Outlines of dairy bacteriology, 10th edition / A concise manual for the use of students in dairying

by H. L. Russell; E. G. Hastings

Fig. 30.—A Pasteurizer for Use in the Home.
A milk bottle with a tumbler for a cover. The cover prevents the formation of the "scalded layer" on the milk during the heating and also protects the mouth of the bottle from dust.

Efficiency of pasteurizing. It is easy to destroy over 99 per cent of the bacteria present by the use of any of the modern types of machines. The number remaining after treatment will be largely dependent, other things being equal, upon the number of bacteria before pasteurization. The pasteurizing process is not one by which poor milk can be changed into good milk, nor is it legitimate to use the process in place of cleanliness, as is sometimes done. There is a legitimate field for the process in the handling of market milk, as well as in the creamery; but it should be used to improve the keeping quality, and to insure the freedom of the milk from pathogenic bacteria, when other protective measures have been carried as far as possible under the prevailing conditions.

Details of process. If the process is to be successful, due attention must be given to certain details. In the treatment of market milk, care should be taken to use only that in which the acidity has not materially increased. A fair standard is about 0.2 per cent. High acid milk usually means old milk or dirty milk, either of which is very likely to contain many more spore-bearing bacteria than clean, fresh milk. The greater the number of spores, the more rapidly will the pasteurized milk spoil. If it is possible to exercise any selection of milk prior to pasteurization, the rapid test for determination of acidity will prove of great advantage.

Care should be taken to prevent fluctuations in the temperature to which the milk is heated. With varying steam pressure and variations in the rate of flow of milk, these fluctuations may be very considerable. Regulators are now made that will control the temperature within narrow limits.

In all pasteurized milk as it flows from the machine, there will remain some living bacteria. The spores will not be destroyed by any pasteurizing process, and under commercial conditions, vegetating bacteria are also present. If the milk is not quickly chilled after heating, these forms will grow, and their development is particularly hastened by the destruction of the lactic bacteria, the acid of which would otherwise hold them in check. The result is that, unless immediately chilled, pasteurized milk spoils almost as rapidly as though it had not been heated at all. Efficient and rapid cooling are, therefore, as essential a portion of the process as the heating itself.

Care should also be taken to protect the milk from contamination after treatment. Every utensil with which it comes in contact should be sterilized. The bottles should be thoroughly washed and sterilized and subsequently protected from dust until used.

Sterilization of milk. It is possible to render milk sterile by the use of temperatures above the boiling point of water, where it is heated in a closed vessel, in which steam under pressure is generated. Such milk is often found in the European markets. In our own country, the only milk of this kind is the so-called "evaporated milk." In this process sweet fresh milk is evaporated in vacuum pans to about one-third of the original volume. This is then placed in tin cans, which are treated, as in the canning of such vegetables as peas and corn, by heating the milk to 230° or 240° F. for a few minutes. In this process, the bacteria (spores as well as vegetating forms) are completely killed, and the milk acquires a brownish tint, due to the caramelization of the sugar. The appearance of the product is very similar to cream, and previous to the passage of the pure food law, it was sold as evaporated cream.

Condensed milk is not wholly free from bacteria, but is sufficiently thick, by reason of its treatment so that the contained bacteria cannot grow. They remain dormant in the milk, but as soon as it is diluted to a normal consistency, growth takes place, and the milk rapidly spoils. Condensed milk is prepared by adding cane sugar to fresh sweet milk, then evaporating the mixture to one-third the original volume, forming a semi-solid product. Syrups owe their keeping qualities to the same factor, as condensed milk, i.e., the high consistency.