A more important test than the acid test, from the standpoint of the butter and cheese maker, and even the milk dealer, is the fermentation test. In its simplest form, it consists in placing a sample of the milk to be tested in a warm place and noting the time required to curdle and the type of curd formed. In this country the fermentation test has been largely supplanted by the Wisconsin curd test which possesses the advantage of detecting the presence of bacteria harmful in cheese making, especially the gas forming bacteria. The curd test is helpful in detecting the source of an abnormal condition in a milk supply coming from diverse sources. The milk furnished by each patron can be tested separately and the trouble located, perhaps in an individual herd; the offending herd determined, the test may then be used on the milk of individual cows. The cheese maker and the milk dealer should be able not only to detect which of the patrons furnish him poor milk, but he should be able to give the patron definite instructions how to avoid the sources of such trouble. This information can be given only when the source is positively known. [Illustration: Fig. 23.--Curd Test. A good curd obtained from milk containing no harmful bacteria but many desirable acid-forming organisms.] The Wisconsin curd test is made as follows: Samples of the milk to be tested are placed in sterile pint fruit jars. The milk is warmed to 90° F., ten drops of rennet are added to each sample, and as soon as the curd is solid, it is cut into small pieces with a case knife so as to facilitate the expulsion of the whey. As the curd settles to the bottom of the vessel, the whey is poured off at intervals so that a pat of firm curd is left. As the milk curdles the bacteria are enmeshed and are carried with the curd. The jars are kept at a temperature of 100° to 105° F., since this temperature is favorable to growth of the bacteria that are sought, the gas-forming organisms. At the end of ten to twelve hours, the jars are examined; if the curd is solid, the texture firm, not mushy or slimy on the surface, if the odor is agreeable, it indicates that the milk contains few or none of the undesirable forms of bacteria. If the curd is full of gas holes, it is apparent that undesirable bacteria are present and under such circumstances the curd will not have an agreeable odor. If the gas-forming bacteria are numerous, the curd may even be spongy from the abundance of gas holes, and the undesirable odor more pronounced. Such curds are tough and rubbery. In some cases a bad flavor or odor is apparent even though the texture of the curd is not open and full of holes. The curd, the surface of which is slimy indicates undesirable organisms. A solid curd of agreeable odor is indicative of the presence of the desirable acid-forming bacteria. Such a milk is excellent from the standpoint of the butter or cheese maker, but may not be so