2^72 SCIENCE AND PRACTICE OF CHEESE-MAKING 



investigation for years, to the neglect of all other 

 possibilities. The general statement of this theory is 

 that the changes observed in proteins during the 

 cheese-ripening process are caused by the direct action 

 of micro-organisms. This has appeared in many dif- 

 ferent forms according to the particular kind of micro- 

 organisms to which the work was attributed. Of the 

 different micro-organisms assigned as the cause of 

 cheese-ripening, we can mention only one, the lactic 

 acid organisms. Freudenreich has been the most 

 prominent champion of this explanation of the 

 changes in cheese-ripening, and he devoted years 

 of investigation to the lactic acid organisms. In 

 favor of this particular theory, we have the fol- 

 lowing facts: (i) The lactic acid species of bacteria 

 are abundant from the start and increase in num- 

 bers enormously for some time, suppressing the 

 growth of those bacteria that are known to have 

 the power of transforming milk-casein and the 

 paracasein of cheese-curd into soluble products. 

 (2) There is a coincidence in time between the 

 early marked advance in the formation of soluble 

 proteins and the period of bacterial increase. Against 

 this theory we have the following facts : ( i ) The 

 lactic acid bacteria that are most useful in cheese- 

 making have not been satisfactorily shown to have 

 the power of changing milk-casein or paracasein into 

 soluble products. (2) Ammonia is found at an early 

 stage of cheese-ripening, but it has not been proved 

 that lactic acid organisms produce ammonia. (3) A 

 large proportion of the chemical changes in cheese 

 proteins appear after the lactic acid bacteria have 

 greatly decreased in number. This has been explained 

 by saying that the bacteria secrete an enzym which 



