Yeast
Yeasts are single-celled (
unicellular)
fungi, a few species of which are commonly used to
leaven bread,
ferment alcoholic beverages, and even drive experimental
fuel cells. Most yeasts belong to the
division Ascomycota, though some are
Basidiomycota. A few yeasts, such as
Candida albicans, can cause
infection in humans (
Candidiasis). More than one thousand
species of yeasts have been described. The most commonly used yeast is
Saccharomyces cerevisiae, which was domesticated for
wine,
bread, and
beer production thousands of years ago.
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 the ethanol evaporates.
An example with
glucose as the substrate is
C
6H
12O
6 (glucose) â†'2C
2H
5OH + 2CO
2The 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.
Baker's yeast is used for bread production,
brewer's yeast is used for beer fermentation, and yeast is also used for wine fermentation. Yeast are also one of the most widely used
model organisms for
genetics and
cell biology.
Yeast can
reproduce asexually through
budding or sexually through the formation of
ascospores. During asexual reproduction, a new bud grows out of the parent yeast when the condition is right, then, after the bud reaches an adult size, it separates from the parent yeast. Under low nutrient conditions yeasts that are capable of sexual reproduction will form ascospores. Yeasts that are not capable of going through the full sexual cycle are classified in the genus
Candida.
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.
*
Fungi*
Nutritional yeast*
Saccharomyces*
Brettanomyces*
Brewer's yeast
