RELATION OF MICROORGANISMS TO CHEESE 491 



increases in acidity. The Cheddar cheese-maker desires milk that shall 

 contain such a number of acid-forming bacteria that during the opera- 

 tions that are carried on in the first part of the cheese-making process 

 large amounts of acid shall be formed in the curd. He thus wishes to 

 know, as accurately as can be determined under the conditions found 

 in the factory, the number of bacteria in the milk which he is to use. 

 This information is gained either by the titration of the milk with a 

 standard alkali solution or by determining the time required for the 

 curdling of a definite quantity of milk at a definite temperature by a 

 known quantity of rennet. Very much smaller increases in acidity 

 can be detected by the so-called rennet test than by titrating 

 the milk. If the milk shows the desired acidity when it reaches the 

 factory, the making process is immediately begun. If the milk is too 

 sweet, or in other words, too low in its bacterial content, bacterial 

 growth is favored by warming the milk to temperatures most favorable 

 for the lactic bacteria, 30 to 32, and by the addition of pure cultures 

 of Bad. lactis acidi which are identical in nature and the method of 

 propagation with those used in butter making. The development of a 

 slight acidity is known as the "ripening" of milk. 



In order to insure proper rennet action the maker of Cheddar cheese 

 desires the milk to have an acidity of about 0.2 per cent. He thus 

 wishes milk in which an appreciable amount of acid has been formed. 



CURDLING OF MILK. Under the influence of a favorable tempera- 

 ture and the slight acidity, the milk is quickly changed by the rennin* to 

 a firm, jelly-like mass that is cut, with appropriate knives, into small 

 cubes. The curd encloses over 80 per cent of the bacteria in the milk. 

 The same factors that favor the curdling of the milk favor the shrinking 

 of the curd and the expulsion of the whey from the cubes. The develop- 

 ment of acid within the curd is rapid, due to the concentration of large 

 numbers of bacteria in a small volume and to the favorable environ- 

 ment. During the six to eight hours that elapse between the curdling 

 of the milk and the pressing of the curd, the increase of acidity is over 

 o.i per cent per hour. The following table gives the acidity of milk and 

 the whey expressed from the curd at various stages in the making of a 

 typical Cheddar cheese. 



The rennet used in cheese-making is obtained by extracting the abomasum, the true diges- 

 tive stomach of the calf, with -a solution of sodium chloride. The extract contains two enzymes, 

 a clotting or curdling enzyme, rennin, and a proteolytic enzyme, Pepsin. 



