In the freezing process, air is incorporated and a proportion of the water is frozen. The resulting product is a three-phase system of air, liquid and solid. As the temperature of the mix is reduced, the water begins to freeze into ice. These ice crystals are composed of practically pure water and the rate of freezing should be rapid enough for these crystals to be as small as possible so the ice cream will be 'icy' in texture. As the water begins to freeze, the sugars and other soluble ingredients become more concentrated and therefore further crystallization will proceed at a slower rate. During the formation of ice, heat has to be removed from the mix. This is known as the latent heat of fusion and is not measured by a thermometer. Consequently, the temperature of the mix does not appear to alter to any great extent during the formation of ice crystals, although the temperature will be seen to drop at a slow rate; this is because the freezing point of the water is reduced as the soluble materials become more concentrated in the water. In general about 30-60% of the water in the mix is frozen, the amount depending upon the formulation and type of freezer used.

Several types of freezer are in use, but in general these can be classified as follows:

  1. Vertical freezers
  2. Horizontal (batch) freezers
  3. Continuous freezers.

Vertical freezers of a simple type are rather slow in operation and the scraper blades are immersed in the mix. Freezing is relatively slow and air is whipped in with difficulty and not in an easily controlled manner. The overrun obtainable is usually in the order of 40-50% and it is preferable not to use this type of machine with mixes of high total solids. Generally, the mix should contain no more than 7% fat and the total solids should be in the order of 30%. Direct expansion vertical freezers, where the freezing area is directly surrounded by the refrigerant coils, are faster and easier to control. Higher overruns of 55-60% can be achieved in more controlled ways and mixes with higher solids (fat content up to 8%) can be used.

Horizontal freezers are more effective and overruns in excess of 100% can be obtained. A wider range of mix formulations can be used but the ice cream will be lacking in body and texture if mixes of too low a solids content are overbeaten.

Continuous freezers tend to produce a more uniform product and because the air is pumped or drawn into the mix rather than beaten in, the overruns can be controlled effectively. Freezing is more rapid and this benefits the ice-cream. When a continuous freezer is used, the air incorporation does not depend so much on the character or the viscosity of the mix. Shorter ageing times will suffice and it has been said that less stabilisers are used.

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