se of these when
they preponderate, or when, through the use of bad cultures, the
lactic-acid-producing bacteria are absent, or present only in small
numbers. By the thorough boiling of the milk, we get rid of all living
bacteria and nearly all spores or germs, and by scrupulous cleanliness
in the vessels used--scalding or even boiling them, and allowing them to
dry naturally in an inverted position--we greatly diminish the
probability of infection with fresh injurious germs from the atmosphere.
All depends on the purity of the culture employed; this can now be
obtained, prepared in the most careful manner, from wholesale chemists
making a speciality of its manufacture.

As we have seen, even the Bulgarian bacillus is ultimately killed by the
products of its own activity, and the natural corollary is, that the
life of cultures cannot be a long one. Only those cultures should be
bought which are labelled with the date to which they are guaranteed to
maintain their efficiency. With fresh good milk, careful boiling,
scalding, and cleanliness with regard to the containing vessels, and the
means of maintaining the incubating temperature for ten or twelve hours,
there is not the slightest difficulty in preparing perfectly reliable
soured milk. There are simple tests which will sufficiently guide the
experimenter; the soured milk should not be too acid to the taste, and
it has a flavour of its own by which its quality can be recognised. The
litmus test-papers mentioned in the chapter on the chemistry of milk are
very useful; both the red and the blue papers should be obtained and
used first of all in testing the quality of the fresh milk. After
incubation the soured milk should turn the blue paper decidedly red; if
this does not occur, test it with the red paper; if the latter turns
blue it is proof that the wrong fermentation has taken place--that
putrefactive germs have gained the upper hand. The most probable
explanation is, that the culture is bad, the Bulgarian bacillus is not
present, or if so, only in small numbers. With these simple tests,
combined with proper care, one cannot go far wrong. The Eastern nations
who prepare soured milk in various forms do not exercise the care we
have predicated, but they seem to make the article of fairly constant
and good quality. It has been suggested in explanation that, as the
ordinary flora differs in different countries, the bacterial flora
varies in a similar manner, and that in