditions for the product. High lighting levels increase the heat load, and the air temperature in the cabinet rises. This rise increases the temperature difference across the evaporator coil, and the air entering the cabinet is dehumidified. Consequently, the rate of evaporative weight loss from foods on display is increased. Studies have shown that changing a lighting combination of 50 W sons and 100 W halogen lights to 100 W sons and a colour 83 fluorescent significantly increased the weight loss (Figure 3). The increase was similar in magnitude to that produced by 20% reduction in relative humidity. On average, the rate of weight loss under the combination of 50 W sons and 100 W halogen (spot) lights was approximately 1.4 times less than the 100 W sons and colour 83 fluorescent lighting. See also: Ascorbic Acid: Properties and Determination; Fish: Processing; Meat: Preservation; Ripening of Fruit; Vitamin B6: Properties and Determination
Further Reading Anonymous (1974) ASHRAE Handbook and Product Directory. Applications. New York: American Society of Heating Refrigeration and Air Conditioning Engineers. Food Refrigeration & Process Engineering Research Centre (FRPERC) (1997) Meat Refrigeration – Why and How? EU Concerted Action Programme CT94 1881. Langford, UK: FRPERC. FRPERC (2000) Publications on the refrigeration and thermal processing of food. University of Bristol. http://www.frperc.bris.ac.uk. Gormley TR (1990) Chilled Foods – The State of the Art. Barking, UK: Elsevier Applied Science. International Institute of Refrigeration (1985) Technology Advances in Refrigerated Storage and Transport. Orlando, FL: International Institute of Refrigeration – Commissions D1, D2 & D3. International Institute of Refrigeration (1986) Recent Advances and Developments in the Refrigeration of Meat by Chilling. Bristol, UK: International Institute of Refrigeration Commission C2. International Institute of Refrigeration (1990) Progress in the Science and Technology of Refrigeration in Food Engineering. Dresden: International Institute of Refrigeration – Commissions B2, C2, D1, D2/3. International Institute of Refrigeration (2000) Recommendations for Chilled Storage of Perishable Produce. Paris: International Institute of Refrigeration ISBN 2-91314909-X James SJ (1999) Food refrigeration and thermal processing at Langford, UK: 32 years of research. Transactions of the IChemE Food and Bioproducts Processing 77(C): 261–279. Zeuthen P, Cheftel JC, Eriksson C, Gormley TR, Lonko P and Paulus K (1990) Chilled Foods: The Revolution in Freshness. Barking, UK: Elsevier Applied Science.
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Predictions made using a mathematical model that calculated bacterial growth from temperature/time relationships indicated that increases of up to 1.8 generations in bacterial numbers (Table 3) can occur during this transport and domestic cooling phase. The model assumes that bacteria require a time to acclimatize to a change in temperature (the lag phase) and that no acclimatization had occurred during display. If this rather optimistic assumption was not made, then up to 4.2 generations of Pseudomonas and growth of both Salmonella and Listeria were predicted. Very small increases in bacterial numbers (< 0.4 generations: Table 3) were predicted when the insulated box was used, due to the lower product temperatures. Chilled foods spend a period between a few minutes and many days stored in domestic refrigerators. The average food temperature measured in a survey of refrigerators in 252 UK households was approximately 6 C (Figure 5a). However, in over 15% of the refrigerators the average food temperature was above 8 C. In a typical UK refrigerator, a chilled food would spend over 35% of its time in an air temperature above 7 C (Figure 5b). See also: Chilled Storage: Principles; Quality and Economic Considerations; Vegetables of Temperate Climates: Leaf Vegetables
Further Reading Anonymous (1972) Meat Chilling – Why and How? MRI Symposium no. 2. Bristol: Langford. Anonymous (1986) Proceedings of the International Institute of Refrigeration. Recent Advances and Developments in the Refrigeration of Meat by Chilling. Bristol. Anonymous (1997) Proceedings of Meat Refrigeration – Why and How? EU Concerted Action Programme CT94 1881. Bristol, Langford: FRPERC, University of Bristol. Anonymous (2001) Proceedings of the International Institute of Refrigeration. Rapid Cooling – Above and Below Zero. Paris: International Institute of Refrigeration. Bøgh-Sørensen L (1980) Product temperatures in chilled cabinets. In: 26th European Meeting of Meat Research Workers, Colorado, paper 22. Cutting CL (1972) Meat Chilling – Why and How? MRI Symposium No 2. Bristol: Langford, FRPERC, University of Bristol. Evans JA, Stanton JI, Russell SL and James SJ (1991) Consumer Handling of Chilled Foods: A Survey of Time and Temperature Conditions. London: Ministry of Agriculture, Fisheries and Food. Gigiel AJ, James SJ and Evans JA (1998) Controlling Temperature During Distribution and Retail. EU Conference on Better and Safer Food. Kalsrhue: Bundesforschungsanstalt fu¨ r Ema¨ hrang.
