Thawing and tempering 167 tic collisions with surrounding molecules and this energy appears as heat Thus electromagnetic radiation may be used to heat foodstuffs. Three regions of the electromagnetic spectrum have been used for such heating: resistive 50Hz; radio frequency 3-300 GHz and microwave 900-3000GHz 8.3.2.1 Resistive thawing A frozen foodstuff can be heated by placing it between two electrodes and applying a low voltage at normal mains frequency. As the electric current flows through the material, it becomes warm(ohmic heating). Electrical contacts are required and product structure must be uniform and homoge eous, otherwise the path of least resistance will be taken by the current resulting in uneven temperatures and runaway heating Frozen meat at a low temperature does not readily conduct electricity, but as it becomes warmer, its electrical resistance falls, a larger current can flow and more heat is generated within the product. In practice, the system is only suitable for thin (5cm) homogeneous blocks such as catering blocks of liver since current flow is very small through thick blocks and inhomogeneities lead o runaway heating problems. 8.3.2.2 Radio frequency During radio frequency thawing, heat is produced in the frozen foodstuff because of dielectric losses when a product is subjected to an alternating electric field. In an idealised case of radio frequency heating the foodstuff, a regular slab of homogeneous material at a uniform temperature is placed between parallel electrodes and no heat is exchanged with its surroundings. When an alternating electro magnetic force is applied through the elec trodes the resulting field in the slab is uniform, so the energy and the resul tant temperature rise is identical in all parts of the food (Sanders, 1966) In practice this situation rarely applies. Foodstuffs are not generally the shape of perfect parallelepipeds, frozen meat consists of at least two components, fat and lean. During loading frozen meats pick up heat from the surroundings, the surface temperature rises and the dielectric system not presented with the uniform temperature distribution required for even heating. By using a conveyorised system to keep the product moving past the electrodes and/or surrounding the material by water, commercial systems ave been produced for blocks of oily fish and white fish(Jason and Sanders, 1962). Successful thawing of 13 cm thick meat blocks and 14cm thick offal blocks have also been reported (Sanders, 1961)but the tempera- ture range at the end of thawing(44 min) was stated to be -2-19C and -24"C, respectively, and the product may not have been fully thawed To overcome runaway heating with slabs of frozen pork bellies, workers Satchell and Doty, 1951) have tried coating the electrodes with lard,tic collisions with surrounding molecules and this energy appears as heat. Thus electromagnetic radiation may be used to heat foodstuffs. Three regions of the electromagnetic spectrum have been used for such heating: resistive 50 Hz; radio frequency 3–300 GHz and microwave 900–3000 GHz. 8.3.2.1 Resistive thawing A frozen foodstuff can be heated by placing it between two electrodes and applying a low voltage at normal mains frequency. As the electric current flows through the material, it becomes warm (ohmic heating). Electrical contacts are required and product structure must be uniform and homoge￾neous, otherwise the path of least resistance will be taken by the current, resulting in uneven temperatures and runaway heating. Frozen meat at a low temperature does not readily conduct electricity, but as it becomes warmer, its electrical resistance falls, a larger current can flow and more heat is generated within the product. In practice, the system is only suitable for thin (5 cm) homogeneous blocks such as catering blocks of liver since current flow is very small through thick blocks and inhomogeneities lead to runaway heating problems. 8.3.2.2 Radio frequency During radio frequency thawing, heat is produced in the frozen foodstuff because of dielectric losses when a product is subjected to an alternating electric field. In an idealised case of radio frequency heating the foodstuff, a regular slab of homogeneous material at a uniform temperature is placed between parallel electrodes and no heat is exchanged with its surroundings. When an alternating electro magnetic force is applied through the elec￾trodes the resulting field in the slab is uniform, so the energy and the resul￾tant temperature rise is identical in all parts of the food (Sanders, 1966). In practice this situation rarely applies. Foodstuffs are not generally in the shape of perfect parallelepipeds, frozen meat consists of at least two components, fat and lean. During loading frozen meats pick up heat from the surroundings, the surface temperature rises and the dielectric system is not presented with the uniform temperature distribution required for even heating. By using a conveyorised system to keep the product moving past the electrodes and/or surrounding the material by water, commercial systems have been produced for blocks of oily fish and white fish (Jason and Sanders, 1962). Successful thawing of 13cm thick meat blocks and 14cm thick offal blocks have also been reported (Sanders, 1961) but the tempera￾ture range at the end of thawing (44min) was stated to be -2–19 °C and -2–4 °C, respectively, and the product may not have been fully thawed. To overcome runaway heating with slabs of frozen pork bellies, workers (Satchell and Doty, 1951) have tried coating the electrodes with lard, Thawing and tempering 167