New York AGRICULTURAL EXPERIMENT STATION. 169 
INTRODUCTION. 
The chemical changes taking place during the early stages of 
manufacture of cheese have hitherto been little investigated or 
understood. This is particularly true of the kind of cheese most 
extensively made in the United States and Canada, commonly 
called American cheddar cheese. One of the first recognized 
changes occurring in the process of cheese manufacture is that 
produced by the action of rennet on milk-casein, resulting in the 
formation of an insoluble curd. The action of rennet, according 
to Hammarsten’s explanation, splits milk-casein into two differ- 
ent compounds!,— one, the insoluble curd, called paracasein, 
formed in large proportions; the other soluble, albumose-like, 
called whey proteid, formed in small proportions. While Ham- 
marsten’s view is still held by many, there is growing evidence? 
that the albumose-like proteid found in whey is not a result of 
the action of rennet, but that it can be produced by actual proteo- 
lytic digestion of casein, due to enzymes either present in the 
milk itself or introduced with the rennet (as pepsin or Glaessner’s 
pseudo-pepsin). Whatever we may uitimately find to be the 
exact nature of rennet action on milk-casein, it is not the only 
factor at work during and after the formation of the insoluble 
curd; for there is present at least one other active process during 
the early stages of cheese-making that has much to do with the 
production of a “good curd”, judged from the practical stand- 
point of a cheesemaker. 
One of the first operations in cheddar cheese-making is to 
“ripen ” the milk previous to adding rennet. This is accom- 
plished by allowing the milk to stand some time at a temperature 
of about 86° F. (30° C.), or the operation may be hastened by 
adding to the milk special cultures of acid-producing organisms. 
This process, commonly known as the “ripening of milk,” has 
for some time been recognized as an acid-producing form of 
fermentation, that is, a fermentation brought about by the action 
of lactic-acid organisms on the milk-sugar, resulting in the forma- 
tion of lactic acid. In ripening milk, the cheesemaker increases 
1Maly’s Ber., 2: 118 (1872); 4: 1385 (1874). 
2Puld. Beitrage zur Chim. Physiol., 2: 169 (1902). 

