igoo-i.] The Ripening of Cheese. 131 



1. The enormous number of lactic acid bacteria in hard cheese, and 

 the very small numbers of liquefying or digesting bacteria. 



2. The existence of galactase, a natural enzyme inherent in fresh 

 milk. 



3. The ability of rennet to cause the change of non-soluble nitro- 

 genous products to soluble ones. 



If we grant that these three facts are proved, and we may safely do 

 so, our inquiry into the cause of the ripening of cheese will be somewhat 

 simplified. 



The lactic acid bacteria seem to be able to cause an increase in the 

 amount of soluble nitrogenous products in the casein of milk (de Freud- 

 enreich). Klein and Kirsten also state that normal cheese may be 

 made from pasteurised milk (hence free from enzymes) with the aid of 

 starters. Russell, before the discovery of galactase, stated " that the 

 addition of a pure lactic acid ferment to the pasteurised milk permits 

 the usual changes to occur in a perfectly normal way." 



On the other hand, Boekhout and Vries were unable to produce 

 normal* cheese (Edam) made from aseptic milk with the addition of a 

 culture of lactic acid bacteria, but at the same time they admit that 

 perhaps some other variety of lactic acid bacteria might bring about the 

 ripening changes. 



Chodat and Bang grew lactic acid bacteria on coagulated casein, but 

 the quantity of soluble nitrogen in this mass did not increase ; so that 

 taking into account these facts, we are bound to admit that there exists 

 more or less doubt as to the ability of the lactic acid bacteria to alone 

 bring about an increase in the amount of soluble nitrogen. 



Babcock and Russell's discovery of galactase led them to consider 

 that the " breaking down of the casein was due in larger part to the 

 action of this enzyme " ; in fact, they attribute to galactase the principal 

 role in the ripening of cheese. Both de Freudenreich and Jensen con- 

 firmed the presence of this enzyme in milk, but they do not consider 

 that it is the all-important factor in the curing process. Boekhout and 

 Vries completely deny its ability to ripen cheese, and Klein and Kir- 

 sten's experiments show that soft cheeses ripen normally, even when 

 made from milk in which the enzyme has been destroyed by heat. 



My experiments show that the amount of acid present in Canadian 

 Cheddar cheese is sufficient to inhibit and perhaps altogether stop its 

 action in cheese ; for, if as shown b}' de Freudenreich, 0.5 per cent, of 



