4Q2 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



Acidity of milk before adding rennet o. 20-0. 21 per cent. 



Acidity of whey immediately after cutting curd. o. 14-0. 145 per cent. 

 Acidity of whey when removed from the curd. . o. 16-0. 18 per cent. 



Acidity of whey when curd is packed o . 24-0 . 30 per cent. 



Acidity of whey when curd is milled 0.65-0. 75 per cent. 



Acidity of whey when curd is salted 0.90-1 . 10 per cent. 



MANIPULATION OF THE CURD.* The curd particles at first show 

 little tendency to cohere; but, as the acidity increases, the nature of the 

 curd changes, and, when the whey is removed, the pieces of curd soon 

 cohere and ultimately form a single mass in which the original cubes of 

 curd cannot be detected. The fusion of the curd particles is known as 

 " matting" and is an important step in the Cheddar process. The lack 

 of acid formation within the curd prevents matting while the curd is in 

 the vat, and may even render difficult the fusion of the particles under 

 pressure. The nature of the change which the curd undergoes at this 

 stage in the manufacture is not well understood, but probably is due to 

 a combination between the paracasein and the lactic acid, the resulting 

 compounds differing from the paracasein in physical properties and in 

 solubilities. 



RIPENING OF CHEESE. Cheese in ripening undergoes profound 

 physical and chemical changes under the influence of a number of 

 factors, which for purposes of discussion may be divided into two groups : 

 those by which the content of soluble nitrogen in the cheese is increased 

 and the digestibility enhanced; and those which cause the formation of 

 flavoring substances. During the ripening of the cheese the maker 

 can do little toward the control of the factors which ultimately deter- 

 mine its commercial value. As in butter, the flavor is the most impor- 

 tant characteristic of the ripened cheese and the most difficult to 

 control. 



Theories of Cheese Ripening. Many theories have been advanced 

 to explain the changes that occur during the ripening process. Duclaux, 

 a French microbiologist, studied the bacterial flora of Cantal cheese 

 by aid of the crude methods available before the introduction of the 

 gelatin-plate method. By the use of the dilution method, using 

 bouillon as the nutrient medium, he isolated a number of kinds of 

 spore-forming bacteria. The organisms formed two enzymes, one 

 a curdling enzyme related to rennin, the other a proteolytic enzyme 



* Cheddar cheese. 



