230 PRACTICAL DAIRY BACTERIOLOGY 



When cheese ripening was first studied by Duclaux, it was be- 

 lieved by him that the ripening was primarily the action of 

 bacteria. We have already seen that certain kinds of bacteria 

 have the power of changing casein to peptone the liquefying 

 type (see page 40) and Duclaux, finding such organisms 

 more or less abundant in cheeses, reached the conclusion that 

 the change from casein to peptone in the cheese ripening was 

 due to the growth of these peptonizing bacteria. This he argued 

 strongly and held for many years, supporting his position by 

 the claim that when treated by heat or antiseptics, so as to 

 destroy bacteria, the bacteria would not ripen. Later the fact 

 was more and more clearly seen that the liquefying bacteria 

 are not common in cheeses, especially in the better grades. 

 If present at the beginning they rapidly decrease in numbers 

 and come to be very few, if not absent entirely, a fact which 

 forced the conclusion that they cannot contribute materially 

 to the ripening of cheeses. More recently, Gorini has advanced 

 the view that certain "acid liquefiers" i.e., peptonizing bac- 

 teria that at the same time produce acid are intimately con- 

 nected with the ripening. 1 But there does not yet appear to be 

 much evidence for this. 



These facts led to a suggestion by Freudenreich that the 

 ripening is due really to the lactic acid bacteria. 2 Lactic acid 

 bacteria, as we have already seen, do not liquefy gelatin and do 

 not ordinarily have any power of changing casein to peptone. 

 As they grow in the milk, they produce lactic acid which curdles 

 the milk, after which they apparently cease to act upon it at all. 

 Hence, it would not seem that they could digest cheese. Freu- 

 denreich, 3 however, showed that if the acid which they produce 



1 Gorini. Cent. f. Bact., II., viii., p. 137, 1902. 

 Gorini. Rev. Gen. d'Lait, iii., pp. 289, 405, 1904, 



2 Freudenreich. Tirage special d. 1'Ann. agricole de la Suisse, 1905. 



3 Freudenreich. Rev. Gen. d'Lait, I., p. 112, 1901. 

 Freudenreich. Land. Jahr. d. Schweiz, 1897, 1899. 

 Freudenreich and Thoni. Cent. f. Bact., II., xiv., p. 34, 1905. 



