KINDS OF FERMENTATION 153 



proteolytic ferment (Lehmann). The production of proteolytic 

 ferments fluctuates with many, perhaps with all, bacteria in a greater 

 degree than might be supposed. Upon albuminous media lique- 

 f\ing bacteria produce bitter-tasting metabolic products, as Hueppe 

 pointed out in the case of milk. 



3. Steatolytic ferments split fats into fatty acids and glycerine. 

 Steapsin of the pancreatic juice is an example. 



4. Inversive ferments are those which convert cane sugar, maltose, 

 and lactose into glucose. In this class is invertin of the intestinal 

 juice, and yeast. According to Fermi and Montesano, B. mega- 

 therium, B. fluorescens liquefaciens, B. vulgare, and several other 

 species produce inversive ferments. 



5. Coagulative ferments are such as convert soluble into in- 

 soluble proteids.-^ Rennet is an example ; and bodies like it, which 

 coagulate milk of neutral or amphoteric reaction unconnected with 

 the action of acid, are not lacking among the products of bacteria 

 {e.g. B. prodigiosus). 



Lehmann states that the presence of rennet ferment may be 

 suspected in all varieties of bacteria which coagulate milk without 

 being able to form lactic acid from milk sugar. Fibrin ferment 

 and myosin ferment are other examples of coagulative fermenta- 

 tion. 



From what has been said, it will be seen that enzymes may 

 be the products of bacteria and yeasts as well as of highly specialised 

 cells.^ 



We may now turn to what are designated organised ferments, 

 although the term is not wholly a suitable one, and we shall confine 

 ourselves to such fermentations set up in milk. They may be 

 classified in five main divisions as follows : — 



1. Lactic acid fermentation. 



2. Butyric acid fermentation. 



3. Coagulative fermentations. 



4. Alcoholic fermentations. 



5. Diseases of milk. 



1. Lactic Acid Fermentation : the Souring- of Milk -^ 



As every one knows, milk readily " turns sour." When so 

 changed it becomes semi-solid and acid. This ordinary common 

 " souring " of milk is generally due to the lactic acid ferment. 

 It does not occur spontaneously, nor is it a necessary and invari- 



' Buchner has proved that fermentation can be produced by adding the 

 expressed ferment of yeast cells alone to sugar solutions. 



