The yeasts are a large group of non-mycelial, single-celled (acellular) Ascomycote fungi, extremely widely distributed in the biosphere but associated especially with the outside surfaces of fruits, vegetables and leaves. They can all break down glucose into carbon dioxide and ethanol, though most are facultative anaerobes, deriving energy from
aerobic respiration ( when oxygen is available and glucose levels reasonably low) but able to switch to the anaerobic process ( when oxygen is in short supply or, in aerobic conditions, when glucose levels are high). Reproduction is by 'budding' Saccharomyces cerevisiae is baker's (or brewer's) yeast and Saccharomyces carlsbergensis is used in brewing lager. certain species of Saccharomyces are involved in soy sauce production. Other yeast genera include Torula and Candida (some species of the latter can be parasitic on humans, whilst others successfully exploited in the production of single-cell protein (SCP)).
Historically, beer and wine-making relied upon whatever 'wild' yeasts happened to find their way into the vat, often with disastrous results for the brew. Over the years however, highly specific strains of yeast species have been identified, maintained and often jealously guarded by brewers for specific products. Greatly improved strains (for example, with high alcohol tolerance) have been produced by highly sophisticated techniques of cross breeding, genetic manipulation and selection. During fermentation, yeasts carry out side reactions which produce small amounts of various organic compounds apart from alcohol; many of these, along with other substances present in ingredients such as malt, grapes and hops, are significant contributors to the flavour and other effects of a particular product.
Further information on developments using yeast is given elsewhere.
It is suggested that students should