232 Meat refrigeration can be important especially with transparent, controlled atmosphere packs During any control cycle, the cabinet temperature rises, heat enters the pack, the atmosphere inside the pack warms with consequent reduction in elative humidity and increase in the surface temperature of the product. As the surface temperature rises so does its saturation vapour pressure(a factor controlling evaporation) and more water evaporates into the sealed atmosphere of the pack. If the cabinet temperature was stabilised ther evaporation would continue until the atmosphere became saturated However, in practice the cabinet air temperature cycles and as it is reduced the wrapping film is cooled. If it reaches a temperature below the dew point of the atmosphere inside the pack, then water vapour will condense on the inner surface of the pack. This film of water can obscure the product and consequently reduce consumer appeal. As the cycling process continues the appearance of the product deteriorates. To maintain product temperatures close to 0C, the air off the coil must typically be -4'C and any ingress of humid air from within the store wil quickly cause the coil to ice up. Frequent defrosts are often required and even in a well maintained unit the cabinet temperature will then rise to 0-12C and the temperature of the product will rise by at least 3C (Brolls 1986). External factors such as the store ambient temperature, the siting of he cabinet and poor pretreatment and placement of products substantially affect cabinet performance. Warm and humid ambient air and loading with insufficiently cooled products can also overload the refrigeration system Even if the food is at its correct temperature, uneven loading or too much product can disturb the air flow patterns and destroy the insulating layer of cooled air surrounding the product. An in-store survey of 299 prepackaged meat products in chilled retail displays found product temperatures in the range-8.0-14.0° C with a mean of 5.3°Cand18% above9°C(Rose,1986) Other surveys(Bogh-Sprensen, 1980: Malton, 1971)have shown that tem- peratures of packs from the top of stacks were appreciably higher than hose from below owing to radiant heat pick up from store and cabinet ghting. It has also been stated that products in transparent film over wrapped packs can achieve temperatures above that of the surrounding refrigerated air owing to radiant heat trapped in the package by the green house'effect. However, specific investigations failed to demonstrate this effect(Gill, 1988) 11.1.1 Factors affecting display life The display life of wrapped meat can be affected by the diet and the treatment of the meat before display During display 4C TBARS values were lower and Huntera' values higher in pork chops from pigs fed with a high 100-200mg a-tocopherol acetate per kilogram diet than those fed with 10mgkg(Monahan et al., 1994). Lipid oxidation and colour deterioration were also faster during display of chops that hadcan be important especially with transparent, controlled atmosphere packs. During any control cycle, the cabinet temperature rises, heat enters the pack, the atmosphere inside the pack warms with consequent reduction in relative humidity and increase in the surface temperature of the product. As the surface temperature rises so does its saturation vapour pressure (a factor controlling evaporation) and more water evaporates into the sealed atmosphere of the pack. If the cabinet temperature was stabilised then evaporation would continue until the atmosphere became saturated. However, in practice the cabinet air temperature cycles and as it is reduced the wrapping film is cooled. If it reaches a temperature below the dew point of the atmosphere inside the pack, then water vapour will condense on the inner surface of the pack. This film of water can obscure the product and consequently reduce consumer appeal. As the cycling process continues the appearance of the product deteriorates. To maintain product temperatures close to 0°C, the air off the coil must typically be -4 °C and any ingress of humid air from within the store will quickly cause the coil to ice up. Frequent defrosts are often required and even in a well maintained unit the cabinet temperature will then rise to 10–12 °C and the temperature of the product will rise by at least 3 °C (Brolls, 1986). External factors such as the store ambient temperature, the siting of the cabinet and poor pretreatment and placement of products substantially affect cabinet performance. Warm and humid ambient air and loading with insufficiently cooled products can also overload the refrigeration system. Even if the food is at its correct temperature, uneven loading or too much product can disturb the air flow patterns and destroy the insulating layer of cooled air surrounding the product. An in-store survey of 299 prepackaged meat products in chilled retail displays found product temperatures in the range -8.0–14.0 °C, with a mean of 5.3°C and 18% above 9°C (Rose, 1986). Other surveys (Bøgh-Sørensen, 1980; Malton, 1971) have shown that tem￾peratures of packs from the top of stacks were appreciably higher than those from below owing to radiant heat pick up from store and cabinet lighting. It has also been stated that products in transparent film over￾wrapped packs can achieve temperatures above that of the surrounding refrigerated air owing to radiant heat trapped in the package by the ‘green￾house’ effect. However, specific investigations failed to demonstrate this effect (Gill, 1988). 11.1.1 Factors affecting display life The display life of wrapped meat can be affected by the diet of the animal and the treatment of the meat before display. During display for 8 days at 4 °C TBARS values were lower and Hunter ‘a’ values higher in pork chops from pigs fed with a high 100–200 mg a-tocopherol acetate per kilogram diet than those fed with 10 mg kg-1 (Monahan et al., 1994). Lipid oxidation and colour deterioration were also faster during display of chops that had 232 Meat refrigeration