=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. Milk is also preserved by wholly evaporating the water, thus leaving a dry powder, which on being mixed with water again will have much the same properties as the original milk. Various methods have been devised for the preparation of these milk powders, all of which have been patented by the inventors. If the powder is to be kept for long periods, skim milk must be used, since the fat slowly undergoes changes which cause it to have a rancid odor. These dry preparations are largely used by bakers in place of fresh milk. CHAPTER VII. BACTERIA AND BUTTER MAKING.