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Small organism responsible for beer, wine, leavened bread, marmite and thrush.
Yeast is a term applied to several different species of single-celled fungi. In beer and wine making the production of alcohol by yeast is the main reason for using yeast in baking it is is the carbon dioxide that is desired to make things rise and alcohol is only present in trace amounts as the alcohol tends to evaporate during baking. Yeasts produce carbon dioxide and alcohol (ethanol) from sugars.
This is reflected in the genus of the most commonly used yeast Saccharomyces cerevisiae which is used in the production of wine, beer and bread; "Saccharomyces" being Greek for "sugar mould".
Glucose and fructose (which is an isomer of glucose) are the only sugars capable of penetrating the cell membrane, in order to be converted into carbon dioxide and ethanol by the enzyme zymase, as they are the simplest sugars (i.e. they have the shortest chain of carbon atoms). Other sugars which are present in the ingredients of bread, wine and beer recipes must first be broken down into simple sugars.
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Types of Yeast
Not all yeasts are of the Saccharomyces genus, for example Thrush is caused by Candida albicans (it is very uncommon, though not impossible, for Saccharomyces cerevisiae to cause an infection in humans).
Brettanomyces
(Translation: "British Fungi")
These yeasts are often considered to be "spoilage yeast" in both wine and beer making. However in small quantities the compounds produced can be desirable as sensory compounds giving flavours to wine such as bacon, spice, cheese and apparantly even barnyards. The name comes from the flavour that used to to be given by such yeasts in British ales, nowadays these yeasts are mostly used in wine making and for lambic beers.
Candida
Saccharomyces
(Translation: "sugar mould")