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 most of the ethanol evaporates.
An example with glucose as the substrate is that
C6H12O6 (glucose) →2C2H5OH + 2CO2
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. 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.