The yeasty or alcoholic fermentation may also be of importance in butter making. In many sections of the country the milk is separated on the farm and the cream is forwarded to the creamery at more or less infrequent intervals. It becomes sour and if it has become contaminated with yeasts, they will find favorable conditions for growth in the acid medium. A large amount of carbon dioxide gas is produced. Cans of gathered cream often foam to such an extent as to run over, and in some cases actual explosions have occurred on account of the great pressure caused by the gas. =Bitter fermentation of milk.= Bitterness in milk may be due to bacteria that enter the milk after it is drawn from the cow, or it may be caused by the feed consumed by the animal. It has been previously shown that certain specific substances contained in the food may be absorbed and reappear in the milk. If the animal eats ragweed, lupines, or other plants containing bitter substances, the milk is likely to have a bitter taste, which will be noticeable at the time the milk is drawn. The milk of cows at certain advanced stages of lactation may show a bitter taste, due to a change in the ash constituents of the milk in which the lime salts are largely replaced by salts of sodium. There are many bacteria that will impart to milk a bitter taste. Milk that has undergone the sweet-curdling fermentation is likely to be bitter, as is the ease with pasteurized milk. Some of the acid-forming bacteria are able to develop a bitter principle, the milk retaining a pleasant odor and having the normal amount of acid, while the taste is intensely bitter. One of the authors (H) found in the case of a Wisconsin brick cheese factory, that the usual acid organism was almost wholly replaced by a bitter type. Storage of milk at very low temperatures is conducive to the appearance of a bitter taste in milk, the explanation in this case being that the acid-forming bacteria are unable to grow at a low temperature, while some of the putrefactive forms can multiply and develop these astringent or bitter by-products.