5 Influence of refrigeration on evaporative weight loss from meat From the moment an animal is slaughtered the meat produced begins to lose weight by evaporation. Under typical commercial distribution condi- tions, it has been estimated that lamb and beef lose from 5.5 to 7% by evap- oration between slaughter and retail sale (Malton, 1984).Weight losses from pork are probably of the same magnitude. In addition to the direct loss in saleable meat there are also secondary losses. Excessive evaporation during initial chilling and chilled storage produces a dark unattractive surface on the meat. Either this has to be removed by trimming, or the meat is down- graded and sold at a reduced price. Freezing does not stop weight loss. After meat is frozen, sublimation of ice from the surface occurs. If the degree of sublimation is excessive, the surface of the meat becomes dry and spongy, a phenomenon called ‘freezer burn’. In the United States, weight loss resulting from a combination of direct evaporative loss and freezer burn in pork bellies stored for one month before curing was estimated to be 500 000 kg (Ashby and James, 1974). Since that report, developments in the use of moisture imperious packaging materials have significantly reduced sublimation in frozen meat. Over 4 000 000 tonnes of meat and meat products are sold in the UK per year (MAFF, 2000). A very conservative estimate is that the use of existing technology in the field of refrigeration could reduce evaporative loss by at least 1%. This would result in a minimum saving to the UK meat industry of ￡60000000 (�96 m) per annum. In this chapter the theoretical factors that govern evaporative loss are briefly discussed. Comparisons are then made between weight losses in commercial practice and those resulting from the use of more closely con- trolled refrigeration techniques throughout the cold chain. The data for this comparison have been obtained from the available literature, and from an unpublished survey and experimental information gathered by the MRI (Meat Research Institute at the Institute of Food Research, Bristol Labo- ratory (IFR-BL)). In the concluding section, areas and systems that require further investigations are discussed. 5.1 Theoretical considerations The rate at which a piece of meat loses weight through its surface depends upon two related processes: evaporation and diffusion. Evaporation is the process that transfers moisture from the surface of the meat to the sur- rounding air. Diffusion transfers water from within the meat to its surface. The rate of evaporation (M e ) from the surface of a food is given by Dalton’s law: [5.1] where m is the mass transfer coefficient, A the effective area and P m and P a the vapour pressure at the surface of the meat and in the surrounding air, respectively. If each term in the right-hand side of the equation is examined in turn, the difficulty of predicting the rate of mass transfer from a meat carcass or joint becomes apparent. In most systems a value for the mass transfer coef- ficient is not obtained directly, but by analogy with the overall surface heat transfer coefficient (h). Some work has been carried out to measure m and h simultaneously (Kondjoyan et al., 1993). The surface heat transfer coeffi- cient itself is a function of the shape of the body and the properties of the medium flowing over it. It can be calculated for simple shapes, but must be obtained experimentally for irregular bodies such as meat joints and car- casses. Arce and Sweat (1980) carried out one of the most comprehensive reviews of publishing values of h for foodstuffs. However, only 4 references relate to meat and these cover a very limited range of refrigeration condi- tions. It is well established for forced air conduction systems that h becomes larger as air velocity increases. Therefore, all other factors being equal, weight loss will increase as air velocity increases. The effective area A can be difficult to measure, for example, the surface area of an irregular shape such as a meat carcass. In many commercial situations joints and/or carcasses are packed tightly together making an estimate of the ‘effective’ area even more problematic. Even meat blocks contain a number of irregularly shaped pieces of meat and do not normally present flat continuous surfaces to the air stream. Only in limited