~Pasteurizing milk for cheese-making.~ If it were possible to use properly pasteurized milk in cheese-making, then practically all abnormal conditions could be controlled by the use of properly selected starters. Numerous attempts have been made to perfect this system with reference to cheddar cheese, but so far they have been attended with imperfect success. The reason for this is that in pasteurizing milk, the soluble lime salts are precipitated by the action of heat, and under these conditions rennet extract does not curdle the casein in a normal manner. This condition can be restored, in part at least, by the addition of soluble lime salts, such as calcium chlorid; but in our experience, desirable results were not obtained where heated milks to which this calcium solution had been added were made into cheddar cheese. Considerable experience has been gained in the use of heated milks in the manufacture of certain types of foreign cheese. Klein[182] finds that Brick cheese can be successfully made even where the milk is heated as high as 185 deg. F. An increased weight is secured by the addition of the coagulated albumin and also increased moisture. ~Bacteria in rennet.~ In the use of natural rennets, such as are frequently employed in the making of Swiss cheese, considerable numbers of bacteria are added to the milk. Although these rennets are preserved in salt, alcohol or boric acid, they are never free from bacteria. Adametz[183] found ten different species and from 640,000 to 900,000 bacteria per cc. in natural rennets. Freudenreich has shown that rennet extract solutions can be used in Swiss cheese-making quite as well as natural rennets; but to secure the best results, a small quantity of pure lactic ferment must be added to simulate the conditions that prevail when natural rennets are soaked in whey, which, it must be remembered, is a fluid rich in bacterial life. Where rennet extract or tablets are used, as is generally the case in cheddar making, the number of bacteria added is so infinitesimal as to be negligible. ~Development of acid.~ In the manufacture of cheddar cheese, the development of acid exerts an important influence on the character of the product. This is brought about by holding the curds at temperatures favorable to the growth of the bacteria in the same. Under these conditions the lactic-acid organisms, which usually predominate, develop very rapidly, producing thereby considerable quantities of acid which change materially the texture of the curds. The lactic acid acts upon the casein in solutions containing salt, causing it to dissolve to some extent, thus forming the initial compounds of digestion.[184] This solution of the casein is expressed physically by the "stringing" of the curds on a hot iron. This causes the curds to mat, producing a close, solid body, free from mechanical holes. Still further, the development of this acid is necessary for the digestive activity of the pepsin in the rennet extract.