ere is material to supply it with the means of growth. While
its growth is rapid, its decay is equally so; and unless some means of
preservation be employed, the yeast will die, and the mixture become
sour and foul. Ordinarily it can be kept good for several days, and
under the best conditions, even three or four weeks. After it has been
kept from four to six hours, it should be placed in some receptacle as
nearly air-tight as possible and set in the cellar or refrigerator,
where it can be kept at a temperature not conducive to fermentation.
Thus the little yeast organisms will remain in a quiescent state, but
yet alive and capable of multiplying themselves when again surrounded
with favorable conditions.

The yeast should be kept in glass or glazed earthen ware. The vessel
containing it should be washed and scalded with scrupulous care before
new yeast is put in, since the smallest particle of sour or spoiled
yeast will ruin the fresh supply in a very short time. It is generally
conceded that yeast will keep longer if the material of which it is made
be mixed with liquid of a boiling temperature, or cooked for a few
minutes at boiling heat before adding the yeast. The reason for this
undoubtedly lies in the fact that the boiling kills foreign germs, and
thus prevents early souring or putrefaction. The yeast must not be
added, however, until the liquid has cooled to a little more than blood
heat, as too great heat will kill the yeast cells.

The starch of the potato is thought to furnish better material for the
promotion of yeast growth than that of wheat flour; but whether the
potato be first cooked, mashed, and then combined with the other
ingredients, or grated raw and then cooked in boiling water, makes
little difference so far as results are concerned, though the latter
method may have the advantage of taking less time. If potatoes are used
for this purpose, they should be perfectly mature. New ones will not
answer.

Sugar assists in promoting the growth of the yeast plant, and a small
amount is usually employed in making yeast. Hops serve to prevent the
yeast from souring, and an infusion of them is frequently used for this
purpose.

While it is essential that the water used should be boiling, it is also
necessary that the mixture should cooled to a lukewarm temperature
before the introduction of the original yeast, as intense heat will kill
the yeast plant. Freezing cold will likewise produced the same result