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

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

Fig. 26.—Fat Globules in Heated Milk.
When milk is heated the masses of globules are broken up and fat globules are uniformly distributed throughout the milk.

Fig. 27.—Creaming of Milk.
The cylinder on the left contains raw milk; that in the center, milk heated to 140° F. for twenty minutes; on the right, milk heated to 160° F. for twenty minutes. The dark line indicates the depth of the cream after twenty-four hours. The breaking up of the fat globule clusters delays greatly the rising of the cream.

Heated milk has a taste unlike that of raw milk; to one not accustomed to it the taste is objectionable. This change is due to some extent to the expulsion of the carbon dioxide from the milk. The insipid taste of boiled water is, in part, due to its freedom from carbon dioxide. The production of this cooked flavor is dependent upon the time and temperature of exposure. It has been claimed that heated milk is less digestible than raw, and a considerable amount of experimental work has been done, both on animals and children, in order to determine the relative digestibility of heated and raw milk. The results obtained have been contradictory. It is claimed that heated milk causes such diseases as rickets, scurvy and marasmus in children. It is probably true that milk heated to the boiling point is less fitted as food for the young child than raw milk, but, on the other hand, it has not been proven that properly pasteurized milk is an unsuitable food for children. The best evidence has been accumulated in recent years, in many of the large cities of this country and of Europe, where pasteurized milk has been used with the greatest success in the feeding of children of all ages.

The heated milk does not curdle readily when rennet is added due to the precipitation of the lime salts by heat. The curdling power can be restored by the addition of soluble lime salts or of acids.

Purpose of pasteurization. There are two reasons for the pasteurization of milk: (1) To improve the keeping quality; (2) To destroy any pathogenic bacteria it may contain. The first may be called the economic reason; the second, the hygienic reason fur pasteurization. In the selection of a proper pasteurizing temperature, two factors must be taken into account: First, the effect of heat on milk, and second, the temperature necessary to destroy those forms of bacteria that are of the greatest importance, as far as the keeping properties are concerned, and the pathogenic bacteria that might possibly be present in the milk. The lactic acid bacteria are non-spore-bearing and are not resistant to heat. Most of them are destroyed when the milk is heated to 140° F. for fifteen minutes or to 160° F. for a moment. To insure proper keeping quality, somewhat higher temperatures must be employed, such as 145° to 150° F. for fifteen to twenty minutes.

Milk pasteurized at these temperatures will, as a rule, undergo an acid fermentation in much the same manner as will raw milk. The rate with which the acid develops is of course much slower than in the raw milk, due to the destruction of 95 to 99 per cent of the acid-forming bacteria. If the milk has been pasteurized at higher temperatures, the acid fermentation may not appear. The spores of the spore-bearing organisms will be left; these may germinate and cause their characteristic change in milk, which, as previously noted, is usually a sweet-curdling or a digesting fermentation. Since the changes they produce in the milk are not evident at first, it might be used as food even though it was so far advanced in decomposition as to be undesirable or even harmful as food. Indeed one of the objections urged against pasteurization is that it destroys the natural safe guard, the acid-forming bacteria. Many people are so accustomed to use this as the indication of spoiled milk that they will use milk long after it should be used if it does not show an acid fermentation.

The butyric acid organisms are spore forming and may at times produce their characteristic fermentation in pasteurized milk. The milk shows gas formation and develops an objectionable odor.