Although milk contains a large amount of nitrogenous substances (casein and albumen), it does not undergo putrid decomposition, as do meat and eggs, not because it is not fitted for the growth of the bacteria causing that type of change, but because the acid formed in it stops the growth of the putrefactive bacteria. If a sample of milk is placed in a stoppered bottle, it will have much the same taste and odor at the end of several months as at the end of a few days. The acid acts as a preservative, like the vinegar in pickles, or the acid in silage and in sauerkraut. Meat placed in a stoppered bottle which is then filled with milk will be preserved. The products formed in the decomposition of meat and eggs are not only offensive but may also be injurious to the health of the consumer. Milk that has been fermented by the desirable kinds of acid-forming bacteria is not harmful. It is consumed in a variety of forms (buttermilk, cottage cheese) as a common article of food and its use is rapidly increasing. The preparation of the pure culture buttermilks or artificially soured milks that are now so frequently recommended for digestive troubles rests upon an acid fermentation of this type. =Undesirable acid-forming bacteria.= Other types of bacteria capable of forming substances that impart to milk an offensive odor and a disagreeable taste not infrequently appear instead of the desirable group. Instead of producing from the sugar of milk large quantities of lactic acid, these types generate other acids, such as acetic and formic, which impart a sharp taste to the milk. Besides the acids the bacteria of this group form gases from the sugar of the milk. Some produce small amounts of gas; others so much that the curd will be spongy and will float on the surface of the whey. The fermentation caused by them is often called a "gassy fermentation" and is dreaded by butter and cheese makers since the gas is indicative of bad flavors that will appear in the product. Gas may also be produced in other types of fermentations to be discussed later. This class of bacteria enters the milk with the dust, dirt, and manure, in which materials they are especially abundant. No spores are formed; hence they are easily killed by heating the milk. They grow both in the presence and in the absence of free oxygen. High temperatures favor their growth, most rapid development taking place at 100° to 103° F.