168 Meat refrigeration placing the bellies in oil, water and saline baths and wrapping the meat in heesecloth soaked in saline solution. Only the last treatment was success- ful but even that was not deemed practical. 8.3.2.3 Microwave thawing Microwave thawing utilises electromagnetic waves directed at the product through waveguides without the use of conductors or electrodes. whilst he heating of frozen meat by microwave energy is potentially a very fast method of thawing, its application is constrained by thermal instability. At its worst, parts of the food may be cooked whilst the rest is substantially frozen. This arises because the absorption by frozen food of electro- magnetic radiation in this frequency range increases as the temperature rises, this dependence being especially large at about -5C, increasing as the initial freezing point is approached. If for any reason during irradiation a region of the material is slightly hotter than its surroundings, propor tionately more energy will be absorbed within that region and the original difference in enthalpy will be increased. As the enthalpy increases so the absorption increases and the unevenness of heating worsens at an ever- increasing rate. Below the initial freezing point the temperature increase is held in check by thermal inertia since for a given energy input the tem is continued after the hot spot has reached its initial freezing poin, th? perature rise is inversely proportional to the thermal capacity. If irradiation temperature rises at a catastrophic rate. A hybrid microwave/vacuum system, in which boiling surface water at a low temperature was used to cool the surface, thawed 15 cm thick cartoned meat in 1-2h without runaway heating, but problems of control and cost would appear to limit the commercial use (James, 1984). Despite a wide- pread belief to the contrary, microwave thawing systems have not been commercially successful. However, microwave tempering systems(see later) have found successful niche applications in the meat industry 8.3.3 Published thawing data for different meat cuts ders and carcasses, beef quarters and boned-out meat blode s, lamb shoul Process design data is available on thawing of frozen pork legs, lamb shoul 8.3.3.7 Thawing of pork legs/hams Bailey et al.(1974)made a comparative experimental study of thawing of frozen pork legs of different weights in air, water and vacuum heat thawing VHT) systems with respect to thawing time, weight loss and appearance A comprehensive chart(Fig. 8.2) was produced for the determination of thawing times over a range of process operating conditions(Bailey and James, 1974a) Thawing time increased almost linearly with leg weight for all systems hawing in water was faster than in air at any given temperature, butplacing the bellies in oil, water and saline baths and wrapping the meat in cheesecloth soaked in saline solution. Only the last treatment was success￾ful but even that was not deemed practical. 8.3.2.3 Microwave thawing Microwave thawing utilises electromagnetic waves directed at the product through waveguides without the use of conductors or electrodes. Whilst the heating of frozen meat by microwave energy is potentially a very fast method of thawing, its application is constrained by thermal instability. At its worst, parts of the food may be cooked whilst the rest is substantially frozen. This arises because the absorption by frozen food of electro￾magnetic radiation in this frequency range increases as the temperature rises, this dependence being especially large at about -5 °C, increasing as the initial freezing point is approached. If for any reason during irradiation a region of the material is slightly hotter than its surroundings, propor￾tionately more energy will be absorbed within that region and the original difference in enthalpy will be increased. As the enthalpy increases so the absorption increases and the unevenness of heating worsens at an ever￾increasing rate. Below the initial freezing point the temperature increase is held in check by thermal inertia since for a given energy input the tem￾perature rise is inversely proportional to the thermal capacity. If irradiation is continued after the hot spot has reached its initial freezing point, the temperature rises at a catastrophic rate. A hybrid microwave/vacuum system, in which boiling surface water at a low temperature was used to cool the surface, thawed 15cm thick cartoned meat in 1–2 h without runaway heating, but problems of control and cost would appear to limit the commercial use (James, 1984). Despite a wide￾spread belief to the contrary, microwave thawing systems have not been commercially successful. However, microwave tempering systems (see later) have found successful niche applications in the meat industry. 8.3.3 Published thawing data for different meat cuts Process design data is available on thawing of frozen pork legs, lamb shoul￾ders and carcasses, beef quarters and boned-out meat blocks. 8.3.3.1 Thawing of pork legs/hams Bailey et al. (1974) made a comparative experimental study of thawing of frozen pork legs of different weights in air, water and vacuum heat thawing (VHT) systems with respect to thawing time, weight loss and appearance. A comprehensive chart (Fig. 8.2) was produced for the determination of thawing times over a range of process operating conditions (Bailey and James, 1974a). Thawing time increased almost linearly with leg weight for all systems. Thawing in water was faster than in air at any given temperature, but 168 Meat refrigeration