160 Meat refrigeration system for industrial use a balance must be struck between thawing time, appearance and bacteriological condition of product, processing problems such as effluent disposal and the capital and operating costs of the respec- tive systems. Of these factors, thawing time is the principal criterion that governs selection of the system. Appearance, bacteriological condition and weight loss are important if the material is to be sold in the thawed cond tion but are less so if the meat is for processing 8.1 Considerations The design of any thawing system requires knowledge of the particular environmental or process conditions necessary to achieve a given thawing time, and the effect of these conditions on factors such as drip, evaporative losses, appearance and bacteriological quality The process of freezing a high water content material such as meat takes place over a range of temperatures rather than at an exact point, because as freezing proceeds the concentration of solutes in the meat fluid steadily increases and progressively lowers the freezing temperature. Thawing simply reverses this process. Thawing time depends on factors relating to the product and the envi onmental conditions that include dimensions and shape of the product, particularly the thickness change in enthalpy thermal conductivity of the product initial and final temperatures surface heat transfer coefficient temperature of the thawing medium. equal to the enthalpy change between the initial temperature and ns The total amount of energy that must be introduced into the product average temperature required within the material after thawing. For the thawing process to be complete, no ice should remain and the minimum emperature has to be above -1C To thaw lkg of meat from a starting temperature of -40C would require the addition of 300kJ of energy if the meat was very lean, falling to about 180kJ if very fat. Frozen meat that requires boning has to be completely thawed. However, an increasing pro- portion of meat is boned before freezing and if it is subsequently used in products such as pies, sausages, and so on, it can be cut by machine in a semi-frozen( tempered) state To temper meat from -40C to an average temperature of-4C requires a heat input of approximately 100kJkg-, only one third of that required for complete thawing Thermal conductivity has an important effect on thawing The conduc t ty of frozen lean meat is three times that of the thawed material. When wing commences, the surface rises above the initial freezing point. Sub-system for industrial use a balance must be struck between thawing time, appearance and bacteriological condition of product, processing problems such as effluent disposal and the capital and operating costs of the respec￾tive systems. Of these factors, thawing time is the principal criterion that governs selection of the system. Appearance, bacteriological condition and weight loss are important if the material is to be sold in the thawed condi￾tion but are less so if the meat is for processing. 8.1 Considerations The design of any thawing system requires knowledge of the particular environmental or process conditions necessary to achieve a given thawing time, and the effect of these conditions on factors such as drip, evaporative losses, appearance and bacteriological quality. The process of freezing a high water content material such as meat takes place over a range of temperatures rather than at an exact point, because as freezing proceeds the concentration of solutes in the meat fluid steadily increases and progressively lowers the freezing temperature. Thawing simply reverses this process. Thawing time depends on factors relating to the product and the envi￾ronmental conditions, that include: • dimensions and shape of the product, particularly the thickness • change in enthalpy • thermal conductivity of the product • initial and final temperatures • surface heat transfer coefficient • temperature of the thawing medium. The total amount of energy that must be introduced into the product is equal to the enthalpy change between the initial temperature and the average temperature required within the material after thawing. For the thawing process to be complete, no ice should remain and the minimum temperature has to be above -1 °C. To thaw 1kg of meat from a starting temperature of -40 °C would require the addition of 300 kJ of energy if the meat was very lean, falling to about 180 kJ if very fat. Frozen meat that requires boning has to be completely thawed. However, an increasing pro￾portion of meat is boned before freezing and if it is subsequently used in products such as pies, sausages, and so on, it can be cut by machine in a semi-frozen (tempered) state. To temper meat from -40 °C to an average temperature of -4 °C requires a heat input of approximately 100 kJkg-1 , only one third of that required for complete thawing. Thermal conductivity has an important effect on thawing. The conduc￾tivity of frozen lean meat is three times that of the thawed material. When thawing commences, the surface rises above the initial freezing point. Sub- 160 Meat refrigeration