applica- tions such as plate freezing or thawing can an accurate estimate be made of the effective surface area. P a is a function of both air humidity and temperature and values are readily available in standard text books. P m is dependent upon the rate of diffusion and thus difficult to determine. After slaughter and flaying, free MmAPP ema =-() 86 Meat refrigeration water is present on the surface of a carcass and the P m can be assumed to equal that of saturated vapour at the same temperature as the surface. As the surface cools, water evaporates and this assumption only remains true if the rate of diffusion is high enough to maintain free water at the surface. Investigations in South Africa (Hodgson, 1970) reported that during chill- ing of a beef side only a part of the surface remained saturated throughout the operation. After flaying, the surface apparently dried, reaching maximum dehydration after ca. 10 h when only 70% of the surface was wet. Diffusion then gradually restored free water to the surface until, after 20 h under the test conditions, 90% of the surface was wet. There was no defi- nition of ‘wet’ in the paper but we interpret the statement to mean that after 10 h the rate of evaporative loss was 70% of that from a saturated surface at the same temperature. No other published work relating to carcasses has been located, but Australian experiments (Lovett et al., 1976) on small samples produced a similar pattern. There is a short initial phase, when the rate of evaporation is the same as that from free water. This is followed by a decreased rate of evaporation below the value expected from a water surface and a final phase where the surface is pro- gressively rewetted. However, Daudin and Kuitche (1995), predicted weight loss from pork carcasses assuming a fully wetted surface to a stated accu- racy of 0.1%. A simple examination of Fick’s law gives an indication of the problems in calculating the rate at which diffusion can occur through meat. It states that: [5.2] Where M d is the rate diffusion of water, K is the diffusion coefficient and dC is the concentration gradient. Meat is a non-homogeneous material consisting of fat, lean and bone and even these three elements are heterogeneous within themselves. Lean, com- mercially the most important component, is the muscle tissue of the live animal and consists of fibre bundles and connective tissue. The fibres have a preferred orientation, and diffusion coefficients and concentration gradi- ents vary with this orientation and the presence of barriers of different permeability within and between muscles. The rate of diffusion cannot therefore be predicted with any great degree of accuracy. 5.2 Weight loss in practice In this section the unit operations present in a meat distribution chain, chill- ing, chilled storage and display, freezing and frozen storage, are considered from the point of view of weight loss. Since the majority of the loss tends to occur during chilling, it is given greater consideration than the other processes. MKAC d =d Influence of refrigeration on evaporative weight loss from meat 87 5.2.1 Chilling Immediately after slaughter the surface of the carcass is hot (ca. 30°C) and wet so the rate of evaporation is high. Pork carcasses lose 0.4% moisture between 0.5 and 1.0 h post-mortem when held at approximately 15 °C (Cooper, 1970). Spray-washed lamb carcasses show an even greater rate of weight change, ca. 1.0%, during this time (James, unpublished work). Con- sequently, the time at which initial hot weight is obtained is crucial in all weight loss measurements. The majority of carcasses in the UK are chilled in a single stage system, pork at a nominal temperature of 4 °C, air velocity of 0.4ms -1 , 85–90% relative humidity (RH), lamb and beef at 0 °C, 0.5ms -1 , 85–90% RH. In practice the majority of chill rooms have under- powered refrigeration plants and are overloaded, so the rooms take several hours to reach their designed operating conditions. Typical weight losses in these single stage systems for beef are 2–3.5%, for lamb 2–2.8%, and for pork 1.8–3.5%. In a single stage chilling process, the factors in equation [5.1] that can be controlled by the refrigeration designer are P a and m, since both are a func- tion of air humidity and temperature. Humidity is controlled by the tem- perature difference (DT) across the evaporator coil. There are two ways of designing a coil to extract the same amount of heat: it can either have a very large surface area and a small DT, or a small area and a large DT.The former is expensive but produces air at a high humidity, whilst the latter is cheap but dries the air. If we assume that in the initial stages of chilling the surface of a carcass is saturated and is above 30°C, then in air at 0°C, 90% RH, P m - P a = 0.054 bar, and at 70% RH, P m - P a = 0.055 bar. The initial effect of RH on weight loss is therefore small, but as cooling proceeds, P m reduces and RH becomes increasingly important. Hodgson (1970) in South Africa showed that beef sides cooled for 20 h in air at a temperature of 1.7 °C, and velocity of 0.75m s -1 lost 2.75% in weight at 90% RH, and 3.4% at 70% RH, i.e. a 0.65% difference. Hodgson also stated that the maximum return on investment was achieved using a large coil with a DT of 5 °C. Since that time the price of beef has risen faster than the capital and the running costs of refrigeration equipment, and it is probable that the DT for a maximum return is now even smaller. The lower the air temperature the faster the rate of fall of the surface temperature, which controls the maximum value of P m . Lower air temper- atures should therefore reduce weight loss during chilling. Beef sides of average UK weight (140 kg) lost 1.2% in air at 4 °C, 0.5 m s -1 , 90% RH and 0.2% less at 0 °C, 0.5 m s -1 , 90% RH when cooled to a maximum centre tem- perature of 10 °C (Bailey and Cox, 1976). The initial weight was recorded ca. 2 h after slaughter. Since air velocity is directly related (via h), to the mass transfer coeffi- cient it would seem from equation [5.1] that increasing the air velocity during chilling would produce a greater weight loss. However, higher air velocities also increase the rate of fall of surface temperature and hence 88 Meat refrigeration decrease (P m - P a ), so the overall effect is not obvious.The results of experi- ments carried out on samples (15 ￥ 15 ￥ 2 cm thick) removed from freshly killed sheep (Lovett et al., 1976), show that the effect depends upon the definition of the completion of chilling, either within a set time (Table 5.1), or to a given maximum temperature (Table 5.2). Independent experiments using beef sides confirmed these findings. When chilling time was defined as that required to a set temperature (10 °C in the deep leg), increasing air velocity from 0.5 to 1.0m s -1 reduced weight loss by 0.15% (Cooper, 1970). When chilling for a set time (20 h), increasing the air velocity from 0.75 to 3 m s -1 increased weight loss from 2.75 to 3.3% (Hodgson, 1970). Minimal weight loss during chilling is therefore attained by using the lowest temperature and highest humidity that are practically feasible, and the minimum air velocity needed to meet the temperature/time require- ments. In single stage chilling the lowest temperature that can be used is -1 °C to avoid freezing at the surface of the meat. Toughening resulting from rapid chilling (‘cold shortening’) limits the use of such methods with lamb and beef. To avoid cold shortening a number of systems have been introduced that involve an initial holding period at a high temperature, con- sequently increasing weight loss. Influence of refrigeration on evaporative weight loss from meat 89 Table 5.1 Percentage weight loss from 15 ￥ 15 ￥ 2cm thick samples of lean mutton cooled from one side in air at 1–2 °C, for a set time, at different air velocities Air velocity (m s -1 ) Cooling time (h) 422 3.7 1.64 4.11 1.4 1.60 3.25 0.56 1.67 3.03 Source: Lovett et al., 1976. Table 5.2 Percentage weight loss from 15 ￥ 15 ￥ 2cm thick samples of lean mutton cooled from one side in air at 1–2 °C, to a set maximum temperature, at different air velocities Air velocity (m s -1 ) Final temperature ( °C) 13 7 4 3.7 0.95 1.14 1.27 1.4 1.09 1.32 1.48 0.56 1.20 1.49 1.69 Source: Lovett et al., 1976. The same restrictions do not apply to pork since the presence of insu- lating fat layers and the more rapid rate of glycolysis minimises the likeli- hood of toughening. Harsher pork chilling treatments are quite com- mon and in Denmark (Hermanson, personal communication) a two-stage system has been used in which the carcass is conveyed for 80min through a tunnel, operating at -15°C, 3ms -1 , then equalised for 12h in a chill room at 4°C, 0.5ms -1 with a very high RH. After the first stage the surface tem- perature of the carcass is below 0°C and moisture therefore condenses onto it in the initial part of the second stage. The average weight loss from 70 kg carcasses in such systems is claimed to be as low as 0.8% over the 14 h period. Work at the MRI produced a single stage 3 h system for 70 kg pork car- casses using air at -30°C, 1ms -1 (James et al., 1983; Gigiel and James, 1983). After chilling the average meat temperature is 0 °C and the carcass can be band sawn into primal joints and vacuum packed for distribution. The overall weight loss at 5 days post-mortem was just over 1%. The principle advantage of such a system is that the chilling can be conveyorised. Since the overall process time can be reduced from 14 to 4h, a three-fold increase in throughput can be achieved without a corresponding increase in chiller space. Commercial trials of a similar system for beef sides using electrical stimu- lation to minimise cold shortening, then air at -15°C, 3ms -1 for 6h, showed an overall chilling loss of 0.8%. However, its application in the production of chilled meat is limited since a proportion of the muscle is frozen. A number of large abattoirs in the USSR (Sheffer and Rutov, 1970) used a two-stage chilling system for beef sides, 4–8 h in air at -10 to -15 °C, 1–2ms -1 followed by 6–8 h at -1 °C and a moderate air velocity. Special jets were used to increase the air velocity over the thickest sections of the sides during the first stage and the overall weight loss was reported to be ca. 1%. 5.2.2 Chilled storage Equation [5.1] also governs weight loss in chilled storage. Since there is no further requirement to extract heat from the product, the relative impor- tance of the factors change and the air velocity should now be the minimum required to maintain a stable uniform temperature around the meat. Any increase in velocity will increase the rate of weight loss. Since there will normally only be a small temperature difference between the meat and the air, it is clear from equation [5.1] that the effect of any change in RH will be marked. If both the air and the surface are at 0 °C, and the surface is assumed to be saturated, a 10% change in RH will produce an equivalent change in the rate of evaporative loss. In commercial storage, -1 °C, 90% RH and 0.3 m s -1 represent near ideal conditions for minimal weight loss. Lower temperatures produce a risk of surface freezing, while a higher RH may reduce shelf-life because of faster 90 Meat refrigeration growth of micro-organisms in moist conditions. Table 5.3 shows the effects of different storage conditions upon weight loss from carcasses. A direct consequence of equation [5.1] is that poor temperature control during chilled storage should increase weight loss, for example poorly designed automatic defrosting systems in storage rooms lead to periodic cycles of condensation and drying on meat (Malton, 1984). These cycles harden and darken the surface of the meat and necessitate extra trimming before sale. Overall losses from beef joints can be as much as 5% per day. 5.2.3 Freezing and frozen storage The rate of sublimation of ice from a frozen surface is considerably slower than the rate of evaporation from a moist surface, and the ability of air to hold water rapidly diminishes as its temperature falls below 0°C. The con- sequent advantage of fast freezing and using low temperatures is shown in the survey summarised in Table 5.4. Influence of refrigeration on evaporative weight loss from meat 91 Table 5.3 Weight loss (% per day) from beef, lamb and pork carcasses stored at different relative humidities and temperatures Temperature (°C) % RH % Loss Beef 2 90 0.1–0.3 80 0.5 Lamb -1 90 0.5 94 0.2 Pork -1 95 0.2 85 0.5 75 0.8 5 95 0.3 85 0.6 75 1.0 Malton and James, 1984. Table 5.4 Percentage loss from stockinet-wrapped meat during freezing Freezing conditions Velocity (m s -1 ) Temperature ( °C) Loss (%) 0.3 -30 0.7–1.2 -20 1.4–1.6 -12 1.2–2.6 1.5 -28 0.6 Malton and James, 1984. More meat is now wrapped in impervious material before freezing, but, despite popular belief to the contrary, such packaging does not completely eliminate weight loss. Evaporative losses from polyethylene-wrapped carcass meat frozen at -30 °C are negligible, but