Yeast

From Wikipedia, the free encyclopedia

Yeasts are unicellular, eukaryotic microorganisms classified in the kingdom Fungi. The term yeast is commonly used to refer to the species Saccharomyces cerevisiae, however more than seven hundred other species of yeasts have been described.^[1] S. cerevisiae has been used in brewing and baking for thousands of years, and is also extremely important as a model organism in modern cell biology research.

Most yeasts belong to the division Ascomycota although some are classified in Basidiomycota. The "true yeasts" are classified in the order Saccharomycetales.^[2] A few yeasts, such as Candida albicans, can cause infection in humans (Candidiasis). Some species of yeast have been used to leaven bread, ferment alcoholic beverages, and even drive experimental fuel cells.^[3]

Reproduction

Yeasts have asexual and sexual reproductive cycles; however the most common mode of vegetative growth in yeast is asexual reproduction by budding or fission. Here a small bud, or daughter cell, is formed on the parent cell. The nucleus of the parent cell splits into a daughter nucleus and migrates into the daughter cell. The bud continues to grow until it separates from the parent cell, forming a new cell. The bud can develop on different parts of the parent cell depending on the genus of the yeast.

The simple splitting of a cell into two daughter cells (Fission) is another means of increasing the population and is characteristic of bacteria and some yeast.

Other yeasts employ Budding, in which a small outgrowth is formed on the parental cell. The nucleus of the parent cell divides and one daughter nucleus migrates into the bud. The bud increases in size and eventually breaks off.

Under high stress conditions haploid cells will generally die, however under the same conditions diploid cells can undergo sporulation, entering sexual reproduction (meiosis) and producing a variety of haploid spores, which can go on to mate (conjugate), reforming the diploid.

Yeast of the species Schizosaccharomyces pombe reproduce by binary fission instead of budding.

Growth

Yeast species can have either obligately aerobic or facultatively anaerobic physiology, and there is no known obligately anaerobic yeast. Yeast are also non-photosynthetic. 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

Many yeasts can be isolated from sugar-rich environmental samples. Some good examples include fruits and berries (such as grapes, apples or peaches), and 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.

Uses

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, yeast in wine fermentation and for xylitol^[4] production. Yeasts are also one of the most widely used model organisms for genetics and cell biology.

Alcoholic beverages

Yeast has been used to produce alcoholic beverages for thousands of years.

Beer

Beer brewers classify yeasts as top-fermenting and bottom-fermenting. This distinction was introduced by the Dane Emil Christian Hansen. Top-fermenting yeasts are so-called because they form a foam at the top of the wort during fermentation. They can produce higher alcohol concentrations and prefer higher temperatures, producing fruitier, sweeter, ale-type beers. An example of a top-fermenting yeast is Saccharomyces cerevisiae, known to brewers as ale yeast. Bottom-fermenting yeasts are used to produce lager-type beers. These yeasts ferment more sugars, leaving a crisper taste, and grow well at low temperatures. An example of a bottom-fermenting yeast is Saccharomyces uvarum.

For both types, yeast is fully distributed through the beer while it is fermenting, and both equally flocculate (clump together and precipitate to the bottom of the vessel) when it is finished. By no means all top-fermenting yeasts demonstrate this behaviour, but it features strongly in many English ale yeasts which may also exhibit chain forming (the failure of budded cells to break from the mother cell) which is technically different from true flocculation.

Brewers of Bavarian-style wheat beers often use varieties of Torulaspora delbrueckii, which contribute to the distinctive flavour profile. Lambic, a style of Belgian beer, is fermented spontaneously by wild yeasts primarily of the genus Brettanomyces.

To ensure purity of strain, a 'clean' sample of the yeast is stored in a laboratory environment at refrigerated temperature. After a certain number of fermentation cycles, a full scale propagation is produced from this laboratory sample. Typically, it is grown up in about three or four stages using sterile brewing wort and oxygen.

Wine

Yeast is used in winemaking where it converts the sugars present in grape juice or must into alcohol. Yeast is normally already present on the grapes, often visible as a powdery film on their exterior. The fermentation can be done with this indigenous yeast, but since this may give unpredictable results depending on the exact types of yeast that are present, cultured yeast is often added to the must. Most added wine yeasts used are Saccharomyces cerevisiae. Some yeasts such as Zygosaccharomyces and Brettanomyces are capable of spoiling wine.

Baking

Yeast, specifically Saccharomyces cerevisiae, is used in baking as a leavening agent, where it converts the fermentable sugars present in the dough into carbon dioxide. This causes the dough to expand or rise as the carbon dioxide forms pockets or bubbles. When the dough is baked it "sets" and the pockets remain, giving the baked product a soft and spongy texture. The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Salt and fats such as butter slow down yeast growth. The majority of the yeast used in baking is of the same species common in alcoholic fermentation.

It is not known when yeast was first used to bake bread. The first records that show this use come from Ancient Egypt.^[5] Researchers speculate that a mixture of flour meal and water was left longer than usual on a warm day and the yeasts that occur in natural contaminants of the flour caused it to ferment before baking. The resulting bread would have been lighter and more tasty than the normal flat, hard cake.

