Acidity of milk before adding rennet .2 to .21 per cent Acidity of whey before heating curd .14 to .145 " Acidity of whey before removing from curd .16 to .18 " Acidity of whey coming from the curd after removal of whey and curd is packed .24 to .30 " Acidity of whey coming from curd before milling .65 to .75 " Acidity of whey coming from curd before salting .90 to 1.10 " If the milk had been kept at the same temperature as the curd, the acidity would have increased much more slowly since the acid would have been distributed through a larger volume. In the cheese curd the same amount of acid is probably formed, as would have been produced in the total amount of milk during the same interval. The acid produced by this bacterial activity has a most marked effect on the curd. At first the curd masses are tough and firm, the particles showing no tendency to adhere to each other. As the acid increases in amount, the curd becomes plastic, the outer surface of the particles adhering or "matting," as the maker expresses it. The result is a solid coalescent mass of curd, which is cut into small pieces, _i.e._, "milled," before it is put to press. The acid allows the blending of the pieces under the influence of the pressure so that a cheese is one single mass. Under certain abnormal conditions, the development of acid may be interfered with and the particles of curd fail to mat, in which case, the cheese will be crumbly when it is cut. The determination of the proper time for pressing is made by the application of what is known as the hot iron test. This is made by determining the length of the "strings" or "threads" which can be drawn from a mass of curd when it is brought in contact with a hot iron at a cherry red heat, the length of the curd threads being a measure of the amount of acid that has been formed in the curd. The rate of acid formation within the curd particles is also measured by determining the acidity of the whey as it comes from the curd at different stages in the making. This test, which is often used in place of the "hot iron" test is carried out in the same manner, as in determining the acidity of milk or cream. The quality of the cheese, both as to texture and flavor, is dependent to a great degree upon the amount of acid that is formed during the various stages in making; hence, the successful maker must follow closely by some means the acid formation in the curd until it is put to press. It is very necessary that the milk shall contain a sufficient number of acid-forming bacteria to produce the required amount of acid. If a sufficient number of bacteria are not present in the milk as it is received, as is the case with very sweet milk, they must be added by the maker in the form of a starter, or the process of making will be much prolonged. [Illustration: Fig. 35.--Bacteria in Cheese.