s in that, while the characteristic
disinfecting action of the former is retained, any
secondary action of the growth of this organism upon the
milk-fat is checked by the growth of _Bacillus
paralacticus_, thus ensuring the production of a more
palatable product.]

CHAPTER VI

THE PREPARATION OF SOURED MILK IN THE HOUSE

There is no great difficulty in making soured milk at home: the
necessary operations are quite simple, but at the same time they must be
conducted with precision and care, otherwise the results may be
unsatisfactory and disagreeable; there may even sometimes be danger in
badly prepared sour milk. It is always an advantage in such matters to
understand the reason of things, and a few notes on the surrounding
conditions, and what has to be accomplished, may be of assistance to the
would-be experimenter.

The majority of intelligent people are now acquainted with the fact that
the germs of bacteria are to be found everywhere on the surface of the
earth, in air, and in water, and that they are the sole cause of the
decay of all manner of perishable articles.

The distribution is unequal--bacteria are much more plentiful where
there is decaying matter--in dirty houses, sewage, or other contaminated
water, etc. Milk is a splendid food for bacteria, and numerous varieties
multiply in it exceedingly, and many of these are injurious, producing
putrefactive changes which render the milk unwholesome, even poisonous
in some cases. Others are beneficial, and are absolutely necessary for
the souring of milk for making butter or cheese and for the ripening of
the latter. The soured milk which is the subject of this book is the
work of certain lactic-acid-producing bacteria, and the problem we have
before us is to encourage the growth of the latter to the uttermost and
to exclude the others.

As bacterial germs are present in the air and readily sow themselves
into any medium with which they come in contact, the first consideration
is to get good fresh milk which has been as little exposed to the air as
possible. The second is to conduct the experiment where the germs are
fewest, and in cleanly surroundings, far removed from decaying matter
and free from taints and smells.

[Illustration: FIG. 38.--Photo-micrograph of smear of
one-month culture of _Bacillus bulgaricus_. In spite of
its age, the culture is perfectly free from any foreign
organisms, which would otherwise lower its valu