In this fermentation, the acidity begins to be evident to the taste when it reaches about 0.3 per cent., calculated as lactic acid. As the formation of acid goes on, the casein is precipitated and incipient curdling or lobbering of the milk occurs. This begins to be apparent when the acidity is about 0.4 per cent., but the curd becomes more solid with increasing acidity. The rapidity of curdling is also dependent upon the temperature of the milk. Thus milk which at ordinary temperatures might remain fluid often curdles when heated. The growth of the bacteria is continued until about 0.8 to 1.0 per cent. acid is formed, although the maximum amount fluctuates considerably with different lactic acid species. Further formation then ceases even though all of the milk-sugar is not used up, because of the inability of the lactic bacteria to continue their growth in such acid solutions. As this acidity is really in the milk serum, cream never develops so much acid as milk, because a larger proportion of its volume is made up of butter-fat globules. This fact must be considered in the ripening of cream in butter-making where the per cent. of fat is subject to wide fluctuations. The formation of lactic acid is a characteristic that is possessed by a large number of bacteria, micrococci as well as bacilli being numerously represented. Still the preponderance of evidence is in favor of the view that a few types are responsible for most of these changes. The most common type found in spontaneously soured milk changes the milk-sugar into lactic acid without the production of any gas. This type has been described by various workers on European as well as American milks, and is designated by Conn as the _Bact. lactis acidi_ type.[51] It is subject to considerable variation under different conditions. Curiously enough if milk which has been drawn with special care is examined immediately after milking, the lactic organisms are not usually found. They are incapable of development in the udder itself, as shown by injections into the milk cistern. They abound, however, on hay, in dust, in the barn air, on the hairy coat of the animal, and from these sources easily gain access to the milk. In this medium they find an exceptionally favorable environment and soon begin a very rapid growth, so that by the time milk is consumed, either in the form of milk or milk products, they make up numerically the larger portion of the bacteria present. Another widely disseminated, although numerically less prevalent, type is _B. lactis aerogenes_. This type forms gas in milk so that the soured milk is torn by the presence of gas bubbles. It also grows more luxuriantly in contact with the air.