small amount of original yeast begins to supply itself with food at once by converting the starch into dextrine, and then into grape sugar, and multiplies itself with great rapidity, and will continue to do so as long as there 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.  While a cool temperature is one of the requisites for keeping yeast fresh, care must be taken, especially in winter, that it does not get chilled.

When yeast is needed for bread, it is always the best plan to take a cup to the cellar or refrigerator for the desired quantity, and re-cover the jar as quickly as possible.  A half hour in a hot kitchen would be quite likely to spoiled it.  Always shake or stir the whole well before measuring out the yeast.  In making yeast, used earthen bowls for mixing, porcelain-lined or granite-ware utensils for boiling, and silver or wooden spoons for stirring.