1175
James SJ and Evans JA (1990) Temperatures in the retail and domestic chilled chain. In: Processing and Quality of Foods, vol. 3. Chilled Foods: The Revolution in Freshness, pp. 3.273–3.278. London: Elsevier Applied Science. James SJ and James C (2002) Meat Refrigeration. Cambridge: Woodhead Publishing Limited. Olsson P (1990) Chill cabinet surveys. In: Processing and Quality of Foods. Volume 3. Chilled Foods: The Revolution in Freshness, pp. 3.279–3.288. London: Elsevier Applied Science. Ramila A, Valin C and Taylor AA (1988) Accelerated Processing of Meat. London: Elsevier Applied Science. Rose SA (1986) Microbiological and temperature observations on pre-packaged ready-to-eat meats retailed from chilled display cabinets. In: Recent advances and developments in the refrigeration of meat by chilling, pp. 463–470. Paris: International Institute of Refrigeration.
Quality and Economic Considerations S James, University of Bristol, Langford, North Somerset, UK Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Introduction Quality and economic considerations are often the most important factors that govern the choice of a chilling system and the subsequent chilled chain for a particular foodstuff. Since the microbiological aspects are covered elsewhere, quality considerations in this chapter relate to organoleptic and nutritional changes. Chemical and biochemical changes within foods cause modifications in the appearance, taste, or texture that can limit its high quality shelf life. These changes are not always detrimental to the eating quality of the food. For example, biochemical changes, referred to as ‘conditioning’ or ‘aging’, that occur in meat after slaughter lead to improvements in both its texture and taste. In general with meat- and fish-based foods, the longest high-quality shelf life is achieved by rapidly reducing the temperature of the food and then maintaining it at a temperature very close to its initial freezing point. However, this is not true of all foods. For example, tropical fruits such as bananas suffer discoloration at temperatures below 12 C, whereas salad vegetables such as cucumbers lose their textural properties at temperatures below 6 C. Different cultivars of tomato suffer damage, called watersoaking and softening at temperatures below 7 or 10 C, pitting, and russeting occur in beans below 7 C and brown core in apples below 2 C.
0001
0002
1176 CHILLED STORAGE/Quality and Economic Considerations
Primary and Secondary Chilling Quality Considerations 0003
0004
0005
tbl0001
In the majority of foods, fast cooling is beneficial, but there are instances where excessively rapid chilling rates cause quality problems in particular foods. For example, a serious defect known as ‘woolly texture’ can be produced in rapidly cooled peaches. Biochemical constraints lead to toughening if the lean tissue of beef or lamb is reduced to 10 C or below within 10 h of slaughter. Due to differences in the biochemistry of pork muscle, the same temperature has to be achieved within 3 h to cause toughening. (See Meat: Preservation.) Different foodstuffs exhibit particular quality advantages because of rapid chilling. In meat, the pH starts to fall immediately after slaughter, and protein denaturation begins. The result of this denaturation is a pink, proteinaceous fluid, commonly called ‘drip’, often seen in pre-packaged joints. The rate of denaturation is directly related to temperature, and it follows therefore that the faster the chilling rate, the less the drip. With both pork (Table 1) and beef, the use of rapid rates of chilling can halve the amount of drip loss. Fish passing through rigor mortis above 17 C are largely unusable because the fillets shrink and become tough. A relatively short delay of an hour or two before chilling can demonstrably reduce shelf life. In a different commodity area, freshly harvested sweetcorn loses 5, 20, or 60% of its sugar content after 24 h in air at 0, 10, and 30 C, respectively. Prompt cooling is clearly required if this vegetable is to retain its desirable sweetness. Similarly, the ripening of fruit can be controlled by rapid cooling, the rate of ripening declining as temperature is reduced and ceasing below about 4 C. (See Fish: Processing; Ripening of Fruit.) Rapid cooling is also often desirable with cooked products to maintain quality by eliminating the overcooking that occurs during slow cooling. For vitamin retention, the time taken to reduce the center temperature from 80 to 15 C is a critical factor. The vitamin C content in nonpasteurized meals is reduced Table 1 Drip loss after 2 days’ storage at 0 C, from leg joints from different breeds of pig cooled at different rates Breed
Landrace Large White Wessex Large White Pietrain
Drip loss (percent by weight) Slow
Quick
0.47 0.73 0.97
0.24 0.42 0.61
1.14
0.62
by 1 to 12% if cooling is carried out in 0.5 h, by 2 to 17% if the time is increased to 2 h, and by 10 to 38% when cooling takes 5 h. (See Ascorbic Acid: Properties and Determination.) Vacuum-cooling is a very effective method of cooling cooked beef joints and hams. However, the process results in high weight losses and some reduction in eating qualities of the meat. Intermittent spraying to replace the water loss is practiced in vacuum-cooling of vegetables. It could have an application with meat.
