with a change in temperature, correct readings will not be obtained if the temperature is much above or below 140° F. This is particularly true of cream because of the greater volume of fat which it contains. For more detailed

Fig. 13. Showing the points from which milk and

cream tests should be measured.

information on testing milk and cream the reader is referred to some special work on that subject.1

After receiving, sampling, and grading the milk and

1 Farrington and Woll's "Testing Milk and Its Products."

cream, the next consideration is to prevent as far as possible the increase of bacteria in the milk or cream before it is used. As ice cream factories usually pay a price somewhat in advance of the regular butter fat price, every can of cream that sours means quite a loss to the factory. To be prepared for any increased demand for ice cream, the manufacturer must carry a supply of sweet cream on hand.

How large this supply should be will depend upon the size of the regular trade, the capacity of the plant, and the facilities for handling and storage.

Where facilities for pasteurization are wanting and storage facilities are limited the reserve cream supply can not be very large. Under such conditions the cream must be cooled as quickly and as thoroughly as possible and held. at a low temperature. The better method, however, is to

pasteurize and cool the cream before placing it in the storage 100ms. In the smaller establishments, which cannot afford to install a pasteurizer, the cream may be pasteurized in ordinary milk cans by placing them in hot water. The cream should be stirred to insure uniform heating. A temperature of 140° F. to 145° F. must be maintained for about 20 minutes.

The process of pasteurization consists in heating the milk

or cream to a temperature sufficiently high to destroy all the vegetative cells of bacteria, yeasts, and molds, and then. cooling quickly to a very low temperature to inhibit the growth of spores which are not destroyed at the temperatures employed in pasteurization. There are two methods. of pasteurization, known as the continuous method and the intermittent method.

The continuous method. In this method there is a constant inflow and discharge of the milk or cream. In these machines the milk or cream is subjected to a very high temperature for a very brief period of time. This system is therefore known as "flash heat." Rapid heating is accomplished by passing the milk or cream over the heated inner wall of the pasteurizer in a very thin sheet. The time during which the milk or cream is subjected to the high temperature is very brief. In most cases a temperature between 170° F. and 180° F. for from one to 11⁄2 minutes is employed. The advantages claimed for the continuous method are greater capacity and greater efficiency.

Greater capacity is claimed for these machines because there is a continuous inflow and discharge of the milk or cream. This continuous operation makes it necessary to heat higher because the milk or cream is exposed to the heat for a very short time. The advantage claimed for rapid and high heating or "flash heat" is that a very sudden increase in temperature is more effective in destroying bacteria. It is also claimed that many more of the spore bacteria are destroyed by this system, because the heat is raised so rapidly that the organism is destroyed before it is able to pass into the spore state. When properly handled and so regulated that all of the milk or cream passing through the machine is heated to the desired point, these machines are very efficient. The machine is not automatic, however, and must be regulated from time to time. Even in the hands of a careful workman there will be considerable variation in the temperature at which the milk or cream is pasteurized. This difficulty may be overcome and all the advantages of rapid heating and continuous operation retained by holding the milk or cream for a short time before cooling.