Yeast
yeast. (yeest')
Microscopic, unicellular, fungi of the genus Saccharomyces, distinct from bacteria because they possess a true nucleus. Yeasts are classed depending on their ability to sporulate and the method of sporulation. Brewing yeasts are classed into three categories: bottom-fermenting yeast, or Saccharomyces carlsbergensis, reclassified Saccharomyces uvarum; top-fermenting yeast, or Saccharomyces cervisiae; and wild yeasts, such as Saccharomyces candida and other species. Each category is further subdivided into strains. Because yeasts belong to the plant family, they are named according to the International Code of Botanical Nomenclature; each yeast is known by a binomial combination, the first name being that of the genus, the second that of the species. More than five hundred types of yeasts have been isolated, not including the numerous wild strains. The rate of reproduction of yeasts in wort varies with the temperature and reaches a maximum at about 86 °F (30 °C). The reproduction rate of Saccharomyces cervisiae is greater than that of Saccharomyces uvarum at high temperatures, and the opposite holds at lower temperatures. Brewer’s yeasts are sensitive to heat and may be killed by exposure to temperatures of 125.6 °F (52 °C) or above for ten minutes or more. During the fermentation process, yeast converts the natural malt sugars into equal parts of alcohol and carbon dioxide gas. Yeast was first viewed under a microscope in 1680 by Dutch scientist Antoine van Leeuwenhoek; later, in 1867, Louis Pasteur discovered that yeast cells lack chlorophyl and that they could develop only in an environment containing both nitrogen and carbon. See also: bottom fermenting yeast; lag phase; reproduction phase; top-fermenting yeast.
Yeast comes in a liquid form and also a dry form. There are different yeast strains used for specific styles of beer, mead, cider, and wine.