248 BACTERIA IN MILK AND MILK PRODUCTS 



to the lactic acid group. This commences in green cheese about the 

 eighth day, and continues more or less for twenty days. In Cheddar 

 cheese it commences about the fifth day, reaches its maximum about 

 the twentieth day, declines rapidly to the thirtieth day, and gradu- 

 ally for a hundred following days. During the first forty days of 

 this period the casein-digesting and gas-producing organisms are 

 present, and at first increasing, but relatively to only a very slight 

 degree. With this rapid increase in organisms the curd begins to 

 lose its elastic texture, and before the maximum number of bacteria 

 is reached the curing is far advanced. Freudenreich has shown that 

 acid inhibits the growth of the casein-digesting microbes, and vice 

 versa. 



3. Period of Final Bacterial Decline. The cause of this decline 

 can only be conjectured, but it is highly probable that it is due to a 

 general principle to which reference has frequently been made, viz., 

 that after a certain time the further growth of any species of 

 bacteria is prevented by its own products. "We may observe that 

 the gas-producing bacteria in Cheddar cheese last much longer than 

 the peptonising organisms, for they are still present up to eighty 

 days. Professor Russell aptly compares the bacterial vegetation of 

 cheese with its analogue in a freshly-seeded field. " At first multi- 

 tudes of weeds appear with the grass. These are the casein-digesting 

 organisms, while the grass is comparable to the more native lactic 

 acid flora. In course of time, however, grass, which is the natural 

 covering of soil, ' drives out ' the weeds, and in cheese a similar con- 

 dition occurs." In milk the lactic acid bacteria and peptonising 

 organisms grow together; in ripening cheese the former eliminate 

 the latter. 



Artificial Ripening. We have seen that the conclusion generally 

 held respecting these lactic acid bacteria is that they are the main 

 agents in curing the cheese. Upon this basis a system of pure 

 " starters " has been adopted, the characteristics of which must be 

 as follows : (a) The organism should be a pure lactic-acid-producing 

 germ, incapable of producing gaseous products ; (&) it should be free 

 from any undesirable aroma ; (c) it should be especially adapted for 

 vigorous development in milk. The " starter " may be propagated in 

 pasteurised or sterilised milk from a pure culture from the labora- 

 tory. The advantages accruing from the uses of this lactic acid 

 culture, as compared with cheese made without a culture, are that 

 with sweet milk it saves time in the process of manufacture ; that 

 with tainted milk, in which acid develops imperfectly, it is an aid to 

 the development of a proper amount of acid for a typical Cheddar 

 cheese ; and that the flavour and quality of such cheese is preferable 

 to cheese which has not been thus produced. Professor Russell is of 

 opinion that the lactic acid organisms are to be credited with greater 



