Yeast
From Freepedia
Yeasts constitute a group of single-celled (unicellular) fungi, a few species of which are commonly used to leaven bread and ferment alcoholic beverages. Most yeasts belong to the division Ascomycota. A few yeasts, such as Candida albicans, can cause infection in humans. 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 sugars into carbon dioxide and ethanol (alcohol). 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
- C6H12O6 (glucose) →2C2H5OH + 2CO2
Yeasts 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 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 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.
Yeast fermentations comprise the oldest and largest application of microbial 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.
Winemakers use a variety of different yeasts depending on the type of wine and the condition of the grapes. Too high a sugar or alcohol concentration slows the growth of yeast, so for very ripe grapes with lots of sugar he or she would use a yeast tolerant of those conditions. If the yeast dies before all the fermentable sugar has been converted to alcohol, the result is a "stuck" fermentation. Some yeast is chosen because it tends to develop certain aromas, such as the distinctive "banana" smells of Beaujolais from Georges Duboeuf. Wild yeasts are naturally present on the skins of grapes, so grape juice will spontaneously ferment unless the wild yeast are arrested by cold temperature or sulfites. Depending on the strain of indigenous yeast, the result may be unpalatable or possibly more complex than if a single cultured strain were used. In general, natural yeasts are riskier than cultured, and tend to be used by tradition-oriented, Old World-style winemakers.
Saccharomyces cerevisiae is extensively used as a model organism by biologists studying genetics and molecular biology
