particles. Under most conditions a mixture of one part of salt to ten or eleven parts of ice will be sufficient, but if the mix is very cold the proportion of salt should be reduced to avoid too rapid freezing.

The ice should be crushed in such a way that the pieces will be of fairly uniform size, small enough to pack well in the tub. Large chunks not only do not pack well and leave large air spaces, but melt too slowly. This fact should be kept in mind, because it is sometimes necessary to produce a very low temperature, in which case the ice must be finely crushed and more salt used. This holds true whether the ice and salt mixture is packed around the freezing can or made in a separate container and the cold brine which runs from the melting ice used as the freezing agent.

Brine freezing has many advantages over the old method. In the first place, it is possible to keep the floor cleaner around the freezer, because a new charge of ice and salt may be added to the brine at any time without interfering with operations in any way. Furthermore, the cold brine comes in contact with all parts of the freezing can, resulting in greater efficiency. Probably the greatest advantage is in the more perfect control over the freezing by regulating the temperature and rate of flow of the brine. This method of freezing may be employed with the ordinary tub freezer by providing a tank in which the cold brine may be made. A rotary pump must also be provided to circulate the cold brine around the freezing can. With this system of brine freezing the temperature of the brine is regulated by the proportion of ice and salt in the mixture.

The temperature of brine cooled by mechanical means,

however, is regulated by the amount of ammonia gas allowed to expand in coils of pipe submerged in the brine. Where the brine is cooled in this way the temperature may be regulated more accurately and as a result the freezing process will be under more complete control.

CHAPTER XIV

REFRIGERATION

Ice cream making is largely a refrigeration process. Natural ice was originally used as the refrigerating agent, but mechanical refrigeration has developed so rapidly in recent years that it has been adopted by many factories because of its greater efficiency and cleanliness. Mechanical refrigeration is no longer confined to large plants exclusively. In fact its use in ice cream factories is becoming so general that a book of this sort would hardly be complete without a brief discussion of this process and its application to ice cream making.

Refrigeration is an interchange of heat between substances of different temperature. This interchange of heat may be accomplished in various ways, but the process is known as refrigeration regardless of the means employed.

Natural ice was originally the only refrigerating agent that could be put into general use. The unit for measuring refrigeration is therefore based upon the refrigerating capacity of ice. When we speak of so many tons of refrigerating capacity we simply mean the equivalent of so many tons of ice melting in 24 hours.

The British thermal unit. Just what is meant by a ton of refrigeration will be better understood by determining the actual absorbing capacity of a ton of ice. Heat is measured by what is known as the British Thermal Unit