《肉制品冷冻技术》（英文版） Part 5 Influence of refrigeration on evaporative weight loss from meat

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 nitial 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 500000kg(Ashby and James, 1974). Since that report, developments in the use of moisture imperious packaging materials have significantly reduced sublimation in frozen meat Over 4000000 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 f60000000(E96m)per annum In this chapter the theoretical factors that govern evaporative loss briefly discussed. Comparisons are then made between weight losses commercial practice and those resulting from the use of more closely con- trolled refrigeration techniques throughout the cold chain. The data for this

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 £60 000 000 (€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

86 Meat refrigeration comparison have been obtained from the available literature, and from an unpublished survey and experimental information gathered by the MR (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- ounding air Diffusion transfers water from within the meat to its surface The rate of evaporation(Me) from the surface of a food is given by Daltons law Me=mA(P-P) where m is the mass transfer coefficient. a the effective area and p and Pa the vapour pressure at the surface of the meat and in the surrounding r,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 coe 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 Pa is a function of both air humidity and temperature and values are readily available in standard text books. Pm is dependent upon the rate of diffusion and thus difficult to determine. After slaughter and flaying, free

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 (Me) 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 Pm and Pa 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. Pa is a function of both air humidity and temperature and values are readily available in standard text books. Pm is dependent upon the rate of diffusion and thus difficult to determine. After slaughter and flaying, free M mA P P e ma = - ( ) 86 Meat refrigeration

Influence of refrigeration on evaporative weight loss from meat 87 water is present on the surface of a carcass and the pm can be assumed to equal that of saturated vapour at the same temperature as the surface. As he 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 nvestigations in South Africa(Hodgson, 1970) reported that during chill ng 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 10h when only 70% of the surface was wet Diffusion then gradually restored free water to the surface until, after 20h under the test conditions. 90% of the surface was wet There was no defi- nition of wetin the paper but we interpret the statement to mean that after 10h 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 re owever 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 Ficks law gives an indication of the problem in calculating the rate at which diffusion can occur through meat. It states Md=KAδC [52] Where Ma is the rate diffusion of water, K is the diffusion coefficient and SC 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 nercially 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 herefore 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

water is present on the surface of a carcass and the Pm 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 Md 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. M KA C d = d Influence of refrigeration on evaporative weight loss from meat 87

88 Meat refrigeration 5.2.1 Chilling Immediately after slaughter the surface of the carcass is hot(ca 30C)and wet so the rate of evaporation is high. Pork carcasses lose 0. 4% moisture between 0.5 and 1.0h post-mortem when held at approximately 15C (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 n a single stage system, pork at a nominal temperature of 4 C, air velocity of 0.4ms-,85-90% relative humidity (RH), lamb and beef at 0C, 0.5ms, 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 pork18-3.5% In a single stage chilling process, the factors in equation [ 5.1] that can be controlled by the refrigeration designer are Pa and m, since both are a func tion of air humidity and temperature Humidity is controlled by the tem- perature difference(AT) 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 AT, or a small area and a large AT 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 0C, 90% RH, Pm-Pa=0.054 bar, and at 70% rh, Pm -Pa= 0.055 bar. The initial effect of RH on weight loss is therefore small, but as cooling proceeds, Pn reduces and RH becomes increasingly important. Hodgson(1970)in South Africa showed that beef sides cooled for 20h in air at a temperature of 1.7C, and velocity of 0. 75ms lost 2.75% in weight at 90%RH, and 3.4% t 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 ATof 5'C. Since that time the price of beef has risen faster than the capital and the runnin costs of refrigeration equipment, and it is probable that the AT 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 Pm. Lower air temper atures should therefore reduce weight loss during chilling. Beef sides of average UK weight(140kg) lost 1. 2% in air at 4C, 0.5ms, 90%Rh and 0.2% less at 0.C0.5ms-l 90% RH when cooled to a maximum centre tem perature of 10C (Bailey and Cox, 1976). The initial weight was recorded ca. 2h 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

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 0.4 ms-1 , 85–90% relative humidity (RH), lamb and beef at 0 °C, 0.5 m s-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 Pa 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, Pm - Pa = 0.054 bar, and at 70% RH, Pm - Pa = 0.055 bar. The initial effect of RH on weight loss is therefore small, but as cooling proceeds, Pm 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 Pm. 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

Influence of refrigeration on evaporative weight loss from meat 89 Table 5.1 Percentage weight loss from thick samples of lean mutton cooled fror de in air at 1-2C. for a set time. at different air Air velocity(ms") Cooling time(h) 1.64 1.60 3.25 0.56 1.67 3.03 Table 5.2 Percentage weight loss from 15 x 15 2cm at 1-2C, to a set maximum temperature at differen air thick samples of lean mutton cooled from one side velocities Air velocity(ms Final temperature(°C 0.56 1.20 Source: Lovett et al 1976 decrease(Pm-Pa), so the overall effect is not obvious. The results of experi- ments carried out on samples(15 x 15 x 2cm 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 (10C in the deep leg), increasing air velocity from 0.5 to 1.0ms- reduced reight loss by 0. 15%(Cooper, 1970). When chilling for a set time(20h), ncreasing the air velocity from 0.75 to 3ms increased weight loss from 2.75to3.3%( 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 1C 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 ntroduced that involve an initial holding period at a high temperature, con sequently increasing weight loss

decrease (Pm - Pa), 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 ¥ 2 cm 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) 4 22 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 ¥ 2 cm 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