The machine is filled with milk containing three per cent. of fat, which has been previously pasteurised to about 190° F., and cooled down to about 90° F.; at this point the pure culture of _Bacillus bulgaricus_ is introduced, and the agitator is kept working, so as to mingle it thoroughly with the milk. The agitator is then stopped until the acidity shows a test of 0.9 to 1.0 per cent., when the agitator is again started, and cold brine from the refrigerating machine is turned on to the cooling pipes, so that the product is thoroughly broken up, and cooled down to 40° F. The milk is then transferred to a bottle-filling machine (Fig. 10), poured into bottles and hermetically sealed, after which it is ready for consumption. When it has to be kept for any time it should be placed in a cold room where there is a temperature not higher than 40° F. The process, therefore, is a simple one, and lends itself to the ordinary dairy business, without involving any great expenditure on account of a new plant. CHAPTER V THE BACTERIOLOGY OF FERMENTED OR SOURED MILK A CHAPTER FOR STUDENTS During the last few years much work has been done in investigating the action of various classes of organisms—bacteria, yeasts, and moulds—upon milk and its products. While, however, the attention of the dairyman has been chiefly directed to the propagation of acid-producing organisms and the use of pure cultures of lactic acid bacteria in their relation to butter and cheese making, a new sphere in micro-biology has been disclosed by the study of the effects caused by the combined growth of two or more different classes of organisms in milk and the consequent production of lactic, alcoholic, and gaseous fermentations. The simultaneous occurrence of these fermentative changes is responsible for the formation of such beverages as keffir, koumiss, milk-wine, etc. It has therefore become essential, in connection with the study of new developments in the milk industry, that we should make a more intimate acquaintance with the bacteriology of the ferments involved. _Keffir_ (_kephir_, _kifyr_, _kiafyr_, _kephor_, _kyppe_) is the name given to an acid, slightly alcoholic drink, which for many centuries has been prepared by the nomadic tribes in the Caucasus. The characteristic fermentation is induced by the addition of so-called keffir grains. These are yellow or golden-yellow, warty, and furrowed flakes or nodules, the former varying in size from that of a rice grain to that of a bean, while the latter are often about an inch across and one eighth of an inch thick. Bearing in mind the fact that the preparation of keffir has been carried on for many centuries, it is not surprising that the origin of these grains should be surrounded by myths.