Yeast

yest, small plants creating fermentation. Yeast, as known to the housekeeper and the brewer, is simply a collection of small oval plants. The yeast plants are related to the molds, but are too small to be seen by the naked eye. They are more like bacteria, but are larger. In the household and in manufacturing, molds and bacteria are often harmful--ruinously so, but neither the breadmaker nor the brewer can get on without yeast.

Many yeasts are known to the botanist, but it is sufficient for our purpose to speak of yeast in the singular. The yeast plant is about 1/2800 of an inch in diameter, and is an egg-shaped, transparent body without color. Under favorable conditions the plant, for we must get rid of the notion that a plant necessarily has a root, stem, branches, and leaves, absorbs nourishment through its walls and grows by budding or throwing out a sort of pocket which becomes a new plant. The new pouch sends out other pouches and so on, all in less time than it takes to describe the process, until, under the microscope, the whole irregular-shaped group or colony looks like a miniature cactus plant. After a while the joints drop apart, each a perfect yeast plant as at the start. Under certain circumstances, as when the yeast plants are grown with a scarcity of food, individuals divide within into grains or spores and become sacs of from two to four dry spores. The sac bursts and the spores are carried away and scattered by the wind. Under ordinary circumstances every breath we draw carries with it spores of yeast, as well as mold and bacteria.

Three stages of yeast growth are recognized:

1. Spores, or the spore-bearing state. 2. The budding or growing stage. 3. The rest stage in which individual plants lie at rest singly or in a mass, inactive for want of moisture, food, and warmth.

The cake of dry yeast sold in a grocery store is composed largely of millions of dry yeast plants in a state of rest. The liquid foaming yeast from the brewer contains millions of yeast plants actively budding. When deprived of most of its water and pressed into a soggy cake, it is sold as compressed yeast. Yeast requires sugar for food, although it cannot live upon pure sugar. The natural sugar solutions, such as molasses, fruit juices, and sirups, contain usually sufficient other material so that yeast will thrive in them. Too large a percentage of sugar, however, is injurious to yeast, hence fruit may be preserved from fermentation by cooking it with a large amount of sugar. "Pound for pound" is the old-fashioned rule for preserves which do not require to be sealed. With air-tight jars so much sugar is unnecessary. Bread dough contains sugar enough for yeast growth.

The flour of which bread is made contains a great deal of starch. When this is "wet up," as the housewife says, a chemical action takes place by which some of this starch is changed into sugar. When yeast is added to bread dough and is mixed in thoroughly, it attacks the sugar of the dough at once and multiplies with marvelous rapidity. As the yeast feeds upon the materials in the dough, the sugar is fermented, producing carbon dioxide and alcohol. The carbon dioxide gas forms tiny bubbles in all parts of the dough, causing it to swell or rise, that is, to become light. When the bread is sufficiently light, it is baked. The action of heat drives off the alcohol which has been produced, swells the bubbles of gas, causing the bread to rise in the pan, but checks the rising process by hardening the dough into the firm texture of bread. Yeast thrives best, that is to say, bread rises best, at a moderately warm temperature of say from 75 deg. to 90 deg. F. At a higher temperature, bacteria are apt to sour the dough. If the temperature be below 75 deg. the yeast will not act, that is, it will not grow rapidly. In long standing, bacteria are apt to sour the dough.

In like manner the brewer and distiller use yeast to convert the sugar of grain into the alcoholic portion of beers, whiskies, and into alcohol proper. Yeast is killed by a boiling temperature. Fermenting in fruit, jelly, or cider may be checked at once by boiling. The brewers and distillers are the largest users of yeast. The best method of preparation is thus described in Popular Mechanics:

The yeast cells reproduce themselves by budding or sprouting and also, under less favorable conditions, by the development of spores. A pure culture of yeast, that is, a culture in which there is just one kind of yeast present, is generally to be preferred to a mixture of varieties. Yeasts are much like human beings. When different races are brought together much of the energy developed is consumed in fighting for supremacy.

The medium in which the yeast is grown usually consists of a mash containing equal parts of rye meal and barley malt. Small portions of other grains are sometimes used, but not in quantities sufficient to change materially the nutritive medium. A proper amount of water is heated to 140 deg-150 deg. F. The rye meal is added and well stirred and then the malt, reducing the temperature to about 120 deg. F. The stirring is continued and the mixture slowly heated until the temperature reaches about 140 deg. F. A higher temperature is apt to impair the diastatic activity of the malt and thus interfere with converting the starch of the rye meal into sugar. The mixture is next made sour by the action of a lactic acid ferment which usually requires about 24 hours. After souring, the temperature is raised to 160 deg-170 deg. F., to destroy any remaining yeasts or bacteria and secure a medium suitable for seeding by a pure culture. The mash is cooled to about 85 deg. F., which is the best temperature to add the yeast culture. After adding the pure seed yeast, the mixture is cooled to about 60 deg. F. As the yeasts begin to proliferate the temperature rises, but it should not be allowed to go above 85 deg. F. The yeast rises with the bubbles produced by the fermentation and is scraped off as a beautiful creamy covering of the fermenting mass.

See Mold; Bacteria; Bread; Brewing; Distilling; Wine; Cider