Thawing and tempering 163 thawing is unlikely, while on commercial blocks it is probable. Hence, the laboratory count reflects the number of microbes on the frozen meat but not necessarily on meat after commercial thawing Microbial counts incubated at 1C and 20-25C assess the storage lif of meat at chill and intermediate temperatures. Counts at 37C give an indi- cation of contamination from human and animal sources. Thawing under conditions that permit growth of bacteria counted at 1C and 25C will result in meat of poorer quality in terms of storage life. Thawing conditions food-poisoning bacteria(such as Salmonella spp. may be capable ce allowing heavy growth of bacteria counted at 37C are undesirable since growth e. The appearance of the surface of thawed meat is similarly related to the he spent in a given environment. Since this time will be a function of the material thickness, it is not possible to define one overall set of conditions for optimal appearance. For example, the air temperature, velocity and rela- tive humidity required to thaw small joints satisfactorily in a reasonably short time would almost certainly cause problems if used to thaw whole quarters of beef. In general both the appearance and final bacterial condi tion in air thawing systems improve as the temperature of the thawing medium falls, but the extended thawing times involved may be unaccept- able for other reasons related to operating requirements. A compromise must therefore be reached which for a given material could well differ from one factory to the next 8.3 Thawing systems There is no simple guide to the choice of an optimum thawing system(Table 8.2). A thawing system should be considered as one operation in the pro- duction chain. It receives frozen material which should be within a known temperature range and of specified microbiological condition. It is expected to deliver that same material in a given time in a totally thawed state. The reight loss and increase in bacterial numbers during thawing should be within acceptable limits, which will vary from process to process. In some circumstances, for example a direct sale to the consumer, the appearance of the thawed product is crucial, in others it may be irrelevant. Apart fre hese factors the economics and overall practicality of the thawing opera tion, including the capital and running costs of the plant, the labour require- nents, ease of cleaning and the flexibility of the plant to handle different products, must be considered 8.3.1 Conduction The main conduction-based thawing methods rely on air, water or steam condensation under vacuumthawing is unlikely, while on commercial blocks it is probable. Hence, the laboratory count reflects the number of microbes on the frozen meat but not necessarily on meat after commercial thawing. Microbial counts incubated at 1 °C and 20–25 °C assess the storage life of meat at chill and intermediate temperatures. Counts at 37 °C give an indi￾cation of contamination from human and animal sources. Thawing under conditions that permit growth of bacteria counted at 1°C and 25 °C will result in meat of poorer quality in terms of storage life. Thawing conditions allowing heavy growth of bacteria counted at 37 °C are undesirable since food-poisoning bacteria (such as Salmonella spp.) may be capable of growth. The appearance of the surface of thawed meat is similarly related to the time spent in a given environment. Since this time will be a function of the material thickness, it is not possible to define one overall set of conditions for optimal appearance. For example, the air temperature, velocity and rela￾tive humidity required to thaw small joints satisfactorily in a reasonably short time would almost certainly cause problems if used to thaw whole quarters of beef. In general both the appearance and final bacterial condi￾tion in air thawing systems improve as the temperature of the thawing medium falls, but the extended thawing times involved may be unaccept￾able for other reasons related to operating requirements. A compromise must therefore be reached which for a given material could well differ from one factory to the next. 8.3 Thawing systems There is no simple guide to the choice of an optimum thawing system (Table 8.2). A thawing system should be considered as one operation in the pro￾duction chain. It receives frozen material which should be within a known temperature range and of specified microbiological condition. It is expected to deliver that same material in a given time in a totally thawed state. The weight loss and increase in bacterial numbers during thawing should be within acceptable limits, which will vary from process to process. In some circumstances, for example a direct sale to the consumer, the appearance of the thawed product is crucial, in others it may be irrelevant. Apart from these factors the economics and overall practicality of the thawing opera￾tion, including the capital and running costs of the plant, the labour require￾ments, ease of cleaning and the flexibility of the plant to handle different products, must be considered. 8.3.1 Conduction The main conduction-based thawing methods rely on air, water or steam condensation under vacuum. Thawing and tempering 163