There are other tests used in scientific dairying as the _Fermentation Test_ to ascertain the relative purity of milk, the _Casein Test_, etc., but the above are those mostly used besides the _Bacterial Count_ which is mentioned under the chapter on “Milk Supply,” and the _Rennet Test_ described under “Cheese Making.” FERMENTS Two classes of ferments are of importance in connection with milk: (1) “unorganized” or chemical ferments, the “enzymes,” and (2) “organized” ferments such as bacteria and yeast. _Enzymes_ =Rennet.=—Among the unorganized ferments, _Rennet_ or _Rennin_ is highly important on account of its power of coagulating or curdling milk by precipitation of the casein. Rennet is extracted from the stomach of the suckling or milk-fed calf, where it serves in digesting the calf’s food. It is in the market in the form of a liquid extract as well as a dry powder compressed into tablets (Rennet Tablets and Junket Tablets). The characteristics and use of rennet are described under “Cheese Making” in Chapter III. =Pepsin= is another enzyme the office of which in the process of digestion is to dissolve albuminoids. It is not considered identical with rennet though in an acid solution it will curdle milk. It occurs in the stomachs of grown animals fed on solid food and is usually produced from hogs’ stomachs. Bacteria Everywhere, in the air, in water, in the soil, and clinging to every object in the world, are minute organisms known under the common names of _bacteria_ or _microbes_. In contrast to rennet and the other “unorganized” ferments, bacteria belong to the “organized” class. Some are harmful, producing putrefaction, dissolution, poisons or disease; others are beneficial, leading to desirable fermentations and changes; others again are indifferent, neither good nor bad, but harmless. As the milk comes from the cow it is almost free from bacteria, but milk makes an excellent soil for many of these organisms to grow in, and they soon get in, to multiply with enormous rapidity at any temperature from 60° to 100° F. =Lactic Acid Bacilli= are bacteria of special importance to the dairyman, for they convert sugar of milk into lactic acid and produce various more or less agreeable flavors. They are also powerful germicides and scavengers, destroying or neutralizing the products of other bacteria which in the absence of these bacteria and the lactic acid produced by them would play havoc with the food and produce putrefaction or disease. Everybody who handles milk knows that pure sour milk or buttermilk in which lactic acid bacteria abound keeps well for a long time, free from other fermentations which have no chance to develop in their presence. It is due to this purifying property that Metchnikoff recommended Bulgarian sour milk as a health food, asserting that it prevents harmful fermentations in the digestive channel.