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

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

The pathogenic bacteria most likely to be present in the milk are the typhoid and the tubercle organisms. The typhoid bacillus is no more resistant to heat than the ordinary acid-forming bacteria, and all milk that has been heated, so as to impart to it satisfactory keeping properties, will certainly be free from typhoid bacilli. It has sometimes been asserted that the tubercle bacillus is very resistant to heat; some claiming that it is necessary to heat milk to 200° F. in order to destroy it. Other experimenters have asserted that lower temperatures would suffice, but the temperatures were still above those at which the milk is physically and chemically changed by the heating process. More recent work has shown that not all sources of error were avoided in the earlier attempts to determine the thermal death point of the tubercle bacillus, as, for example, it has been shown by the authors that the "scalded film" that forms on the surface of milk when heated in an open vessel will protect the bacteria imbedded in it. It has also been shown by the authors that a temperature of 140° F., for twenty minutes or 160° F. for one minute will destroy the tubercle bacilli in milk, in case the heating is done with sufficient thoroughness to insure all particles of the milk being heated to the same temperature for these periods of time.

The pasteurization of milk can be done in such a manner as to impart to it good keeping qualities and to insure its freedom from pathogenic bacteria, and yet not impair its physical and chemical properties, but much of the so-called pasteurized milk placed on the market is not treated in accordance with proper hygienic methods.

Fig. 28.—The Pott's Discontinuous Pasteurizer.
The milk is placed in the inner compartment. For heating and cooling, hot or cold water is passed between the jackets.

Methods of pasteurization. In order to destroy the bacteria in milk, it is necessary that the milk be heated for a varying time dependent upon the temperature employed. A lower temperature for a considerable period may exert the same effect on the bacteria as a higher temperature for a shorter time. In practice, two types of pasteurizing machines are employed, depending on the temperature at which the milk is to be treated. The discontinuous machines or intermittently operated pasteurizers are those in which the milk is heated for any desired time at any temperature. Such machines consist of jacketed containers the inner receptacle being filled with milk, while the outer space between the walls is filled with circulating hot water or steam. The milk is kept agitated by the rotation of the machine. After it is heated, it is cooled in the same container by replacing the hot water first with cold water, then ice water. The disadvantage of this process is that the capacity of the machine is limited which precludes its use in places where large quantities of milk or cream are handled; for the pasteurization of limited quantities, it is very successful, as every particle of milk or cream is under the direct control of the operator and may be thoroughly and efficiently treated.

As pasteurization was introduced for the treatment of market milk, and for the preparation of cream for butter, machines have been devised which permit large quantities, as thousands of pounds, to be handled per hour. It is evident under these conditions that the milk must be heated for only a short time, and hence a higher temperature must be employed. These machines are called "continuous flow" pasteurizers since the milk passes through them in a constant stream. The period of exposure is very short, in some only a few seconds; hence, they are sometimes called "flash" pasteurizers.

Fig. 29.—A Continuous Pasteurizer.
The milk is exposed but a short time since it flows through the heater in a constant stream.

All machines of this type possess the obvious disadvantage that it is impossible to heat all of the milk for a uniform period. The milk in contact with the walls of the machine flows much more slowly than in the middle of the stream, just as the current near the bank is less rapid than in mid-stream. In none of the machines yet devised have the designers been able to overcome this disadvantage. In a test of one of the most widely used pasteurizers of this type, it was found that some of the milk passed through the machine in 15 seconds, while the larger part of it was held for about 30 seconds, and some as long as forty-five to sixty seconds. If the temperature employed had been such as to destroy the bacteria in that part of the milk heated for the minimum time, hygienic safety would be assured, but in order to avoid injuring the physical properties of the milk, the tendency is to use as low a temperature as possible, so that the milk heated for the minimum time may often contain organisms that have passed through the machine uninjured.