Commercial Compressed Yeast is obtained from distillery fermenting vats by skim ming or separating from the "wort" and then cleansing, etc., or by sowing selected yeast cells in vats filled with a mixture of malt and rye or corn, or boiled potato mash, etc., and water, held at a moderately warm temperature. Boiling the water with a small quantity of hops, followed by straining, frequently precedes the addi tion of the malt or potato mash, partly for the agreeable flavor but principally to retard the growth of any bacteria present. As the cells multiply, they collect in a thick coat ing on the surface. This is skimmed off from time to time, washed, freed from part of the water and made into the soft, rather soggy cakes sold in such enormous quanti ties for both household and bakers' use. When fresh, nearly all the cells will be found alive and vigorous—there are millions of them in each cake, mixed with starch, etc.— but after two or three days many of them die and the yeast will show less and less vigor. In time, bacteria, another form of micro-organism, will develop in the cake and spoil its flavor. It is consequently best to use Compressed Yeast while perfectly fresh. If this is impossible, its life may be prolonged by placing the cakes in cold water and setting in the ice chest. It should never be exposed to temperature below the freezing point.
Next after Compressed Yeast in strength and utility for bread making, is Brewer's Yeast, the brownish frothy top yeast from the fermenting vats of ale or beer. It answers the same purpose, but is not so vigorous, and sometimes gives a slightly bitter taste.
Beer yeast is also used to a limited extent by physicians. It makes an appetizing "Bouillon," somewhat resembling beef tea, and is given as a stimulant in low fevers when inflammatory symptoms make the use of wine inadvisable.
Dried Yeast, in cakes or powder, is Compressed Yeast dried at low heat. The process kills some of the plants and thus lessens the vigor of the yeast, but a good many are left in a dormant condition and the advantage is that it will, under suitable conditions, keep for several weeks, and sometimes months. It must, however, be handled with care, as its vitality is destroyed or lessened by falls, bruises, etc., and by excessive heat or. cold.
The live cells of Dried Yeast begin growing again when moistened. The best plan is to put the cake or powder in a little sweetened warm water shortly before using.
Both Compressed and Dried yeasts vary in purity and hence in value, so that it is advisable to purchase from concerns of recognized experience and integrity.