140 Meat refrigeration Table 7.3 Drip loss(%)from 776g samples of longissimus lumborum et thoracis frozen under different methods and thawed at 4 c eez Freezing time Freezing rate Storage tim (cmh-) at-20°C 48h 2.5 months Cryogenic,-90°C 3.342 Cryogenic,-65°C 4.70Pb 972 Blast freezer.-21°C 1274 Walk-in-freezer. -21C 1.88h Domestic freezer. -25C 0.5 1172 Means in the same column with different superscripts are different at P>0.05h difference could be seen in cooking losses, even after storage for 2 months. Ice crystals were significantly larger in hamburgers frozen in SF compared to LN and IF. Sensory analysis revealed no difference in eating quality between the three freezing methods, even after storage for 2 months. Slow freezing from a high initial temperature can provide conditions for microbial growth compared with a very rapid freezing process. Castell- Perez et al.(1989) predicted that slow freezing from an initial product temperature of 30C could result in an 83% increase in bacterial numbers compared with a 4% increase from 10%C. 7.2 Freezing systems 7.2.1 Air Air is by far the most widely used method of freezing food as it is eco- nomical, hygienic and relatively non-corrosive to equipment Systems range from the most basic, in which a fan draws air through a refrigerated coil and blows the cooled air around an insulated room(Fig. 7.1), to purpose- built conveyerised blast freezing tunnels or spirals. Relatively low rates of heat transfer are attained from product surfaces in air systems. The big advantage of air systems is their versatility, especially when there is a requirement to freeze a variety of irregularly shaped products or individ ual products. In practice, air distribution is a major problem, often overlooked by the stem designer and the operator. The freezing time of the product is re- duced as the air speed is increased. An optimum value exists between the decrease in freezing time and the increasing power required to drive the fans to produce higher air speeds. This optimum value can be as low as 1.0ms" air speed when freezing beef quarters rising to 15ms or more for thin products.difference could be seen in cooking losses, even after storage for 2 months. Ice crystals were significantly larger in hamburgers frozen in SF compared to LN and IF. Sensory analysis revealed no difference in eating quality between the three freezing methods, even after storage for 2 months. Slow freezing from a high initial temperature can provide conditions for microbial growth compared with a very rapid freezing process. Castell￾Perez et al. (1989) predicted that slow freezing from an initial product temperature of 30°C could result in an 83% increase in bacterial numbers compared with a 4% increase from 10 °C. 7.2 Freezing systems 7.2.1 Air Air is by far the most widely used method of freezing food as it is eco￾nomical, hygienic and relatively non-corrosive to equipment. Systems range from the most basic, in which a fan draws air through a refrigerated coil and blows the cooled air around an insulated room (Fig. 7.1), to purpose￾built conveyerised blast freezing tunnels or spirals. Relatively low rates of heat transfer are attained from product surfaces in air systems. The big advantage of air systems is their versatility, especially when there is a requirement to freeze a variety of irregularly shaped products or individ￾ual products. In practice, air distribution is a major problem, often overlooked by the system designer and the operator. The freezing time of the product is re￾duced as the air speed is increased. An optimum value exists between the decrease in freezing time and the increasing power required to drive the fans to produce higher air speeds. This optimum value can be as low as 1.0 m s-1 air speed when freezing beef quarters rising to 15 m s-1 or more for thin products. 140 Meat refrigeration Table 7.3 Drip loss (%) from 77.6 g samples of longissimus lumborum et thoracis frozen under different methods and thawed at 4 °C Freezing conditions Freezing time Freezing rate Storage time to -2.2 °C (cm h-1 ) at -20 °C 48 h 2.5 months Cryogenic, -90 °C 15 min 6.4 3.34a 9.49a Cryogenic, -65 °C 22 min 4.4 4.70ab 9.72a Blast freezer, -21 °C 1.83 h 0.55 5.53b 12.74b Walk-in-freezer, -21 °C 1.88 h 0.53 4.71ab 13.18b Domestic freezer, -25 °C 1.96 h 0.51 5.26b 11.72b Means in the same column with different superscripts are different at P > 0.05 h Source: Sacks et al., 1993