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YEAST: is the family name of those tiny plants or micro-organisms belonging to the Fungi class which reproduce themselves by budding. There are many varieties. but they are all oval or round, colorless and nearly transparent, except when great numbers are bulked together, and so small that singly they cannot be discerned by the naked eye. 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 from the parent plant, and the latter may produce a second bud before the first is detached—resulting often in the formation of clusters of several cells before dis integration. The multiplication is very rapid where the food supply is favorable.

Yeast plants are present everywhere. They flourish best 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 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, unregu lated growth often spoils it by producing undesired fermentation. The cultivated yeast plant is, on the other hand, one of the most valuable of human food assistants. • The usefulness of properly controlled yeast is found in the fact that the action of its "enzymes" or secretions on the sugary contents of the matter, whether bread dough or grape juice, etc., in which it falls or is placed, is to convert the sugar into alcohol and carbon-dioxide (carbonic acid gas). Of these two, only the alcohol is retained in still wines, etc., but sparkling wines, beer, etc., include also a small amount of the carbon-dioxide. In bread making, the alcohol, comparatively unimportant in quln tity and effect, is lost by evaporation in baking. The carbon-dioxide also passes away in the oven—but in the dough set to rise, it produces the hundreds of little bubbles or cavities which give the loaf the desired porous character (see article on BREAD).

Another good example of the action of yeast is seen in the home manufacture of root beer and similar beverages. The extract purchased provides the agreeable herb flavor, but the directions for making require the addition of both yeast and sugar, and it is the action of the yeast on the sugar which gives the slightly exhilarating quality (from the small percentage of alcohol produced) and the effervescence (from the action of the carbon-dioxide).

It is the wild yeast in grape and apple skins which produces the bulk of our wine and cider, by causing fermentation in the juice of the grapes and apples, but with these exceptions nearly all the yeast plants utilized are those of carefully selected, specially cultivated varieties. The fermentative process which precedes the manufacture of whisky, rum, etc., is always produced by cultivated yeast. 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 the use of cultivated yeast.

Yeast grows most freely between 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 progressed 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. It must be remembered, however, that unless the food thus freed is effec tually covered or placed in a refrigerator, it is just as liable as before to suffer fermen tation from new wild yeast getting into it.

Cultivated yeast consists of selected wild cells, propagated in appropriate food material. If undisturbed, they will multiply until the whole is a mass of practically pure yeast. Different kinds are grown for special purposes ; as a variety which may be very good for beer, for example, may not be desirable in color or taste for bread.

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