Yeast
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- For the baking ingredient, see Yeast (baking).
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Yeasts are unicellular microorganisms classified in the kingdom Fungi. Most yeasts belong to the division Ascomycota although some are classified in Basidiomycota. A few yeasts, such as Candida albicans, can cause infection in humans (Candidiasis). More than one thousand species of yeasts have been described[citation needed]. Some species of yeast have been used to leaven bread, ferment alcoholic beverages, and even drive experimental fuel cells[citation needed]. The most common yeast used in industry is Saccharomyces cerevisiae, which has been used in wine, bread, and beer production for thousands of years.
Yeast species can have either obligately aerobic or facultatively anaerobic physiology. There is no known obligately anaerobic yeast. In the absence of oxygen, fermentative yeasts produce their energy by converting carbohydrates into carbon dioxide and ethanol (alcohol) or lactic acid. In brewing, the ethanol is bottled, while in baking the carbon dioxide raises the bread, and most of the ethanol evaporates.
An example with glucose as the substrate
C6H12O6 (glucose) →2C2H5OH + 2CO2
[edit] Use in biotechnology
The useful physiological properties of yeast have led to their use in the field of biotechnology. Fermentation of sugars by yeast is the oldest and largest application of this technology. Many types of yeasts are used for making many foods: Baker's yeast in bread production, brewer's yeast in beer fermentation, and yeast in wine fermentation. Yeasts are also one of the most widely used model organisms for genetics and cell biology.
[edit] Growth environment
Many yeasts can be isolated from sugar-rich environmental samples. Some good examples include fruits and berries (such as grapes, apples or peaches), exudates from plants (such as plant saps or cacti). Some yeasts are found in association with soil and insects.
A common medium used for the cultivation of yeasts is called potato dextrose agar (PDA) or potato dextrose broth. Potato extract is made by autoclaving (i.e. pressure-cooking) cut-up potatoes with water for 5 to 10 minutes and then decanting off the broth. Dextrose (glucose) is then added (10 g/L) and the medium is sterilized by autoclaving.
A yeast reproduces through budding instead of through spores like other fungi in the "true fungi" category.
[edit] See also
