The Fermentations of Milh. 
803 
The rennet ferment has been found in other places besides the 
stomach of mammals. Roberts has found it in birds, Benger in 
fishes, and other observers in a variety of plants. Duclaux and 
others have found it produced by the growth of many bacteria. 
Before turning to some of the minor fermentations of milk, it 
will be well to get an idea of the number of bacteria to be found in 
it under various circumstances. In the udder it contains none : it 
is possible to withdraw the milk into a vessel still containing none. 
On the other hand, it has sometimes been found to contain a million 
and a half in a cubic inch only a few minutes after milking. Six 
hours afterwards there may be sixty times this quantity. Whatever 
be the number to start with, the temperature at which the milk 
is kept will have an enormous influence on the rate of multiplica- 
tion. 
In a sample of milk kept at 60 deg. F. there was no perceptible 
increase in the bacteria on one hour’s standing ; in the same sample 
kept at 95 deg. the increase was 7-i-fold. On standing six hours 
at the two temperatures the bacteria had increased 435 times and 
.3,800 times respectively. The souring of milk so frequently observed 
during a thunderstorm is attributed, as the result of the experi- 
ments that have been made on the subject, not to any direct 
electrical influence, but simply to the sultry atmosphere which 
generally precedes a thunderstorm favouring the rapid multiplica- 
tion of bacteria. It is found that milk properly refrigerated or kept 
surrounded by cold water does not sour any more quickly during a 
thunderstorm. 
Butyric acid fermentation is sometimes set up in milk, giving 
it the odour of rancid butter. The butyric acid is, however, produced 
from a diflerent source and by different means in the two cases. In 
milk it is the sugar which undergoes this butyric fermentation, 
under the influence of quite a number of different microbes. Most 
of them agree amongst themselves and differ from the lactic ferments 
in growing best in the absence of air or oxygen — that is, they are 
anaerobic microbes. In other respects they differ much one from 
another. Some render the milk acid, and some alkaline ; some 
produce gas, and some do not, and the by-products differ in different 
cases. One of the best known of them, the Bacillus hutyricus of 
Prazmowski, produces spores that are not killed in boiling milk, and 
hence the fact that milk once boiled in a plugged flask generally 
develops butyric acid after a time, although the lactic fermentation 
and consequent curdling have been prevented. 
The production of butyric acid in rancid butter is quite a 
different matter. Here the source is the butyrin ' of the butter fat, 
which gradually splits up into butyric acid and glycerin from causes 
which seem to have nothing to do with bacterial growth. Butter 
fat does not turn rancid if kept from the air. If sterilised butter 
’ Or, at any rate, the mixed glycerides of butyric acid, since many 
chemists deny that butyrin in a separate state exists in butter fat. 
