What is Yeast? What are the types of yeast? Dive into the realm of yeast with our informative post. Discover what yeast is, its various types, and the intriguing roles they play in fermentation, baking, and more.
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Yeasts; microscopic, single-celled, colorless plants. They are true fungi, with round or oval cells that are much larger than bacteria. The characteristic method of reproduction is budding, in which a small swelling occurs at the edge of the cell, gradually enlarges, and at maturity separates from the parent cell by constricting at its base. In some species, buds are slow in breaking free and thus form clusters of cells. The spores produced by true yeasts are a means of survival rather than reproduction. Definite classification of the yeasts and imperfect yeasts is still unsatisfactory from a botanical standpoint, and the term “yeast” often refers to species in both groups.
Yeasts, widely distributed in nature, are found on the fruit and leaves of plants and in the soil. Nearly all yeasts adapted to use by man are in one genus, Saccharomyces, with S. cerevisiae the most prominent species. Uses of yeasts are based on their ability to ferment sugar solutions or their nutritional and medicinal values.
Fermentation processes are as old as history and were highly developed before the advent of scientific methods. The modern industries of bread making, brewing, spirits and industrial alcohol manufacture, and wine making, however, owe their foundations to the work of Louis Pasteur in the mid-19th century. The food, feed, and medicinal uses of yeasts were found later as knowledge of vitamins and metabolism unfolded. The quantities of yeasts grown annually are measured in the hundreds of million pounds.
The procedures for growing yeasts are fundamentally the same in each industry. A pure culture is obtained by isolating a single yeast cell and growing it on a nutrient medium consisting of sugars (molasses, starchy grains, potatoes), a source of nitrogen (ammonia, grain steepwater), minerals, and water of controlled acidity. As growth proceeds, the culture is transferred to successively larger batches of medium until the final batch often exceeds 100,000 gallons. When growth is completed, the yeast cells are separated from the spent medium.
Yeasts grown for bread making, food, feed, and medicinal purposes are the primary product of the fermentation process and produce little alcohol. The spent medium is the byproduct and is discarded after centrifuging out the yeast cells. Yeasts grown during fermentations for beer, ale, spirits, wine, and industrial alcohol, however, are the byproducts, while the spent medium becomes the final consumer product.
Types
Baker’s yeast is a strain of S. cerevisiae grown on a medium of molasses and ammonia or molasses and grain extracts. The yeast rapidly ferments the sugars derived from the starch in the flour to produce carbon dioxide gas which leavens the dough.
Brewer’s yeasts are strains of S. cerevisiae that slowly ferment the extract of malt, cereals, and hops to produce beer (bottom-fermenting yeast) or ale (top-fermenting yeast). After separation from the spent medium, these yeasts are dried for animal feed or refined before drying for use in food and medicine.
Spirits and industrial alcohol yeasts are strains of S. cerevisiae, and sometimes other species, that ferment molasses, grains, or other sugary material. After separation from the spent medium, yeast and fermentation residues are dried together for animal feed.
Wine yeasts are strains of S. cerevisiae var. ellipsoideus that ferment grape juice. Here the yield of yeast is too small to be salvaged economically.
Food (and feed) yeast production, stimulated by shortages in two world wars, reaches large proportions in Europe. Although several species of yeasts have been grown for food use, only strains of S. cerevisiae and Candida utilis are cultivated in the United States. C. utilis is better able to utilize sugars from waste sulphite liquor from wood pulp manufacture. Dried yeasts are used as nutritional supplements in a number of foods in the United States.
Medicinal yeast was first recognized by the United States Pharmacopeia in 1944 as the non-fermentable “dry cells of any suitable strain of S. cerevisiae,” with a second species, strains of C. utilis, added in 1956. Yeast is a rich source of the B-complex vitamins, and its protein is well balanced in amino acids. Yeast or a yeast derivative, ergosterol, acquires vitamin D2 activity after irradiation with ultraviolet light. Other yeast derivatives are used in the medium in which antibiotic-producing organisms are grown and as a starting point in the synthesis of steroid hormones.
The chemical composition of yeasts varies with the species and growing medium. Dried yeasts have been found to assay: protein, 45 to 55 percent; fat, 0.5 to 2.5 percent; ash (minerals), 5 to 7 percent; carbohydrates, 35 to 45 percent; and moisture, 4 to 6 percent.
