Rate at which the temperature is lowered. The rate at which the temperature of the brine and thus also that of the mix to be frozen is lowered depends on the proportion of ice and salt, and on the fineness of division of the ice and salt. Small pieces of ice provide greater surface and thus melt faster than large pieces. Fine salt dissolves more quickly than coarse salt, but a fine table salt is likely to lump. Crushed rock salt is good to use.
Two procedures are used in freezing. The smooth, velvety texture of ice cream and ices depends on several factors. One important factor is the size of the crystals formed. The proportion of ingredients, their treatment, and the incorporation of air bubbles into the mixture while it is freezing are also important. Small crystals may be obtained by the quick-freezing method. A very low temperature is used, the mass being frozen so rapidly that many nuclei are formed and there is little crystal growth because of the short time. This process is easier to use commercially than in the home, because of the very low temperature employed for freezing. Judging from reports in the ice-cream trade journals, it is successful in some instances and not in others.
The other procedure is to freeze at a slower rate, depending on agitation to form many nuclei. Air is incorporated in the mix in both processes. A temperature of – 8° to – 10°C. is sufficiently low to freeze most ice creams, sherbets, and ices. For home freezing, 1 part of salt to 8 of ice is a good proportion. Commercially, 1 part of salt to 12 of ice is often used. Washburn states that a proportion of 1 part of salt to 40 parts of ice will freeze if given ample time. The air globules are retained in the mixture better after it has nearly reached the freezing point. The air is beaten in by the dasher paddles. In freezing very rapidly there is not time to incorporate as much air as when the mix is frozen at a somewhat slower rate.
Packing the freezer. Commercially most ice cream is now held in mechanically operated freezing units or rooms, although the same proportions of ice and salt may be used for packing that are used for freezing. For home-made ice cream the same proportion of ice and salt may be used as for home freezing, or about 1 of salt to 8 of ice. In packing, the ice should be pressed down tight in the can. This will force the surplus water out of the opening in the container. If the water is drained off it only takes a longer time for the brine to form again, and cooling is slower. A little salt should be added to the ice left in the freezer before the new ice is added. For home-made ice cream that is hot to stand very long before serving, a larger quantity of salt may be used to advantage. This will cause the temperature to drop quickly and hardening of the cream will take place more rapidly.
The ice and salt may be mixed together before putting in the freezer, but a better way of packing is to fill the freezer half full of ice, then add a layer of salt. The ice and salt are then added in alternate layers until the top is reached. The water from the melting ice washes the salt towards the bottom of the freezer, and once it reaches the bottom it is of small use for cooling purposes. Therefore, it is preferable to pack the freezer with the salt towards the top.
Mousse. For packing home-made mousse, the proportion of salt to ice must be large, 1 to 3 or even 1 to 2. Whipped cream which has air beaten into it is added to a mousse. The mousse is molded and often is not stirred while freezing, although it may be stirred very slowly. Heat or cold penetrates more slowly to the center of a mixture that is not stirred.
In addition, the air and the high fat content of the whipped cream are poor conductors of heat and cold. Thus a mousse requires a low temperature and long time to cool. The ice and salt may need to be repacked unless set where the air is cold.