Freezing of meat 145 Table 7.6 Advantages and disadvantages of total loss refrigerants in comparison with mechanical refrigeration Advantages Disadvantages Low capital investment High operating cost Igh refrigerating High refrigerating capaci Low weight when out of use High weight at start of use No residual weight(dry ice) Limited duration without fillin No noise or temperature control Advantageous storage atmosphere(N2) Reduced humidi Bacteriostatic affect(CO2) Suffocation hazard Low maintenance requirements Limited availability Foolproof once installed (dry ice) Impingement technologies are being used to increase heat transfer further(Newman, 2001). Newman states that when comparing the overall heat transfer coefficients of cryogenic freezing tunnels, impingement heat transfer is typically 3-5 times that of a conventional tunnel utilizing axial flow fans With the increased overall heat transfer coefficient. one can either increase the freezing temperature to increase overall cryogen efficiency of continue to run at very cold temperatures and dramatically increase the overall production rate. Impingement freezing is best suited for products with high surface area to weight ratios, for example hamburger patties or products with one small dimension. Testing has shown that products with a thickness of less than 20mm freeze most effectively in an impingement heat transfer environment. When freezing products thicker than 20mm, the benefits of impingement freezing can still be achieved, however, the surface heat transfer coefficients later in the freezing process should be reduced to balance the overall process efficiency. The process is also very attractive for products that require very rapid surface freezing and chilling 7.5 Freezing of specific products 7.5.1 Meat block James et al.(1979) showed that air temperatures below -30C and air velocities exceeding 5ms are required to freeze 15cm thick meat blocks in corrugated cardboard cartons in less than 24h(Fig. 7.3).Creed and James (1981)carried out a survey which indicated that only 58% of industrial throughput is frozen in times within +% of the actual freezing time 7.5.2 Beef quarters James and Bailey(1987a)reported that brine spray and liquid nitrogen immersion systems had been used to freeze beef quarters. However, mostImpingement technologies are being used to increase heat transfer further (Newman, 2001). Newman states that when comparing the overall heat transfer coefficients of cryogenic freezing tunnels, impingement heat transfer is typically 3–5 times that of a conventional tunnel utilizing axial flow fans.With the increased overall heat transfer coefficient, one can either increase the freezing temperature to increase overall cryogen efficiency or continue to run at very cold temperatures and dramatically increase the overall production rate. Impingement freezing is best suited for products with high surface area to weight ratios, for example hamburger patties or products with one small dimension. Testing has shown that products with a thickness of less than 20 mm freeze most effectively in an impingement heat transfer environment. When freezing products thicker than 20mm, the benefits of impingement freezing can still be achieved, however, the surface heat transfer coefficients later in the freezing process should be reduced to balance the overall process efficiency. The process is also very attractive for products that require very rapid surface freezing and chilling. 7.5 Freezing of specific products 7.5.1 Meat blocks James et al. (1979) showed that air temperatures below -30 °C and air velocities exceeding 5 m s-1 are required to freeze 15 cm thick meat blocks in corrugated cardboard cartons in less than 24 h (Fig. 7.3). Creed and James (1981) carried out a survey which indicated that only 58% of industrial throughput is frozen in times within ±20% of the actual freezing time required. 7.5.2 Beef quarters James and Bailey (1987a) reported that brine spray and liquid nitrogen immersion systems had been used to freeze beef quarters. However, most Freezing of meat 145 Table 7.6 Advantages and disadvantages of total loss refrigerants in comparison with mechanical refrigeration Advantages Disadvantages Low capital investment High operating cost High refrigerating capacity High refrigerating capacity Low weight when out of use High weight at start of use No residual weight (dry ice) Limited duration without filling No noise Poor temperature control Advantageous storage atmosphere (N2) Reduced humidity Bacteriostatic affect (CO2) Suffocation hazard Low maintenance requirements Limited availability Foolproof once installed (dry ice)