Yeast, a microscopic, one-celled organism belonging to the group of organisms called fungi. There are many kinds of yeasts, some of them of great importance to humans. Yeast is necessary to make leavened bread, beer, cheese, wine, and whiskey. It is rich in B vitamins; a form of yeast called brewer's yeast is used as a diet supplement. Yeast is also used in genetic engineering to produce large quantities of certain hormones and enzymes, which are used for such medical purposes as healing wounds and reducing inflammation. Some types of yeast, however, cause disease; candidiasis, a skin infection, is an example.

Yeasts are found in the soil, in water, on the surface of plants, and on the skin of humans and other animals. Like other fungi, yeasts obtain food from the organic matter around them; they secrete enzymes that break down the organic matter into nutrients they can absorb.

The yeast cell is oval or round and has a thin membrane. Under ideal conditions of moisture, temperature, and food supply, it reproduces asexually, by budding. When a yeast cell reaches full growth, a budlike swelling forms on its surface. Part of the parent cell's nucleus goes into this bud, and a wall is formed between the parent cell and the bud, which then becomes a separate cell. This new cell may break off when it is full grown. It may, however, remain attached as it produces another bud. In this way, chains or clusters of cells are formed. Budding is a rapid process, requiring about 20 minutes to produce a new organism. (See Reproduction of Living Organisms, illustration titled Budding.)

When conditions are adverse, a yeast cell typically reproduces sexually, by fusing with another yeast cell. The combined cell develops into an ascus, a thick-walled, protective structure containing one to eight spores. When favorable conditions return, the spores are released and each develops into a new yeast cell.

Yeast and Fermentation

Yeasts obtain food from fructose, glucose, and other monosaccharides (simple sugars), which are found in most fruits. Yeast enzymes chemically break down the sugars into products that the cell can use. Other yeast enzymes can make simple sugars out of disaccharides (double sugars), which are found in certain organisms.

The breaking down of sugars, or fermentation, produces alcohol and carbon dioxide as by-products. Fermentation turns fruit juices into wine and helps turn wort (diluted grain mash) into beer or whiskey. The carbon dioxide produced by fermentation makes the bubbles in beer and some kinds of wine, and causes bread to rise. As bread bakes, the alcohol produced by fermentation evaporates.

Yeast enzymes do not act upon starches. However, enzymes found in malt (germinating barley) can change starches into sugar. For this reason, malt as well as yeast is added when it is necessary to bring about the fermentation of substances containing starch.

Commercial Yeasts

Fruits and grains carry "wild" yeasts that have settled on them. However, wine-makers, brewers, and bakers do not rely on these yeasts because they may produce undesirable qualities in the finished products. Instead, the wild yeasts are washed off or killed by high temperatures, and pure strains of cultivated yeasts are added.

Yeasts that produce desirable qualities in beverages or bread are isolated and cultivated in a fluid (usually a solution of sugar and mineral salts). As masses of yeast form, they are skimmed off For further processing. Compressed yeast consists of a mass of yeast that has been washed and mixed with starch. The mass is then pressed to remove about 30 per cent of the moisture, cut into cakes, and packaged. Due to its moisture content, compressed yeast must be refrigerated. It remains fresh for up to five weeks. Dry yeast is made by removing over 90 per cent of the moisture from the yeast mass at a low temperature. It does not need to be refrigerated, and has a shelf life of six months. Both compressed and dry yeast are classed as active yeast because they are made up of living yeast. The yeast is in a dormant state when packaged but becomes active when combined with hot water or milk.

Brewer's yeast is a type of inactive yeast, composed of yeast that is not living. It may be formed during the making of beer, or may be manufactured by a process similar to that used in making dry or compressed yeast. It is rich in B vitamins, iron, phosphorus, and protein. Brewer's yeast comes in powdered form and is mixed with ground meat or used in beverages.

Yeasts are sac fungi, class Ascomycetes. Commercial yeasts belong to the family Saccharomycetaceae.