losses of up to 0.5% have been recorded at -10 °C. The slower freezing time allowed water to migrate from the meat to the inner surface of the polyethylene. No published information has been located of the effect of RH on weight loss during frozen storage presumably because of the difficulty of measur- ing RH at temperatures below 0 °C. Figure 5.1 shows clearly the detrimen- tal effect of both air movement and high storage temperatures on weight loss. Although weight losses per day in frozen storage are small, storage times can be long with consequent overall losses as high as 10% (Roussel and Sarrazin, 1970). The importance of temperature control as well as actual temperature is supported by French experiments (Gac et al., 1970). Lean beef stored in cartons at -11°C lost 20mgcm -2 when the temperature was controlled to ±1 °C, but the losses increased by over three-fold to 72mgcm -2 when the temperature fluctuated by ±6 °C. Both losses were measured over 220 days. 5.2.4 Retail display During retail display meat is particularly vulnerable to evaporative losses. The surface of meat displayed (without refrigeration) either hanging from rails or on shelves rapidly warms, and then quickly loses weight in dry ambient conditions. The problem of rapid weight loss is exacerbated by fluctuations in temperatures and by draughts from doorways or fans. 92 Meat refrigeration 30 80 120 225 340 8.1 6.4 4.2 3.6 5.2 7.2 2.2 3.2 5.1 1.8 2.8 4 0.8 1.7 2.8 W eight loss (%) Storage (days) 10 8 6 4 2 0 –20 °C forced ventilation –16 °C free convection –26 °C free convection Fig. 5.1 Weight loss from unwrapped hams in frozen storage (source: Malton and James, 1984). Although refrigerated display lowers weight loss (Table 5.5), the design of many cabinets has paid little or no attention to product evaporation. Improved designs should take greater account of the factors that control evaporation and could significantly reduce losses at this stage of dis- tribution. For example, the importance of continuous refrigeration was shown in work where lambs cut into retail portions and displayed for 7 h under refrigeration lost 0.3% when refrigerated before cutting and 0.8% when not. 5.3 Overall The previous sections have shown the importance of refrigeration and its control in minimising weight loss at various stages in the distribution chain. Table 5.6 estimates the total evaporative loss during cooling and distribu- tion using information gathered from industry and published data. It shows clearly the importance of ‘good’ refrigeration design at all stages of the chilled distribution chain. However, it must be viewed with some caution since there is very little published information to indicate whether low initial weight loss could lead to higher weight loss at a later stage. Work in New Zealand (MIRINZ, 1983) shows a complicated relationship. Maximum freezing losses on lamb carcasses occurred when the previous chilling loss had been ca. 1%. Chilling losses both above and below this value resulted in lower losses during freezing.The minimum overall loss occurred in lambs that had experienced the minimum chilling loss. Following the path of weight loss through total distribution chains requires further investigation. Limited data have been gathered for chilled lamb. Refrigerated carcasses lost 2.2% during a 24 h chilling process increasing to a total of 3.4% after 3 days subsequent refrigerated distribu- tion. Similar carcasses lost 3.1% during ambient cooling for 24 h rising to 4.8% after a further 3 days of refrigerated distribution. This indicates that initial weight savings are maintained. Influence of refrigeration on evaporative weight loss from meat 93 Table 5.5 Percentage weight loss from unwrapped meat during display for 6 h Unrefrigerated Refrigerated Lamb Joints 1.0 0.7 Pork Chops 1.5 1.1 Beef Joints 0.4 0.4 Slices 1.5 1.2 Cubes 1.9 1.5 Mince 2.8 2.1 Source: Malton and James, 1984. 5.4 Conclusions 1 Meat distributed without refrigeration loses twice as much weight as commercially refrigerated meat. 2 The best refrigeration systems found in industry produce a further two- fold reduction in weight loss when compared with the average. 3 Application of the best current established technology could probably save a further 1% weight loss. 4 In an industry where profits are low, typically 1–2% of the value of the throughput at the wholesale stage, the relative effect on profitability would be large. 