22 MARKET DAIRYING 
NORMAL FERMENTATIONS. 
We speak of normal fermentations because milk always 
contains certain classes of bacteria even when drawn and 
kept under cleanly conditions. These fermentations will 
be discussed in the following pages. 
I. LACTIC FERMENTATION, 
This is the most common and by far the most important 
fermentation of milk. Indeed it is indispensable in the 
manufacture of butter of the highest quality. The germ 
causing this fermentation is called Lactici Acidi. It is 
non-spore bearing and has its optimum growth tempera- 
ture between go° and 98° F. At 40° its growth ceases. 
Exposed to a temperature of 140° for fifteen minutes 
it is killed. 
The souring of milk and cream, as already mentioned, 
is due to the action of the lactic acid bacteria upon the 
milk sugar changing it into lactic acid. Acid is therefore 
always produced at the expense of milk sugar. But the 
sugar is never all converted into acid because the pro- 
duction of acid is limited. When the acidity reaches 
about .g% the lactic acid bacteria are either checked or 
killed and the production of acid ceases. Owing to the 
universal presence of these bacteria it is almost impossible 
to secure milk free from them. 
Under cleanly conditions the lactic acid type of bacteria 
always predominates in milk. When, however, miik is 
drawn under uncleanly conditions the lactic organisms 
may be outnumbered by other species of bacteria which 
give rise to the numerous taints often met with in milk. 
Contradictory as it may seem, the lactic acid bacteria 
are alike friend and foe to the butter maker. Creamery 
