561 ANNUAL REPORT OF THE Off. Doc. 



and not because the sui)ply of railk-sngar rung out, for, when they 

 cease their activity, about three-quarters of the milk sugar still 

 remains uuconsumed. There are several different kinds of bac- 

 teria tliat can convert milk sugar into lactic acid. The tempera- 

 ture most favorable to the growth of lactic acid organisms in 90 

 degrees F. to 95 degrees F. Below 80 degrees F. they gradually 

 lose their activity and practically cease at 50 degrees F. At 105 

 degrees F. the}' are fairly inactive, many are killed at 135 degrees 

 F. to 140 degrees F., and at 150 degrees F. to IGO degrees F. all are 

 killed. While this fermentation spoils milk for the taste of most 

 people, it is a very essential factor in the manufacture of butter 

 and cheese, as will be seen later. It is quite commonly thought that 

 milk is peculiarly liable to sour during thunder storms, as the 

 result of some peculiar electrical or other influence. The hot 

 weather preceding such storms favors the more rapid growth of the 

 lactic acid germs, and this is the proper explanation. Wi\k free 

 from such germs never sours during thunder storms. 



Some of the bacteria that act upon milk-sugar form large quan- 

 ities of gases, especially carbon dioxide and hydrogen, and these 

 gases are responsible in cheese-making for "floating'' curds and 

 "huffing" cheese. 



(2.) Fermentations Affecting Milk-Casein. — Occasionally milk- 

 curdles without souring. This is caused by the action or rennet-like 

 enzymes or ferments which certain bacteria produce. The samj 

 bacteria usually produce a second enzyme, which dissolves the coagu- 

 lated casein. Some bacteria form only the dissolving or digesting 

 enzyme, in which case the caseio is slowly rendered soluble without 

 previous coagulation. The bacteria that affect casein do not grow 

 in the presence of lactic acid, and as the lactic acid bacteria usually 

 develop more rapidly, they soon stop the growth of the former. 

 In the absence of lactic acid bacteria, the digesting organisms are 

 apt to grow. There are other forms of fermentation that act upon 

 milk-casein and result in the production of substances having ex- 

 tremely offensive odors and disagreeable taste. In some cases 

 poisonous products are formed, as, for example, tyrotoxicon, com- 

 monly known as cheese poison. 



(3.) Butyric Acid, or "Rancid" Fermentations. — Some organisms 

 act upon milk-fat op butter-fat, forming free butyric acid, thus pro- 

 ducing the odor and taste of "rancid" butter. These bacteria grow 

 slowly under ordinary conditions and do not commonly become ac- 

 tive in milk, unless kept a long time, but they find their way into 

 buiter and, under favorable conditions, develop there to the dis- 

 advantage of the flavor of the butter. 