5 Low temperatures and high relative humidity will minimise weight loss from unwrapped meat. 6 To minimise weight loss in chilling, the air velocity should be just suffi- cient to attain the desired chilling time. 7 A better understanding of water diffusion through meat and mass trans- fer from the surface are required before we can optimise refrigeration systems. 94 Meat refrigeration Table 5.6 Estimates of total evaporative losses (%) in cooling and distribution Cooling Storage Transport Shop store Total Carcass Cut Display Lamb Ideal refrigeration Days 0.5 3 0.25 1 1 0.25 6 Loss (%) 1.2 0.6 0.1 0.2 0.5 0.3 2.9 Typical refrigeration Days 0.5 3 0.25 1 1 0.25 6 Loss (%) 2.0 1.5 0.1 0.5 0.9 0.6 5.6 Unrefrigerated Days 1 1 0.25 1 0.2 0.25 3.7 Loss (%) 3.7 3 0.2 2.0 1.5 1.5 9.2 Beef Ideal refrigeration Days 1 3 0.25 3 1 0.25 8.5 Loss (%) 1.4 0.3 0.1 0.3 1.0 0.6 3.7 Typical refrigeration Days 2 2 0.25 3 1 0.25 8.5 Loss (%) 2.5 0.6 0.1 0.9 1.5 1.5 7.1 Unrefrigerated Days 2 2 0.25 2.5 0.5 0.25 7.5 Loss (%) 3.8 4.0 0.2 4.0 1.5 2.0 15.5 Source: Malton and James, 1984. 5.5 References arce j and sweat v e (1980), Survey of published heat transfer coefficients encoun- tered in food refrigeration processes, ASHRAE Trans, 86(2) 235–260. ashby b j and james g n (1974), Effects of freezing and packaging methods on shrink- age and freezer burn on pork bellies in frozen storage, J Food Sci, 39 1136–1139. bailey c and cox r p (1976), The chilling of beef carcasses, Proc Inst Refrigeration, 72, session 1975–1976, 76–90. cooper t j r (1970), Control of weight losses during chilling, freezing, storage and transport of pigmeat, Weight losses in foodstuffs, Meeting of IIR Commissions II, IV, V & VII, Leningrad (USSR), Annexe 1970–3 Bulletin IIR, 175–191. daudin j d and kuitche a (1995), Chilling of pork carcasses with time-variable con- ditions: analysis and modelling, Proceedings of the XIXth International Congress of Refrigeration, Le Hague (Netherlands), I 129–136. gac a, francois o and schiltz j (1970), Early results concerning frost formation in sealed frozen food packages, Weight losses in foodstuffs, Meeting of IIR Com- missions II, IV, V & VII, Leningrad (USSR), Annexe 1970–3 Bulletin IIR, 73–82. gigiel a j and james s j (1983), The refrigeration aspects of rapid pork processing, Progress in Refrigeration Science and Technology, Proceedings of the XVIth Inter- national Congress of Refrigeration, Tome III, Paris (France), 3 417–423. hermanson p, personal communication. hodgson t (1970), The effect of air velocity and evaporator size on product weight losses in carcass-chilling rooms, Weight losses in foodstuffs, Meeting of IIR Com- missions II, IV, V & VII, Leningrad (USSR), Annexe 1970–3 Bulletin IIR, 161–167. james s j, unpublished work. james s j, gigiel a j and hudson w r (1983), The ultra rapid chilling of pork, Meat Sci, 8 63–78. kondjoyan a, daudin j d and bimbenet j j (1993), Heat and mass transfer coeffi- cients at the surface of elliptical cylinders placed in a turbulent air flow, J Food Eng, 20 339–367. lovett d a, herbert l s and radford r d (1976), Chilling of meat – experimental investigation of weight losses, Towards an ideal refrigerated food chain, Meeting of IIR Commissions C2, D1, D2, D3 & E1, Melbourne (Australia),Annexe 1976–1 Bulletin IIR, 307–314. malton r (1984), National Cold Storage Federation Handbook, 17–25. malton r and james s j (1984), Using refrigeration to reduce weight loss from meat, Proc. Symp. Profitability of Food Processing – 1984 Onwards – The Chemical Engineers Contribution, Pub Ichem E, 207–217. Ministry of Agriculture Fisheries and Food (2000), http://www.maff.gov.uk. mirinz (1983), Meat Industry Research Institute of New Zealand Annual Research Report 1982–83, 22. roussel l and sarrazin p l (1970), Weight losses in unwrapped hams after freez- ing and storage, Weight losses in foodstuffs, Meeting of IIR Commissions II, IV, V & VII, Leningrad (USSR), Annexe 1970–3 Bulletin IIR, 209–214. sheffer a p and rutov d c (1970), Reduction of weight losses in meat during chill- ing, freezing and storage, Weight losses in foodstuffs, Meeting of IIR Commissions II, IV, V & VII, Leningrad (USSR), Annexe 1970–3 Bulletin IIR, 143–151. Influence of refrigeration on evaporative weight loss from meat 95