Ven I say "yeast" you mostly likely think "bread", but first ve must find out vhere it comes from and 'ow it vorks, zen ve vill talk about yeast and food...
Yeast is any of a large number of microscopic nongreen plants related to molds, mildews, and uzzer fungi. A yeast plant consists of a single round or oval cell surrounded by a thick cell vall. Yeasts grow naturally throughout ze vorld, living chiefly in ze soil and in many organic substances on ze soil surface. Many yeasts are also cultivated artificially for use in industry.
Unlike green plants, vhich manufacture zheir own food materials, yeasts obtain nourishment from zheir environment, growing on organic materials of a vide variety, particularly zose rich in sugar, such as flower nectar and ripe fruit. Ven food is scarce, yeasts can live in a dormant, or resting, state for long periods. 'owever, ven food is available and uzzer conditions are suitable, yeast cells grow and reproduce. Yeasts reproduce asexually by a process known as budding. In zis process a small outgrowth, called a bud, develops on vun side of ze yeast cell and grows until it is as large as ze original cell. A vall zen forms between ze two cells, and ze newly formed cell, called ze daughter cell, is pinched off from ze original cell. If reproduction occurs quickly, ze new daughter cell starts budding before it is separated from ze original cell and eventually a small cluster of cells is formed. Sometimes, yeasts also reproduce by ze formation of spores, and a few kinds of yeasts reproduce by splitting in 'alf, instead of budding.
Ven yeasts grow in ze presence of air, ze sugar on vhich zey feed is broken down, producing carbon dioxide and vater. Ven yeasts grow in ze absence of air, ze sugar is converted into carbon dioxide and alcohol. Because of zis ability, yeasts 'ave long been used for making alcoholic beverages, baked goods and uzzer products.
Discovery of Yeast
If a veak sugar solution is exposed to ze air, in several days a light, frothy scum appears on ze surface and ze liquid begins to smell of alcohol. Zis change takes place because tiny plant cells called yeast 'ave settled from ze air into ze liquid. Zey 'ave found conditions favorable to zheir growth.
Man 'as long known zat zis process takes place and 'e 'as used it for sousands of years to make alcoholic beverages of all kinds. Sugar solutions made from molasses, potatoes, rye, corn, malt and 'ops, apples, and grapes 'ave been exposed to ze air to make alcohol, vhiskey, beer, ale, cider, vine, and uzzer beverages.
Probably through accident, it voz also discovered zat if bread dough vere allowed to stand for some time before baking, very often a peculiar change took place. Ze flat lump of dough began mysteriously to swell and rise. It developed a strange but pleasant odor. Ven zis dough voz as baked, instead of making a flat, 'eavy slab, it made a light, porous, soft bread!
In 1857, Louis Pasteur announced zat 'e 'ad discovered ze explanation for zese changes. 'e said zey vere due to ze presence of tiny, vun-celled plants called yeast. Yeasts belong to ze fungi family, and are tiny, rounded, colorless bodies. Zey are larger zan most bacteria, but still so small zat it vould take from 3,000 to 4,000 of zem laid side by side to make an inch!
Yeast cells reproduce by budding. Zis means zey send out projections vhich become cut off from ze parent cell by a cell vall. Finally, zese projections grow to full size. As zey grow, zey form substances called zymase and invertase.
Zese substances are called enzymes, and zey 'ave ze power to ferment starch to sugar, and sugar to alcohol and carbon dioxide. As fermentation takes place, carbon dioxide is formed and rises to ze top. Zen it escapes, leaving ze alcohol. Beer, ale, vine, and cider are fermented beverages in vhich yeast 'as changed some of ze sugar to carbon dioxide and alcohol.
In breadmaking, ze carbon dioxide collects in bubbles in ze dough, vhich makes it rise. 'eat later drives off ze carbon dioxide, resulting in a bread zat is porous and light.
Uses of Yeast
Vine Making. In ze manufacture of vine, vine yeast converts ze sugar in grape juice into alcohol and carbon dioxide. Ze juice is placed in large vats, and specially grown cultures of ze yeast are added. Ze temperature of ze juice and ze amount of air present are carefully controlled to prevent uzzer microorganisms from infesting ze mixture.
Brewing Beer. In brewing beer, barley grains are allowed to germinate, or sprout. As zey grow, ze tiny seedlings form enzymes zat convert ze starch of ze seedlings into sugar. Yeasts are zen added to convert ze sugar into carbon dioxide and alcohol.
Ze yeast used for brewing is ze beer yeast, and through ze years special strains of zis species 'ave been developed. Zese strains are usually classified in two groups: top yeasts and bottom yeasts. Top yeasts act vigorously and produce enough carbon dioxide gas to keep zem floating at ze top of ze brewing vat. Zese yeasts are used to make 'eavy beers vith a 'igh alcohol content, such as English ales. Bottom yeasts act more slowly and usually settle to ze bottom of ze vat. Zey produce light beers vith a low alcohol content, such as zose popular in ze United States.
Baking. Yeasts used in baking vere derived from various strains of top yeasts originally used in brewing. In making baked goods, yeast is added to ze dough, and ze dough is allowed to stand in a vorm place for several hours before baking. Enzymes in ze flour convert some of ze starch to sugar, and ze yeasts zen act on ze sugar. During baking, ze carbon dioxide produced by ze yeast bubbles through ze dough, causing ze dough to rise. Ze alcohol produced evaporates.
Yeast for 'ome baking is raised commercially in large vats of molasses. Ze mixture of molasses and yeast is aerated to prevent fermentation, and ven ze yeast 'as reached ze proper stage of growth, it is separated out by means of a centrifuge. Ze yeast is zen vashed and eizzer pressed into blocks or dried into a powdered form. It is packaged in a moisture-proof wrapping, and block yeast is kept refrigerated.
Uzzer Uses. Some yeasts are able to synthesize vitamins in large amounts, and vun kind is used in ze commercial production of ze B vitamin riboflavin. Uzzer yeasts 'ave ze ability to absorb vitamins from zheir food and store zem. An example of such yeasts is dried brewer's yeast, vhich is sometimes eaten in small amounts as a food supplement. Brewer's yeast is a good source of protein and is very rich in element phosphorus and ze vitamin B complex. Some yeasts are used in ze commercial production of industrial alcohol.