Thawing and tempering 169 Temperature of thawing medium 40℃c30°c20℃10° bE365885 6 kg leg.-30°to0°c of-10C under the fastest and slowest thawing conditions) Thawing time(h) A= Vacuum steam heat b= Water, 0.023ms C= Water 0. 006ms Fig 8.2 Prediction of thawing times of frozen pork legs from-30-0C(source Bailey and James, 1974a) Table 8.3 Mean percentage weight losses(fresh to thawed states)for pork legs thawed in air. water or vacuum Thawing Velocity of Thawing temperature (C) medium(ms) 10 20 25 0.9(18) +1.0(27)+11(20 0.9(18) 1.0(14) 1.0(15) +1.0(8) +1.1(10) +12(8) 0023 +12(8) +11(13) +0.7(6) VHT +13(14) 0.2(14) 0.6(13) O Number of samples. Source: Bailey and James, 1974a. increasing the water velocity had very little additional effect. VHT was not appreciably faster than water thawing at any temperature, demonstrating that for materials of this thickness conductivity is the rate controlling factor The pork legs increased in weight by 1 +0. 3% under any of the condi- tions of thawing in air or water, with the exception of high velocity air where losses of 1% were recorded (Table 8.3). A similar increase was recorded in VHT at 10C but there were small losses of weight at 20 and 30C
increasing the water velocity had very little additional effect. VHT was not appreciably faster than water thawing at any temperature, demonstrating that for materials of this thickness conductivity is the rate controlling factor. The pork legs increased in weight by 1 ±0. 3% under any of the conditions of thawing in air or water, with the exception of high velocity air where losses of l% were recorded (Table 8.3). A similar increase was recorded in VHT at 10 °C but there were small losses of weight at 20 and 30 °C. Thawing and tempering 169 5000 4000 3000 2000 1000 500 400 300 200 100 40 30 20 10 50 A B C D E 0 10 20 30 40 50 Thawing time (h) A = Vacuum steam heat B = Water, 0.023ms –1 C = Water, 0.006ms –1 D = Air, 5ms –1 E = Air, 0.25ms–1 3 kg leg. –30 ° to 0 °C 6 kg leg. –30 ° to 0 °C Thawing from –10 ° to 0 °C (Shaded area shows reduction in thawing time for initial temperature of –10 °C under the fastest and slowest thawing conditions) 40 °C 30 °C 20 °C10 °C 5 °C Temperature of thawing medium Surface film heat transfer coefficient (W m–2 °C–1) Fig. 8.2 Prediction of thawing times of frozen pork legs from -30–0 °C (source: Bailey and James, 1974a). Table 8.3 Mean percentage weight losses (fresh to thawed states) for pork legs thawed in air, water or vacuum Thawing Velocity of Thawing temperature (°C) medium medium (m s-1 ) 10 20 30 Air 0.25 +0.9 (18) +1.0 (27) +1.1 (20) 5.5 +0.9 (18) -1.0 (14) -1.0 (15) Water 0.006 +1.0 (8) +1.1 (10) +1.2 (8) 0.023 +1.2 (8) +1.1 (13) +0.7 (6) VHT +1.3 (14) -0.2 (14) -0.6 (13) () Number of samples. Source: Bailey and James, 1974a
170 Meat refrigeration Table 8.4 Bacterial changes in pork legs thawed in air, water or VhT awing Mean bacterial count (log1ocm-) medium at incubation tempe mediummedium samples freezing) thawing) 37 1°37°C25°C1 0.25 4.45.04.74.34.65.2 200.25 9495.1 .65.2 0.25 3.44.21.14.65.03.9 3.54.12.54.2444.6 Water 0.006 5.175 4.5)(5 0.023 105.777-5.07.0一 (56) 0.006 104.874 0.023 105.16.6 0.006 0.023 104.86.9 (61)(6.6) 666 3.74.84.83.34.15.0 4.3513.6586.26.0 3.54.83.14.8523.4 Source: Bailey et aL. 1974 The surface of legs thawed in air at 10C, 85% RH at low velocity was moist but not wet and the colour of both skin and cut surface was good At the appearance was less attractive. The skin thawed at high air velocity was light brown in colour and rather dry and parchment like. This condition did not improve with storage. Legs from the water and vhT systems were very wet and the colour of the cut surface extremely pale. However, considerable improvements in the condition of the surface and in the colour of legs thawed at 10"C were noted after holding for some time in a chill room. Changes in bacterial numbers during thawing in air or water could not be related solely to the temperature of the thawing medium or its velocit n both cases, the interactions of thawing medium and its velocity were significant(Table 8.4 ) Air at 10.C/0.25 ms- gave the best result and both 10C/5.5ms and 20"C/.25 ms were satisfactory. In water, decreases in bacterial counts were obtained at 10.C/0.023 ms-I and small increases at 10%C/0.006ms-. In vhT. the difference between initial and final bacterial numbers increased with increased thawing temperature
The surface of legs thawed in air at 10°C, 85% RH at low velocity was moist but not wet and the colour of both skin and cut surface was good. At higher temperatures, the appearance was less attractive. The skin of legs thawed at high air velocity was light brown in colour and rather dry and parchment like. This condition did not improve with storage. Legs from the water and VHT systems were very wet and the colour of the cut surface was extremely pale. However, considerable improvements in the condition of the surface and in the colour of legs thawed at 10 °C were noted after holding for some time in a chill room. Changes in bacterial numbers during thawing in air or water could not be related solely to the temperature of the thawing medium or its velocity; in both cases, the interactions of thawing medium and its velocity were significant (Table 8.4). Air at 10°C/0.25 m s-1 gave the best result and both 10 °C/5.5 m s-1 and 20 °C/0.25 m s-1 were satisfactory. In water, decreases in bacterial counts were obtained at 10°C/0.023 m s-1 and small increases at 10 °C/0.006 m s-1 . In VHT, the difference between initial and final bacterial numbers increased with increased thawing temperature. 170 Meat refrigeration Table 8.4 Bacterial changes in pork legs thawed in air, water or VHT Thawing Temp. Velocity Number Mean bacterial count (log10 cm-2 ) medium of of of at incubation temperature medium medium samples (°C) (m s Initial (before Final (after -1 ) freezing) thawing) 37 °C 25 °C 1 °C 37 °C 25 °C 1 °C Air 10 0.25 6 4.4 5.0 4.7 4.3 4.6 5.2 5.5 6 4.1 4.9 5.0 4.6 5.5 5.6 20 0.25 6 4.2 4.9 4.9 5.1 5.4 5.1 5.5 6 2.7 3.3 1.6 5.2 6.3 6.1 30 0.25 6 3.4 4.2 1.1 4.6 5.0 3.9 5.5 6 3.5 4.1 2.5 4.2 4.4 4.6 Water 10 0.006 10 4.8 7.0 – 5.1 7.5 – (4.5) (5.6) 0.023 10 5.7 7.7 – 5.0 7.0 – (4.4) (5.6) 20 0.006 10 4.8 7.4 – 6.0 6.9 – (6.3) (7.1) 0.023 10 5.1 6.6 – 6.1 6.7 – (6.0) (6.2) 30 0.006 10 5.2 6.9 – 6.6 7.2 – (6.6) (7.7) 0.023 10 4.8 6.9 – 7.0 7.4 – (6.1) (6.6) VHT 10 – 6 3.7 4.8 4.8 3.3 4.1 5.0 20 – 6 4.3 5.1 3.6 5.8 6.2 6.0 30 – 6 3.5 4.8 3.1 4.8 5.2 3.4 Source: Bailey et al., 1974