66 MILK ANALYSIS 



— when the lactic acid is the agent — or by renrtet. The first- 

 named method is mainly applied to the manufacture of so- 

 called Dutch or sour-milk cheese, green Swiss cheese, and 

 cottage cheese. More commonly cheese is obtained by means 

 of rennet derived from the fourth stomach of the calf. The 

 action is due to an enzym which acts directly on the proteids 

 and does not produce its effect through the intervention of 

 acids. The curd (cheese) undergoes, by keeping, various 

 decompositions, some essentially putrefactive, and due to the 

 action of microbes. The decomposition of the cheese is termed 

 "ripening." 



In the sour milk cheeses, ripening is restricted intention- 

 ally, since there is liability to an irregular and miscellaneous 

 bacterial growth by which the fermentations may be carried 

 too far, undesirable and even harmful products being formed. 

 Such cheeses are intended for prompt use. 



Cheese contains no casein, if by this term is meant the proteid 

 as it exists in milk, or when precipitated from milk by acids. 

 When milk is coagulated by rennet, only a part of the proteids 

 enter into the curd; true casein contains about 15.7 per cent, 

 of nitrogen, but the proteid matter of cheese contains about 

 14.3 per cent. Under the process of ripening this is further 

 decomposed, amido- and ammonium compounds, peptones 

 and albumoses being formed. 



The following figures, obtained by Van Slyke, will serve 

 to give some idea of the extent to which the curd is changed 

 in ripening. The figures represent average percentage on the 

 total nitrogen. The cheese was an American cheddar: 



Green Cheese, After Five Months. 



Soluble nitrogen compounds, 4.23 35-52 



" amido " none 11.66 



" ammonium " none 2.92 



Van Slyke's experiments seem also to indicate that the cheese 

 ripened more rapidly when the curd was precipitated by a 



