2. A temperature above the thermal death-point destroys bacteria, and thereby stops all changes. This temperature varies, however, with the condition of the bacteria, and for spores is much higher than for vegetative forms. 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. [Illustration: FIG. 23. Microscopic appearance of milk heated above 140 deg. 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 deg. 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 deg. 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 deg.-160 deg. 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 deg. 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.