162 Meat refrigeration (Cutting, 1974: Love, 1966) concluded that fast thawing rates would produce increased drip, while others showed(Finn, 1932; Singh and Essary, 1971) the opposite. Thawing times from -7 to 0C of less than 1 min or greater han 2000 min led to increased drip loss (James et al., 1983). The results are therefore conflicting and provide no useful design data for optimising a thawing system. The principle criteria governing quality of thawed meat are the appear- ance and bacteriological condition. These are major factors if the product is to be sold thawed but are less important if the food is destined for pro- cessing and heat treatment Microbiological problems can arise during thawing of food in bulk. while centre temperatures may not exceed0oC, the exterior surface may be held at 10-15C for many hours, or even days. During this time extensive growth of spoilage organisms can occur on the surface. The time required for micro- biological numbers to reach'spoilage' levels will largely be dependent upon he number of microbes initially present and the temperature. Since freez ing and frozen storage have little effect on the number of viable microbes present, material of poor microbiological quality before freezing is likely to spoil more quickly during thawing(Roberts, 1974). The use of high thawing (10C) temperatures for carcass meats tends to lead to large increases in microbial numbers(James and Creed, 1980; Bailey et al., 1974) Little published data exist on microbiological effects of thawing meat. Buttiaux(1972)reported that water thawing was more successful for beef than for pork if the meat was to be stored. Consequently, care must be exer cised in extrapolating from one meat species to another. Results with po suggested that air thawing gives final counts about ten times higher tha thawing in 3% brine, whereas with beef Heinz(1970) reported counts lower by a factor of about 10 for air(4-5ms": 10C)as opposed to flowing water (10%C). Kassai (1969)also found no significant increase in bacteriological numbers when thawing beef carcasses in air(0. 2-0.3ms", 15-20C,96% elative humidity, (RH)). Shoulders of lamb( Vanichseni et al., 1972)thawed in air(0.2ms: 18C)or water(45C) had bacterial counts that increased respectively by factors of 1.74 and 1. 12: humidity and air velocity also influ enced the results of air thawing It is often asserted that thawed food is more perishable than fresh or chilled produce, but experiments(Kitchell and Ingram, 1956; Kitchell and Ingram, 1959) have failed to demonstrate any difference of practical significance between the growth of meat spoilage organisms on fresh or thawed slices of meat. Greer and Murray(1991) found that the lag phase of bacterial growth was shorter in frozen/thawed pork than in fresh pork while the generation time was unaffected. Under commercial conditions, microbiological sampling of frozen meat may be of limited relevance. Small frozen samples will be thawed in a la boratory, probably under conditions unlike those used later to thaw whole blocks On the laboratory samples, extensive microbial growth during(Cutting, 1974; Love, 1966) concluded that fast thawing rates would produce increased drip, while others showed (Finn, 1932; Singh and Essary, 1971) the opposite. Thawing times from -7 to 0°C of less than 1min or greater than 2000 min led to increased drip loss (James et al., 1983). The results are therefore conflicting and provide no useful design data for optimising a thawing system. The principle criteria governing quality of thawed meat are the appear￾ance and bacteriological condition. These are major factors if the product is to be sold thawed but are less important if the food is destined for pro￾cessing and heat treatment. Microbiological problems can arise during thawing of food in bulk.While centre temperatures may not exceed 0 °C, the exterior surface may be held at 10–15 °C for many hours, or even days. During this time extensive growth of spoilage organisms can occur on the surface.The time required for micro￾biological numbers to reach ‘spoilage’ levels will largely be dependent upon the number of microbes initially present and the temperature. Since freez￾ing and frozen storage have little effect on the number of viable microbes present, material of poor microbiological quality before freezing is likely to spoil more quickly during thawing (Roberts, 1974). The use of high thawing (>10 °C) temperatures for carcass meats tends to lead to large increases in microbial numbers (James and Creed, 1980; Bailey et al., 1974). Little published data exist on microbiological effects of thawing meat. Buttiaux (1972) reported that water thawing was more successful for beef than for pork if the meat was to be stored. Consequently, care must be exer￾cised in extrapolating from one meat species to another. Results with pork suggested that air thawing gives final counts about ten times higher than thawing in 3% brine, whereas with beef Heinz (1970) reported counts lower by a factor of about 10 for air (4–5ms-1 ; 10 °C) as opposed to flowing water (10 °C). Kassai (1969) also found no significant increase in bacteriological numbers when thawing beef carcasses in air (0.2–0.3m s-1 , 15–20 °C, 96% relative humidity, (RH)). Shoulders of lamb (Vanichseni et al., 1972) thawed in air (0.2 m s-1 ; 18 °C) or water (45°C) had bacterial counts that increased respectively by factors of 1.74 and 1.12; humidity and air velocity also influ￾enced the results of air thawing. It is often asserted that thawed food is more perishable than fresh or chilled produce, but experiments (Kitchell and Ingram, 1956; Kitchell and Ingram, 1959) have failed to demonstrate any difference of practical significance between the growth of meat spoilage organisms on fresh or thawed slices of meat. Greer and Murray (1991) found that the lag phase of bacterial growth was shorter in frozen/thawed pork than in fresh pork, while the generation time was unaffected. Under commercial conditions, microbiological sampling of frozen meat may be of limited relevance. Small frozen samples will be thawed in a la￾boratory, probably under conditions unlike those used later to thaw whole blocks. On the laboratory samples, extensive microbial growth during 162 Meat refrigeration