166 Meat refrigeration Meat racks in working section Water Water sump Vacuum pump Fig.8.1 APV-Torry vacuum thawing plant(source: Bailey and James, 1974b) 8.3.1.3 Vacuum-heat thawing A vacuum-heat thawing(VHT) system(Fig 8.1)operates by transferring he heat of condensing steam under vacuum to the frozen product. Theo- retically, a condensing vapour in the presence of a minimum amount of a non-condensable gas can achieve a surface film heat transfer coefficient far higher than that achieved in water thawing. The principle of operation is hat when steam is generated under vacuum, the vapour temperature will correspond to its equivalent vapour pressure. For example, if the vapour pressure is maintained at 1106Nm-2, steam will be generated at 15.C.The steam will condense onto any cooler surface such as a frozen product. The benefits of latent heat transfer can be obtained without the problems of cooking which would occur at atmospheric pressure With thin materials, thawing cycles are very rapid, enabling high daily throughputs to be achieved. The advantage of a high h value becomes less marked as material thickness increases and beef quarters or 25 kg meat blocks require thawing times permitting no more than one cycle per day Under these conditions, the economics of the system and the largest capac ity unit available(10-12 tonnes)severely restrict its application 8.3.2 Electrical methods In all of the methods described above, the rate of thawing is a function of he transfer of heat from the thawing medium to the surface of the meat and the conduction of this heat into the centre of the carcass or cut. In heory, electrical systems should overcome these problems because heat is generated within the material and the limitations of thermal conductivity are circumvented. In such systems the kinetic energy imparted to molecules by the action of an oscillating electromagnetic field is dissipated by inelas8.3.1.3 Vacuum-heat thawing A vacuum-heat thawing (VHT) system (Fig. 8.1) operates by transferring the heat of condensing steam under vacuum to the frozen product. Theo￾retically, a condensing vapour in the presence of a minimum amount of a non-condensable gas can achieve a surface film heat transfer coefficient far higher than that achieved in water thawing. The principle of operation is that when steam is generated under vacuum, the vapour temperature will correspond to its equivalent vapour pressure. For example, if the vapour pressure is maintained at 1106 N m-2 , steam will be generated at 15 °C. The steam will condense onto any cooler surface such as a frozen product. The benefits of latent heat transfer can be obtained without the problems of cooking which would occur at atmospheric pressure. With thin materials, thawing cycles are very rapid, enabling high daily throughputs to be achieved. The advantage of a high h value becomes less marked as material thickness increases and beef quarters or 25 kg meat blocks require thawing times permitting no more than one cycle per day. Under these conditions, the economics of the system and the largest capac￾ity unit available (10–12 tonnes) severely restrict its application. 8.3.2 Electrical methods In all of the methods described above, the rate of thawing is a function of the transfer of heat from the thawing medium to the surface of the meat and the conduction of this heat into the centre of the carcass or cut. In theory, electrical systems should overcome these problems because heat is generated within the material and the limitations of thermal conductivity are circumvented. In such systems the kinetic energy imparted to molecules by the action of an oscillating electromagnetic field is dissipated by inelas- 166 Meat refrigeration ~~ ~ ~ ~~ ~ ~~~ ~~~~ ~~~~~~ ~~ ~~~ ~~~~~~~~ Meat racks in working section Water sump Vacuum pump Steam Water Air In-place cleaning Fig. 8.1 APV-Torry vacuum thawing plant (source: Bailey and James, 1974b)