i8o Report of the Chemical Department of the 



(g) Traces of putrescine. (g) No putrescine. 



(3) Analysis of cheese. (3) Analysis of cheese. 



(a) Ammonia formed. (a) No ammonia formed. 



(b) Amido compounds (b) Amido compounds less 



more abundant. abundant. 



DISCUSSION OF RESULTS. 



We have seen above that results varying in a most marked 

 manner were obtained from the two cheeses used in our investi- 

 gation. We will now consider some of these differences with a 

 view to finding some satisfactory explanation of the facts pre- 

 sented. 



THE SOURCES OF CARBON DIOXIDE IN CHEESE. 



What was the source of the carbon dioxide produced in each 

 cheese? Why did the normal cheese produce relatively so large 

 quantities of carbon dioxide over so long a period of time and 

 why did the chloroformed cheese produce so small quantities and 

 for so brief a period? 



As possible sources of carbon dioxide in cheese, we have (i) 

 the milk used in making cheese, (2) the decomposition of milk- 

 sugar in the formation of lactic acid, (3) the respiration of living 

 cells present in the cheese and (4) the chemical decomposition of 

 compounds present in the cheese. We will consider these 

 separately. 



(i) Milk as a source of carbon dioxide in cheese. — According 

 to Marshall,^ fresh milk, before exposure to air, contains on an 

 average about 4 per ct. of free carbon dioxide by volume and 

 this is reduced one-half by aeration. In the amount of milk 

 used by us in making each cheese, we should have about 0.800 

 gram of carbon dioxide. Some of this is of necessity lost in the 

 process of cheese-making, but we could expect to retain in the 

 cheese 0.200 to 0.300 gram of the carbon dioxide originally 

 present in the milk. 



(2) The decomposition of milk-sugar as a source of carbon 

 dioxide in cheese. — -E. Kayser^ has shown that certain lactic acid 

 bacteria produce, as the result of their action on milk-sugar, not 



8 Special Bull, No. 16, Mich. State Agr. Coll. Exp. Sta. (1902). 



9 Ann. Past., 8: 779 (1894). 



