90 Meat refrigeration The same restrictions do not apply to pork since the pr lating fat layers and the more rapid rate of glycolysis minimises the likeli- hood of toughening. Harsher pork chilling treatments are quite com non and in Denmark(Hermanson, personal communication) a two-stage system has been used in which the carcass is conveyed for 80 min through a tunnel, operating at-15oC, 3ms, then equalised for 12h in a chill room at 4C, 0.5ms with a very high RH. After the first stage the surface tem perature of the carcass is below 0C and moisture therefore condenses onto it in the initial part of the second stage. The average weight loss from Okg carcasses in such systems is claimed to be as low as 0.8% over the 14h period Work at the mri produced a single stage 3 h system for 70kg pork car casses using air at-30.C, 1ms James et al, 1983; Gigiel and James, 1983) After chilling the average meat temperature is 0C 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 Commercial trials of a similar system for beef sides using electrical stimu- lation to minimise cold shortening, then air at -15C, 3ms- 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, 4-8h in air at -10 to -15C, 1-2ms- followed by 6-8h at -1C and a moderate air velocity. Special jets vere used to increase the air velocity over the thickest sections of the side luring 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 etween 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 0C, 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 ms 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 fasterThe 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, 3 m s-1 , then equalised for 12h in a chill room at 4 °C, 0.5 m s-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, 1 m s-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, 3 m s-1 for 6 h, 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, 1–2 m s-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