The speed of freezing is a relative matter, but ‘quick frozen’ produce is generally frozen in 5-10 minutes in an air blast, and depending on the thickness this
Can be somewhat quicker if immersed. Various methods have evolved, depending on the available resources, the product concerned and the premium value it might earn in an improved frozen state.
Where the product shape is irregular, the only way to extract its heat will be by using a cold fluid surrounding it. Air is the obvious choice — it is economical, hygienic and relatively non-corrosive to equipment. The air temperature will be of the order of -40°C and the air speed over the product will be high, to get good heat transfer. Circulation of air around stacked product in a cold room is used for batch processes. This requires good air distribution and an optimum value exists between the decrease in freezing time and the increasing power required to drive the fans to produce higher air speeds.
Discrete pieces of product, such as peas, slices of carrot, beans and items of this size, can be conveyed on a perforated belt. This may be a fluidized bed where cold air is blasted up through the holes, to both cool the product and agitate it, to prevent it sticking either to the belt or to other similar pieces. Flat pieces of product, such as fish fillets, would suffer a change in shape in a free air blast and are better on a flat moving belt. Here, some of the heat goes direct to the cold air and some by conduction to the belt, which is usually of stainless steel. The tunnel can be designed to absorb much less fan power which improves efficiency, because fans are located in the cooled space and the heat generated by their motors adds to the cooling load.
Linear tunnels are restricted by the length of belt necessary to achieve the cooling time required and on the space available. Conveyors wound into a spiral shape and contained within a cold-room with an air blast coil offer a more compact arrangement (see Figures 14.1 and 14.2). Spiral freezers are very good for larger items, such as tubs of ice-cream which take a long time to harden and where a straight conveyor would be too long for convenience.
Very cold, high-velocity air Product out
Figure 14.2 View of spiral freezer (Star Refrigeration)
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