butable to Anthony Van Leeuwenhoek, a Dutchman, whose vocation was
the polishing of lenses, and who lived between 1632 and 1723, altered
our point of view, not only of disease, but of all the functions carried
on by the lower organisms. Since Van Leeuwenhoek's time, the germ theory
has grown to vast proportions and has more especially been applied with
splendid results to the study of milk. As we shall see later, the
researches of modern investigators have led them to the conclusion that
micro-organisms play such an important part in the milk supply, that it
is impossible to carry it on safely without a knowledge of the
bacteriology of the subject. This view began to prevail about 1890, some
twenty years after Pasteur had shown what fermentation really meant.
Since that time, the progress in dairying has been continuous, and,
during recent years, attention has been directed to soured milk to such
an extent that it has become necessary for all who are interested in the
handling of milk and milk products to have a knowledge of the subject,
as it seems clearly demonstrated that, under proper direction, there is
every possibility of its forming an important element in the
prolongation of life.

CHAPTER II

FERMENTED MILKS

There is considerable variety in the number of soured or fermented
milks, and they are known by various names, such as Koumiss or Koomiss,
which is prepared from mares' milk; Keffir, which was originally
discovered in the mountains of the Caucasus, and which is prepared with
Keffir grains; Leben, an Egyptian product prepared from the milk of the
buffalo, cow, or goat; Matzoon, a soured milk which is prepared in
Armenia from ordinary cows' milk; Dadhi, an Indian preparation from
cows' milk. All of these owe their special characteristics to the fact
of their having undergone lactic and alcoholic fermentation.

"Milk left to itself," says Blyth,[14] "at all temperatures above 90 deg.
F. begins to evolve carbon dioxide, and this is simply a sign and result
of fermentation. If this fermentation is arrested or prevented, the fluid
remains perfectly sweet and good for an indefinite time. Besides the
production of carbon dioxide during decomposition, a certain portion of
milk sugar is converted into lactic acid, some of the casein and albumen
are broken up into simpler constituents, and a small proportion of
alcohol produced, which by oxidation appears as acetic acid, while the
fat is in part separate