104 EXPERIMENTAL DAIRY BACTERIOLOGY 



it. This effect is due to the presence in the rennet extract 

 of the enzyme rennin, or caseinase, according to modern termi- 

 nology. Rennet extract also contains pepsin (acid proteinase), 

 which exerts its peculiar action when acid is present, as is 

 normally the case in cheese, due to the action of the lactic- 

 acid bacteria on the milk sugar of the whey. An increase 

 in the amount of rennet used in cheese making has been 

 found to hasten the rate at which the water-soluble products 

 are formed, and hence the rapidity of ripening. 



In addition to pepsin there is an enzyme normal to milk, 

 known as galactase (alkali proteinase), which also exerts a 

 proteolytic action on casein or paracasein. What r61e the 

 bacteria play in the gradually increasing solubility of the 

 curd is not definitely determined. 



Role of acid-forming bacteria. In cheddar cheese the 

 initial stages of ripening are closely related to the develop- 

 ment of the lactic-acid group of bacteria, as indicated in 

 the following experiment. 



Exercise. Melt two tubes of lactose agar, cool to 50 C., and add 

 to each tube about 15 per cent of sterile skim milk, which is well 

 mixed with the melted agar. If the milk is added to the hot agar, 

 the casein of the milk will be precipitated in flocks. Pour one tube 

 of the mixed agar and milk into a sterile Petri dish and allow it to 

 solidify. Inoculate the remaining tube heavily from a pure culture 

 of a lactic-acid organism and pour into a Petri dish. Keep both 

 plates for twenty -four hours at temperatures favorable for the growth 

 of the lactic-acid organism used. At the end of the period of incu- 

 bation place on the surface of each plate a strip of filter paper mois- 

 tened with rennet extract. Incubate the plates at 37 C. for from two 

 to four hours. At the expiration of this period the following changes 

 are to be noted : On the uninoculated plate the opacity produced by 

 the milk is increased in the immediate neighborhood of the paper, 

 due to a physical change in the casein. On the inoculated plate the 

 casein is changed to soluble compounds, and the opacity is thus de- 

 stroyed in the immediate neighborhood of the paper. Between the 