0006
Economic Considerations
It is clear from restrictions already detailed that attempts to increase chilling rates are complicated by many factors, but there are a number of clear advantages in production economics if faster cooling can be achieved. Most foods are of high value, and any increase in rate of product throughput will improve cash flow and utilize expensive plants more efficiently. For example, the cooling time of 400-g pies can be reduced by 10 min if the air velocity is raised from 6 to 10 m s1. In high-throughput baking lines (> 1000 items per hour), the 7% increase in throughput could justify the higher capital and running costs of larger fans. The power required by the fans to move the air within a chill room increases with the cube of the velocity. A fourfold increase in air velocity from 0.5 to 2 m s1 results in a 4.4-h (18%) reduction in chilling time for a 140-kg beef side, but requires a 64-fold increase in fan power. Increasing air velocity to 3 m s1 only achieves an extra 6% reduction in chilling time. In most practical situations, where large items (e.g., meat carcasses, tuna, bins of vegetables) are being cooled, it is doubtful whether an air velocity greater than 1 m s1 can be justified. Efficient chilling produces a reduction in weight loss, which results in a higher yield of saleable material. Most foods have a high water content, and the rate of evaporation depends on the vapor pressure at the surface. Vapor pressure increases with temperature, and thus any reduction in the surface temperature will reduce the rate of evaporation. The effect of air temperature and velocity on evaporative weight loss during chilling is dependent upon the endpoint of the chilling process (Figure 1a and b). When chilling for a set time, weight loss increases as the temperature decreases and velocity increases. The opposite effect is found when chilling to a set temperature. If specified cooling schedules are to be attained, refrigeration machinery must be designed to meet the required heat extraction rate at all times during the chilling cycle. Heat enters a chill room via open doors, from personnel, through the insulation, from lights and cooling fans, and from the cooling
0007
0008
0009
CHILLED STORAGE/Quality and Economic Considerations (a) Velocity
5
4 hours 22 hours
Weight loss (%)
4 13 ⬚C
3
7 ⬚C 4 ⬚C
2 1
0 0
1
2
3
4
Air velocity (m/s)
(b) Temperature Thin 18 h Thin 10 ⬚C Fat 18 h
Weight loss (%)
3.0
2.5
2.0
2
4
6
8
10
Air temperature (⬚C) Figure 1 Relationship between weight loss and (a) air velocity for lamb carcasses and (b) air temperature for beef sides when chilling either for a set time or to a final temperature.
Most packaging systems substantially reduce evaporative weight loss from the surface of the product, and many retard the rate of chemical degradation. The use of modified atmospheres or opaque packaging can limit color changes during the chilled storage of meat, fish, and fruits. Losses in vitamin content tend to be reduced at lower storage and display temperatures. Vitamin C losses from lettuce average 4.8, 5.6, and 6.6% per day at temperatures of 1, 5, and 10 C, respectively.
