800 
The Fermentations of Mill-. 
It is well to note that abnormal qualities in the milk which are 
really due to something in the food consumed — such as the flavours of 
garlic or turnip — are readily distinguished from changes brought about 
by bacteria by the mere fact that they are most noticeable when the 
milk is freshest, whereas it is of the essence of bacterial or fer- 
mentative change to commence some time afterwards, and to attain 
a maximum more or less gradually. 
Let us glance now at some of the best known of the fermenta- 
tive changes which occur in milk under diflferent circumstances. 
The most familiar and universal change is the curdling or preci- 
pitation of the casein, which invariably occurs on keeping milk ex- 
posed to the air. Casein can be precipitated, or caused to take the 
insoluble form, by a great variety of purely chemical reagents, which, 
indeed, act on other soluble albuminoids in a similar manner. 
Amongst these are the mineral acids and the stronger organic 
acids, which act simply in virtue of the fact that casein cannot 
retain the soluble form in presence of an acid, unless the latter be 
very small in amount.* The real cause of the curdling of sour milk 
is the production in it of lactic acid in sufficient quantity, and this 
acid is produced by transformation of the milk sugar contained in 
the milk. Milk sugar, indeed, has the same percentages of the 
elements carbon, hydrogen, and oxygen as lactic acid, so that the 
transformation is purely a molecular one. That this transformation 
is effected by the growth of a bacterium in the milk has been long 
admitted, also that the multiplication of the bacteria continues 
until the lactic acid produced is sufficient in quantity to act as an 
antiseptic and prevent their further growth. In the same way we 
see some specimens of silage produced so rich in lactic and perhaps 
other acids as to resist the growth of the common mildews or 
moulds, even though kept exposed to the air. 
The first experimenter to obtain a pure cultivation of an organism 
producing a lactic fermentation in milk was Sir J oseph Lister, who 
in 1873 isolated and studied a bacterium which he called Bacterium 
lactis. This organism he found to be common around the dairy, but 
not anywhere else, even in the barn. Hence the rather surprising 
result that sterilised milk exposed to the air, in any place away 
from a daiiy, will not turn sour or usually curdle, although it will 
eventually undergo some other form of fermentation. The fact 
that ordinary milk will turn sour wherever it is kept is therefore 
a consequence of the lactic bacterium inevitably gaining access to it 
before it leaves the dairy. 
But it was soon to be shown that Lister’s bacterium is not the 
only one having the power of forming lactic acid from milk sugar. 
Hueppe in 1884 made a thorough study of a bacterium which he 
found to be the most common one in milk, called by him Bacterium 
* According to recent experiments, milk containing as little as two parts 
of lactic acid in a thousand will curdle on being boiled ; it often acquires as 
much as this ten hours after milking. 
