BACTERIA AND MILK FERMENTATIONS 21 
which an exceedingly varied and active life is possible. 
In nature and in many of the arts and industries, 
bacteria are of the greatest utility, if not indispensable. 
They play a most important part in the disintegration of 
vegetable and animal matter, resolving compounds into 
their elemental constituents in which form they can again 
be built up and used as plant food. In the art of butter 
and cheese making bacteria are indispensable. The to- 
bacco, tanning, and a host of other industries cannot 
flourish without them. 
II. MILK FERMENTATIONS. 
Definition. In defining fermentation processes, Conn 
says that, “In general, they are progressive chemical 
changes taking place under the influence of certain 
organic substances which are present in very small 
quantity in the fermenting mass.” 
With few exceptions, milk fermentations are the result 
of the growth and multiplication of various classes of 
bacteria. The souring of milk illustrates a typical fer- 
mentation, which is caused by the action of lactic acid 
bacteria upon the milk sugar breaking it up into lactic 
acid. Here the chemical change is conversion of sugar 
into lactic acid. 
The most common fermentations of milk are the fol- 
lowing: 
Lactic. 
[ Normal...:. { Curdling and Digesting. 
{| Butyric. 
Milk Fermentations Pee: 
Slimy or ‘Ropy. 
Abnormal... 4 Gassy. 
| Toxic. 
| Chromogenic. 