Table 2 The weight loss, saving (£) over conventionally chilled controls, total work done at 24 h post mortem, cooling time to 10 C for each treatment, drip loss from loin chops and annual saving for each treatment compared with the conventional chill used as a control for an abattoir slaughtering an average of 1000 pigs per week (74 kg dead weight at £0.90 kg1) Chilling treatment
(1) Ultrarapid Side Whole carcass (2) Ultrarapid two-stage (3) Immersion (4) High humidity (5) Delayþhigh humidity (6) Delayþspray (7) Rapidþhigh humidity (8) Rapidþconventional a
0010
0011
Packaged Food
0
tbl0002
products. The product load is the major component of the total heat to be extracted from a fully loaded chill room. The rate of heat release from a food varies with time. It is at a peak immediately after loading and then falls rapidly. The peak value is primarily a function of the environmental conditions during chilling. In commercial systems, the peak load imposed on the refrigeration plant is also a function of the rate at which hot food is introduced into the chill room. Increasing the air velocity, decreasing the air temperature, or shortening the loading time increases the peak heat load. In beefchilling, for example, there is a fourfold difference in peak load between a chill room operating at 8 C, 0.5 m s1 loaded over 8 h and the same room operating at 0 C, 3 m s1 and loaded over 2 h. Alternative chilling treatments can produce large increases in throughput and reductions in weight loss over conventional treatments (Table 2). The cash savings that result from the reduced weight losses can substantially increase the overall profits of many slaughtering operations (Table 2).
Commercial Storage, Retail Display, and Domestic Storage
Fat 10 ⬚C
1.5
fig0001
1177
Weight loss at 24 h (%)
1.13 (0.85) 1.10 (1.01) 2.00 (0.29) 0.32 (2.08) 1.96 (0.21) 1.93 (0.24) 0.95 (1.22) 1.53 (0.64) 1.67 (0.50)
Measured at intersection of lean and fat.
Cooling time to10 C Deep leg (h)
Surface leg (h)
>7.0 >8.0 7.0 5.4 12.9 14.8 14.1 11.6 11.5
5.0a 4.0a 3.6 8.7 9.1 9.2 1.3 1.6
Drip (%)
Texture work (J)
Saving (£)
2.3 0.9 0.2 1.5 0.8 1.0 1.0 0.9 1.0
0.18 0.21 0.23 0.30 0.20 0.20 0.20 0.21 0.24
28 300 33 600 9 600 69 200 7 000 8 000 40 600 21 300 16 600
0012
0013
1178 CHILLED STORAGE/Quality and Economic Considerations
0014
0015
Larger effects of temperature are found in French beans with equivalent losses of 1.9, 3.0, and 5.1% per day. No losses were reported in carrots at the three temperatures. Oranges and pineapples are examples of foods that have an optimum temperature for vitamin C retention. In oranges, a 10% loss per month at 10 C increases to 12% at 5 C and 26% at 0 C. Raising the storage temperature above 10 C also increases the rate of loss to 13% at 15 C and 22% at 30 C. A similar optimum at 10 C is found in pineapples. Vitamin B6 retention is also a function of both storage temperature and species. Loss from lettuce and French beans increases twofold and threefold, respectively, as the temperature is increased from 1 to 10 C. No change was reported in parsley or carrots. (See Vitamin B6: Properties and Determination.) Unwrapped Food
0016
0017
Changes in appearance are normally the criteria that limit storage and display of unwrapped foods, commercial buyers or domestic consumers selecting fresh or newly loaded product in preference to that displayed or stored for some time. Deterioration in appearance has been related to degree of dehydration in red meat (Table 3), and similar changes are likely to occur in other foods. Apart from any relationship to appearance, weight loss is of considerable importance in its own right. The direct cost of evaporative loss from unwrapped foods in chilled display cabinets in the UK is in excess of £5 million per annum. In the equation that governs weight loss, already described, the mass-transfer coefficient, m, is a function of air velocity. Typical values range from approximately 2 108 kg m2 s1 Pa1 at 0.25 m s1
to 14 108 kg m2 s1 Pa1 at 1.5 m s1. The vapor pressure difference (PsawPm) is a function of surface temperature and dryness, and the temperature and relative humidity of the air. In storage rooms at temperatures in the range of 0–2 C, 76–90% RH, the average weight loss from cauliflower, French beans, and green peas is in the range of 0.1–0.4% per day. Under conditions typical of domestic refrigeration, 4–8 C and 70–90% RH, weight losses from the same products range from 0.3 to 3.0% per day. In a nonrefrigerated ambient, 16–24 C and 50–70% RH, losses are even higher in the range of 1.0–4.0% per day. The relative effects of air temperature, velocity, and humidity on weight loss in retail display are demonstrated in Figure 2. Changes in relative humidity have a substantial effect with a reduction from 95 to 40%, increasing weight loss over a 6-h display period by a factor of between 14 and 18. The effect of air velocity on weight loss is confounded by that of relative humidity. Raising the air velocity from 0.1 to 0.5 m s1 has little effect on weight loss at 95% RH but
Table 3 Relationship between change in appearance and evaporative weight loss (g cm2) Evaporative loss
Change in appearance
Up to 0.01
Red, attractive, and still wet; may lose some brightness Surface becoming drier; still attractive but darker Distinct obvious darkening; becoming dry and leathery Dry, blackening Black
0.015–0.025 0.025–0.035 0.05 0.05–0.10
2 ⬚C, 0.1 m/s
2 ⬚C, 0.3 m/s
2 ⬚C, 0.5 m/s
6 ⬚C, 0.1 m/s
6 ⬚C, 0.3 m/s
6 ⬚C, 0.5 m/s
0.09
Weight loss (g/sq.cm)
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 40% RH fig0002
60% RH
Figure 2 Weight loss from samples of beef steak under simulated display conditions.
