Freezing of meat 141 Fig. 7.1. Example of a freezing tunnel with longitudinal air circulation. The use of impingement technology to increase the surface heat trans fer in air and other freezing systems has received attention recently (Newman, 2001; Sundsten et al., 2001; Everington, 2001). Impingement is the process of directing a jet or jets of fluid at a solid surface to effect a change. When the jets of fluid are very cold gas, the change is a dramatic ncrease in convective surface heat transfer coefficients. The very high velocity(20-30ms-)impingement gas jets, "breakup' the static surface boundary layer of gas that surrounds a food product. The resulting medium around the product is more turbulent and the heat exchange through this zone becomes much more effective 7.2.1.1 Batch systems Placing food items in large refrigerated rooms is the most common method of freezing. Fans circulate air through refrigerated coils and around the products in an insulated room. Large individual items such as meat car casses are hung from overhead rails, smaller products are placed either unwrapped or in cartons on racks, pallets, or large bins. 7.2.1.2 Continuous systems In a continuous system, meat is conveyed through a freezing tunnel or refrigerated room usually by an overhead conveyor or on a belt. This over omes the problem of uneven air distribution since each item is subjected to the same velocity/time profile. Some meat products are frozen on racks of trays(2m high), pulled or pushed through a freezing tunnel by me- chanical means. For larger operations, it is more satisfactory to use feed meat on a continuous belt through linear tunnels or spiral freezers. LinearThe use of impingement technology to increase the surface heat trans￾fer in air and other freezing systems has received attention recently (Newman, 2001; Sundsten et al., 2001; Everington, 2001). Impingement is the process of directing a jet or jets of fluid at a solid surface to effect a change. When the jets of fluid are very cold gas, the change is a dramatic increase in convective surface heat transfer coefficients. The very high velocity (20–30 m s-1 ) impingement gas jets, ‘breakup’ the static surface boundary layer of gas that surrounds a food product. The resulting medium around the product is more turbulent and the heat exchange through this zone becomes much more effective. 7.2.1.1 Batch systems Placing food items in large refrigerated rooms is the most common method of freezing. Fans circulate air through refrigerated coils and around the products in an insulated room. Large individual items such as meat car￾casses are hung from overhead rails, smaller products are placed either unwrapped or in cartons on racks, pallets, or large bins. 7.2.1.2 Continuous systems In a continuous system, meat is conveyed through a freezing tunnel or refrigerated room usually by an overhead conveyor or on a belt. This over￾comes the problem of uneven air distribution since each item is subjected to the same velocity/time profile. Some meat products are frozen on racks of trays (2 m high), pulled or pushed through a freezing tunnel by me￾chanical means. For larger operations, it is more satisfactory to use feed meat on a continuous belt through linear tunnels or spiral freezers. Linear Freezing of meat 141 Evaporator Reversible fan False ceiling Product on trolleys Fig. 7.1. Example of a freezing tunnel with longitudinal air circulation