86 Chilled foods cooling. Although this can achieve good results, there is considerable risk of surface freezing which would be unacceptable for many products. Liquid nitrogen is another total-loss' refrigerant that may be used to cool cabinets. As the temperature of liquid nitrogen at atmospheric pressure is -196C, careful control is necessary. An alternative may be synthetic liquid air (SLA)(Waldron and Pearce 1998), which overcomes the danger of asphyxiation that exists with other cryogens Alltotal-loss systems depend on the availability of compressed, liquefied ases, and it should be noted that the total energy use of such systems(including that needed for liquefaction) is much greater than that of equivalent mechanical refrigeration systems, so running costs may be high. In some applications, either reduced capital costs or increased chilling speed may make such systems 4.7.3 Hydrocoolers The use of chilled water, either sprayed down through a chamber or in an immersion tank, provides very rapid cooling with no risk of freezing. It is normally only applicable to fresh fruits and vegetables that can withstand water immersion, and so is a little outside the scope of the general range of chilled foods, though it may be applied to vacuum packs of prepared foodstuffs. Water is normally recirculated in such systems, so great care is necessary to ensure continued cleanliness by regular flushing out, addition of fungicides,or whatever may be necessary for the particular product. It is of course possible to combine a degree of hydrocooling with normal cleaning operations for items such as root vegetab 4.7.4 Vacuum coolers Vacuum coolers are highly specialised and expensive pieces of equipment, well suited to the rapid cooling of pre-packaged leafy vegetables. They operate at low pressure with wet produce in a sealed chamber, under which conditions the ooling is mostly achieved by low temperature evaporation of moisture. The process is in batches, with cooling times of about 15-30 minutes, and typical equipment can accommodate several tonnes of produce, normally on pallets or trolleys 4.7.5 Store cooling For large volumes of live produce, particularly fresh fruits and vegetables, cooling may be achieved by placing cartoned or binned produce in a cool store and allowing the circulation of air in the store to provide all the cooling that is necessary. This is a slow process, taking several days and dependent on the store air circulation and the stacking of the produce. In many fruit stores,a combination of store extract fans, curtains, and planned stacking as showncooling. Although this can achieve good results, there is considerable risk of surface freezing which would be unacceptable for many products. Liquid nitrogen is another ‘total-loss’ refrigerant that may be used to cool cabinets. As the temperature of liquid nitrogen at atmospheric pressure is
196ºC, careful control is necessary. An alternative may be synthetic liquid air (SLA) (Waldron and Pearce 1998), which overcomes the danger of asphyxiation that exists with other cryogens. All ‘total-loss’ systems depend on the availability of compressed, liquefied gases, and it should be noted that the total energy use of such systems (including that needed for liquefaction) is much greater than that of equivalent mechanical refrigeration systems, so running costs may be high. In some applications, either reduced capital costs or increased chilling speed may make such systems attractive. 4.7.3 Hydrocoolers The use of chilled water, either sprayed down through a chamber or in an immersion tank, provides very rapid cooling with no risk of freezing. It is normally only applicable to fresh fruits and vegetables that can withstand water immersion, and so is a little outside the scope of the general range of chilled foods, though it may be applied to vacuum packs of prepared foodstuffs. Water is normally recirculated in such systems, so great care is necessary to ensure continued cleanliness by regular flushing out, addition of fungicides, or whatever may be necessary for the particular product. It is of course possible to combine a degree of hydrocooling with normal cleaning operations for items such as root vegetables. 4.7.4 Vacuum coolers Vacuum coolers are highly specialised and expensive pieces of equipment, well suited to the rapid cooling of pre-packaged leafy vegetables. They operate at low pressure with wet produce in a sealed chamber, under which conditions the cooling is mostly achieved by low temperature evaporation of moisture. The process is in batches, with cooling times of about 15–30 minutes, and typical equipment can accommodate several tonnes of produce, normally on pallets or trolleys. 4.7.5 Store cooling For large volumes of live produce, particularly fresh fruits and vegetables, cooling may be achieved by placing cartoned or binned produce in a cool store and allowing the circulation of air in the store to provide all the cooling that is necessary. This is a slow process, taking several days and dependent on the store air circulation and the stacking of the produce. In many fruit stores, a combination of store extract fans, curtains, and planned stacking as shown in 86 Chilled foods