Attempts have been made to employ the first principle in shipping milk by rail, viz., prolonged heating above growing temperature, but when milk is so heated, its physical appearance is changed.[132] The methods of heating most satisfactorily used are known as sterilization and pasteurization, in which a degree of temperature is used approximating the boiling and scalding points respectively.
Fig. 23.
Microscopic appearance of milk heated above 140° F., showing the homogeneous distribution of fat-globules. The physical change noted in comparison with Fig. 22 causes the diminished consistency of pasteurized cream.]
Effect of heat on milk. When milk is subjected to the action of heat, a number of changes in its physical and chemical properties are to be noted.
1. Diminished "body." When milk, but more especially cream, is heated to 140° F. or above, it becomes thinner in consistency or "body," a condition which is due to a change in the grouping of the fat globules. In normal milk, the butter fat for the most part is massed in microscopic clots as (Fig. 22). When exposed to 140° F. or above for ten minutes these fat-globule clots break down, and the globules become homogeneously distributed (Fig. 23). A momentary exposure to heat as high as 158°-160° may be made without serious effect on the cream lime; but above this the cream rises so poorly and slowly that it gives the impression of thinner milk.
2. Cooked Taste. If milk is heated for some minutes to 160° F., it acquires a cooked taste that becomes more pronounced as the temperature is further raised. Milk so heated develops on its surface a pellicle or "skin." The cause of this change in taste is not well known. Usually it has been explained as being produced by changes in the nitrogenous elements in the milk, particularly in the albumen. Thoerner[133] has pointed out the coincidence that exists between the appearance of a cooked taste and the loss of certain gases that are expelled by heating. He finds that the milk heated in closed vessels from which the gas cannot escape has a much less pronounced cooked flavor than if heated in an open vessel. The so-called "skin" on the surface of heated milk is not formed when the milk is heated in a tightly-closed receptacle. By some[134] it is asserted that this layer is composed of albumen, but there is evidence to show that it is modified casein due to the rapid evaporation of the milk serum at the surface of the milk.
3. Digestibility. Considerable difference of opinion has existed in the minds of medical men as to the relative digestibility of raw and heated milks. A considerable amount of experimental work has been done by making artificial digestion experiments with enzyms, also digestion experiments with animals, and in a few cases with children. The results obtained by different investigators are quite contradictory, although the preponderance of evidence seems to be in favor of the view that heating does impair the digestibility of milk, especially if the temperature attains the sterilizing point.[135] It has been observed that there is a noteworthy increase in amount of rickets,[136] scurvy and marasmus in children where highly-heated milks are employed. These objections do not obtain with reference to milk heated to moderate temperatures, as in pasteurization, although even this lower temperature lessens slightly its digestibility. The successful use of pasteurized milks in children's hospitals is evidence of its usefulness.
4. Fermentative changes. The normal souring change in milk is due to the predominance of the lactic acid bacteria, but as these organisms as a class do not possess spores, they are readily killed when heated above the thermal death-point of the developing cell. The destruction of the lactic forms leaves the spore-bearing types possessors of the field, and consequently the fermentative changes in heated milk are not those that usually occur, but are characterized by the curdling of the milk from the action of rennet enzyms.
5. Action of rennet. Heating milk causes the soluble lime salts to be precipitated, and as the curdling of milk by rennet (in cheese-making) is dependent upon the presence of these salts, their absence in heated milks greatly retards the action of rennet. This renders it difficult to utilize heated milks in cheese-making unless the soluble lime salts are restored, which can be done by adding solutions of calcium chlorid.