YEAST

the popular title of those tiny fungi, or micro-organisms, belonging mainly to the Saccharomyces, which rapidly reproduce themselves when immersed in a sugar-solution of suitable temperature, and by their enzymes, or secretions, cause the fermentation of the liquid—ie., the conversion of its sugar into carbon dioxide and alcohol. There are many species and varieties, of divers shapes—round, oval, ete.—but they are all unicellular and nearly transparent, except when great numbers are bulked together, and so small that singly they cannot be discerned by the naked eye. Reproduction is m nearly all varieties by budding. The mature plant, or cell, develops a bud which rapidly increases in size and. detaches itself from the parent plant, to itself bud and develop another cell—and so on, indefinitely. The new cell frequently commences to bud before it detaches itself, and this may be more than once repeated, resulting often in the formation of clusters of several cells. Yeast-plants are present everywhere. They flourish, as already noted, in foods containing sugary solutions in moderate amount, or substances convertible into sugar, but sugar itself is immune, except some moist varieties such as maple-sugar. Thus in the household they will speedily multiply in the jelly that is left exposed in a warm room and cause it to ferment, but they cannot grow in the common dry sugar nor in the heavily sugared jam standing at its side. These ‘‘wild’’ yeast-cells must be kept out of food, for their uncontrolled, unregulated growth often spoils it by producing undesired fermentation. The cultivated yeast-plant is, on the other hand, a valuable a istant in the preparation of human food, one of its most familiar uses being the raising of Breap (which see). Another good example is found in the home-manufacture of Roor Brrr. Tt is the wild veast on grape and apple skins which produces wine and cider, by causing fermentation of the sugar of the fruit juice, but with these exceptions nearly all the veast-plants utilized are those of carefully selected, specially cultivated varieties. The distillation of grain-aleohol is always preceded by a fermentative proce of which cultivated yeast is the prime agent. Bread dough, if left to stand in a warm room, will generally ‘‘rise,’’? as a result of the activity of the wild yeast which has; fallen in it, but the results are uncertain and irregular compared with those obtained by means of cultivated yeast: The cells grow most freely between 70° and 95° Fahr., so the temperature of a good refrigerator will prevent propagation. Food in which wild yeast has begun to grow, but in which it has not progre ed sufficiently to do considerable damage, can be saved by boiling, or its equivalent heat in other forms of cooking. Heat is the only effective destroyer. Tt must be remembered, however, that unle the food thus freed is well covered or placed in a refrigerator, it is just as liable as before to suffer fermentation from new wild yeast getting into it. Cull i rated yeast consists of selected wild cells, propagated in appropriate food material. Different kinds are grown for special purposes -a variety which may be very good for beer, for example, may not be desirable in color or flavor for bread. Commercial compre ed yeast is obtained by sowing selected cells in vats filled with a warm, sweet, filtered mash prepared by malt-conversion of starch. As the cells multiply, they collect in a thick, yellowish, frothy coating 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 quantities 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 eake, mixed with starch, ete.—but after two or three days many of them die and the yeast will show le and le vigor. In time, bacteria, another cla of micro-organism, will develop in the cake and spoil its flavor. It is consequently best to use compre ed veast while perfectly fresh. If this is impo ible, its life may he prolonged by placing the cakes in cold water and setting in the ice chest. They should never be exposed to temperature below freezing point. Dried yeast, in cakes or powder, is compre ed yeast dried at low heat. The proce kills some of the plants and thus le ens the vigor of the yeast, but a good many are left in a dormant condition and it holds the advantage that it will, under suitable conditions, keep for several weeks, and sometimes months. It must, however, be handled with care, ifs vitality being destroyed or le ened by falls and bruises, and by exce ive heat or cold. As the live cells of dried yeast begin growing again when moistened, it is a good plan to put the cake or powder in a little sweetened warm water shortly before using. Both compre ed and dried yeasts vary in purity and hence in value, so that it is advisable to purchase from concerns of recognized experience and integrity. Next after compre ed yeast in strength and utility for bread-making, is breier’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 flavor. Beer yeast is also used to a limited extent by physicians. It makes an appetizing ‘“houillon,’? somewhat resembling beef tea, and is sometimes prescribed as a stimnu- Jant in hospital practise. It was in former vears frequently substituted for meat extract or used to adulterate it. Recent experiments in the search for definite knowledge concerning vitamins have revived the old-time use of veast as a curative and recuperative agent. It has been found that fresh yeast (including fresh compre ed yeast) contains a plentiful supply of the B-vitamin (see further on this subject in the general article on Foon Vauves).