95% RH
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0019
tbl0003
CHILLED STORAGE/Quality and Economic Considerations
40% RH, 100 W spots + 50 W son
65% RH, color 83 + 100 W son
65% RH, 100 W spots + 50 W son
85% RH, 100 W spots + 50 W son
1179
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0
100
200
300
400
500
Time on display (m) fig0003
0020
0021
0022
Figure 3 Weight loss from delicatessen products under different humidity and lighting conditions.
increases the loss by a factor of between 2 and 2.4 at 60% RH. A temperature change from 2 to 6 C has a far smaller effect on weight loss than changes in either relative humidity or velocity. In further work, a model developed to predict the rate of weight loss from unwrapped meat under the range of environmental conditions found in chilled retail displays showed that it was governed by the mean value of the conditions. Fluctuations in temperature or relative humidity had little effect on weight loss, and any apparent effect was caused by changes in the mean conditions. There is a conflict between the need to make the display attractive and convenient to increase sales appeal and the optimum display conditions for the product. High lighting levels increase the heat load, and the air temperature in the cabinet rises. This rise increases the temperature difference across the evaporator coil, and the air entering the cabinet is dehumidified. Consequently, the rate of evaporative weight loss from foods on display is increased. Studies have shown that changing a lighting combination of 50 W sons and 100 W halogen lights to 100 W sons and a colour 83 fluorescent significantly increased the weight loss (Figure 3). The increase was similar in magnitude to that produced by 20% reduction in relative humidity. On average, the rate of weight loss under the combination of 50 W sons and 100 W halogen (spot) lights was approximately 1.4 times less than the 100 W sons and colour 83 fluorescent lighting. See also: Ascorbic Acid: Properties and Determination; Fish: Processing; Meat: Preservation; Ripening of Fruit; Vitamin B6: Properties and Determination
Further Reading Anonymous (1974) ASHRAE Handbook and Product Directory. Applications. New York: American Society of Heating Refrigeration and Air Conditioning Engineers. Food Refrigeration & Process Engineering Research Centre (FRPERC) (1997) Meat Refrigeration – Why and How? EU Concerted Action Programme CT94 1881. Langford, UK: FRPERC. FRPERC (2000) Publications on the refrigeration and thermal processing of food. University of Bristol. http://www.frperc.bris.ac.uk. Gormley TR (1990) Chilled Foods – The State of the Art. Barking, UK: Elsevier Applied Science. International Institute of Refrigeration (1985) Technology Advances in Refrigerated Storage and Transport. Orlando, FL: International Institute of Refrigeration – Commissions D1, D2 & D3. International Institute of Refrigeration (1986) Recent Advances and Developments in the Refrigeration of Meat by Chilling. Bristol, UK: International Institute of Refrigeration Commission C2. International Institute of Refrigeration (1990) Progress in the Science and Technology of Refrigeration in Food Engineering. Dresden: International Institute of Refrigeration – Commissions B2, C2, D1, D2/3. International Institute of Refrigeration (2000) Recommendations for Chilled Storage of Perishable Produce. Paris: International Institute of Refrigeration ISBN 2-91314909-X James SJ (1999) Food refrigeration and thermal processing at Langford, UK: 32 years of research. Transactions of the IChemE Food and Bioproducts Processing 77(C): 261–279. Zeuthen P, Cheftel JC, Eriksson C, Gormley TR, Lonko P and Paulus K (1990) Chilled Foods: The Revolution in Freshness. Barking, UK: Elsevier Applied Science.