Define "yeast."

Yeast are a type of fungi. Fungi include several thousand species of yeast, mold, and mushrooms. They are single-celled organisms that are usually larger than bacteria. Individually, yeast are invisible to the naked eye, but large masses can be easily seen. Yeast come in a variety of forms and shapes; they can be spherical, oval, lemon-shaped, pear-shaped, cylindrical or triangular. The size and shape of the 350 known types of yeast are used to classify them into groups for identification.

Yeast are commonly found on plants, grains, fruits, and other foods containing sugar. They are present in soil, in the air, on the skin and in the intestines of animals and in some insects. They are transferred from place to place by carriers (people, equipment, food) and air currents.

Yeast are the most important and widely used microorganism in the food industry. Although they were first observed by early scientists in the 1600's, they have been used for centuries to ferment fruit juices, leaven bread, and make foods palatable and nutritious.

Types of Yeast

Although some yeast are known to cause disease in plants, animals, and man and others can cause spoilage of foods, yeast are primarily beneficial to the food industry.

Beneficial Yeast

Yeast fermentations are involved in the manufacture of foods such as bread, beer, wine, liquor, surface ripened cheese, soy sauce, vitamins, and enzymes.

Alcohol Fermentation. The best known and one of the most important uses of yeast is the production of ethyl alcohol from carbohydrates (sugar or starch). This fermentation process is used in the manufacture of beer, wine, liquor, (rum, scotch, bourbon, gin) bread, chemicals, and many other products.

Baking. The use of yeast as a leavening (rising) agent in baking dates back to early civilizations. Today, selected yeast are mixed with bread dough and are allowed to grow and ferment. This fermentation produces carbon dioxide, which is responsible for the leavening of the dough. The quality of the product depends on the type of yeast; choice of raw materials; and growth conditions (time and temperature of incubation)

Yeast as Food. Several types of yeast have been grown in mass culture and used as a source of food for humans and animals. Because they are easy to grow and have a high nutritive value, yeast have been used to supplement diets and to improve a variety of food products.

Spoilage Yeast

Yeast are undesirable when they cause spoilage of sauerkraut, molasses, meats, fruit juices, honey, beer, syrups, jellies, and wine. Salt tolerant yeast can spoil salted meats, fish and soy sauces; they can also grow in curing brines containing cucumbers and meats.

Pathogenic Yeast

The great majority of yeast are beneficial to man, but some yeast are capable of causing disease in humans and plants. Yeast infections in humans usually affect the skin, nails and mucous membranes.