The discovery of micro-organisms in perishable products, which is attributable to Anthony Van Leeuwenhoek, a Dutchman, whose vocation was the polishing of lenses, and who lived between 1632 and 1723, altered our point of view, not only of disease, but of all the functions carried on by the lower organisms. Since Van Leeuwenhoek's time, the germ theory has grown to vast proportions and has more especially been applied with splendid results to the study of milk. As we shall see later, the researches of modern investigators have led them to the conclusion that micro-organisms play such an important part in the milk supply, that it is impossible to carry it on safely without a knowledge of the bacteriology of the subject. This view began to prevail about 1890, some twenty years after Pasteur had shown what fermentation really meant. Since that time, the progress in dairying has been continuous, and, during recent years, attention has been directed to soured milk to such an extent that it has become necessary for all who are interested in the handling of milk and milk products to have a knowledge of the subject, as it seems clearly demonstrated that, under proper direction, there is every possibility of its forming an important element in the prolongation of life. CHAPTER II FERMENTED MILKS There is considerable variety in the number of soured or fermented milks, and they are known by various names, such as Koumiss or Koomiss, which is prepared from mares' milk; Keffir, which was originally discovered in the mountains of the Caucasus, and which is prepared with Keffir grains; Leben, an Egyptian product prepared from the milk of the buffalo, cow, or goat; Matzoon, a soured milk which is prepared in Armenia from ordinary cows' milk; Dadhi, an Indian preparation from cows' milk. All of these owe their special characteristics to the fact of their having undergone lactic and alcoholic fermentation. "Milk left to itself," says Blyth,[14] "at all temperatures above 90° F. begins to evolve carbon dioxide, and this is simply a sign and result of fermentation. If this fermentation is arrested or prevented, the fluid remains perfectly sweet and good for an indefinite time. Besides the production of carbon dioxide during decomposition, a certain portion of milk sugar is converted into lactic acid, some of the casein and albumen are broken up into simpler constituents, and a small proportion of alcohol produced, which by oxidation appears as acetic acid, while the fat is in part separated into free fatty acids, which ultimately unite with the ammonia produced by the breaking up of the albuminoids. The main fermentation of milk is a special kind which of late years has been much studied, and is known as _lactic fermentation_. Accompanying lactic fermentation there is nearly always a weak butyric and a weak alcoholic fermentation."