Today there are several retailers of baker's yeast, one of the best-known being Fleischmann’s Yeast, which was developed in 1868. During World War II Fleischmann's developed a granulated active dry yeast, which did not require refrigeration and had a longer shelf life than fresh yeast. The company created yeast that would rise twice as fast, cutting down on baking time. Baker's yeast is also sold as a fresh yeast compressed into a square "cake". This form perishes quickly, and must be used soon after production in order to maintain viability. A weak solution of water and sugar can be used to determine if yeast is expired. When dissolved in the solution, active yeast will foam and bubble as it ferments the sugar into ethanol and carbon dioxide.

Industrial ethanol production

The ability of yeast to convert sugar into ethanol has been harnessed by the biotechnology industry, which has various uses including ethanol fuel. The process starts by milling a feedstock, such as sugar cane, corn, or cheap cereal grains, and then adding dilute sulfuric acid, or fungal amylase enzymes, to break down the starches in to fermentable sugars. Yeasts are then added to convert the fermentable sugars to ethanol, which is then distilled off to obtain ethanol up to 96% in concentration.^[6]

Saccharomyces yeasts have been genetically engineered to ferment xylose, one of the major fermentable sugars present in cellulosic biomasses, such as agriculture residues, paper wastes, and wood chips.^[7] Such a development means that ethanol can be efficiently produced from more inexpensive feedstocks, making cellulosic ethanol fuel a more competitively priced alternative to gasoline fuels.^[8]

Kombucha

Yeast in symbiosis with acetic acid bacteria is used in the preparation of Kombucha, a fermented sweetened tea. Species of yeast found in the tea can vary, and may include: Brettanomyces bruxellensis, Candida stellata, Schizosaccharomyces pombe, Torulaspora delbrueckii and Zygosaccharomyces bailii.^[9]

Nutritional supplements

Yeast is used in nutritional supplements popular with vegans and the health conscious, where it is often referred to as "nutritional yeast". It is a deactivated yeast, usually Saccharomyces cerevisiae. It is an excellent source of protein and vitamins, especially the B-complex vitamins, whose functions are related to metabolism as well as other minerals and cofactors required for growth. It is also naturally low in fat and sodium. Some brands of nutritional yeast, though not all, are fortified with vitamin B12, which is produced separately from bacteria.

Nutritional yeast has a nutty, cheesy, creamy flavor which makes it popular as an ingredient in cheese substitutes. It is often used by vegans in place of parmesan cheese. Another popular use is as a topping for popcorn. Some movie theaters are beginning to offer it along with salt or cayenne pepper as a popcorn condiment. It comes in the form of flakes, or as a yellow powder similar in texture to cornmeal, and can be found in the bulk aisle of most natural food stores. In Australia it is sometimes sold as "savory yeast flakes". Though "nutritional yeast" usually refers to commercial products, inadequately fed prisoners have used "home-grown" yeast to prevent vitamin deficiency.^[10]

Probiotics

Some probiotic supplements use the yeast Saccharomyces boulardii to maintain and restore the natural flora in the large and small gastrointestinal tract. S. boulardii has been shown to reduce the symptoms of acute diahorrea in children,^[11][12] prevent reinfection of Clostridium difficile,^[13] reduce bowel movements in diarrhea predominant IBS patients,^[14] and reduce the incidence of antibiotic,^[15] traveler's,^[16] and HIV/AIDS^[17] associated diarrheas.

Yeast extract

Main article: Yeast extract

Yeast extract is the common name for various forms of processed yeast products that are used as food additives or flavours. They are often used in the same way that monosodium glutamate (MSG) is used, and like MSG often contain free glutamic acids. The general method for making yeast extract for food products such as Vegemite and Marmite on a commercial scale is to add salt to a suspension of yeast making the solution hypertonic, which leads to the cells shrivelling up. This triggers autolysis, where the yeast's digestive enzymes break their own proteins down into simpler compounds, a process of self-destruction. The dying yeast cells are then heated to complete their breakdown, after which the husks (yeast with thick cell walls which would give poor texture) are separated. Yeast autolysates are used in Vegemite (Australia), Marmite and Bovril (the United Kingdom and Republic of Ireland), Oxo (South Africa, United Kingdom, and Republic of Ireland), and Cenovis (Switzerland).

Pathogenic yeasts

Some species of yeast are opportunistic pathogens, where they can cause infection in people with compromised immune systems.

Cryptococcus neoformans, is a significant pathogen of immunocompromised people, causing the disease termed Cryptococcosis. This disease occurs in about 7-8% of AIDS patients in the USA, and a slightly smaller percentage (3-6%) in western Europe.^[18] The cells of the yeast are surrounded by a rigid polysaccharide capsule, which helps to prevent them from being recognised and engulfed by white blood cells in the human body.

Yeasts of the Candida species are another group of opportunistic pathogens, which causes oral and vaginal infections in humans, known as Candidiasis. Candida is commonly found as a commensal yeast in the mucus membranes of humans and other warm-blooded animals. However, sometimes these same strains can become pathogenic. Here the yeast cells sprout a hyphal outgrowth, which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues.^[18] The pathogenic yeasts of candidiasis in probable descending order of virulence for humans are: C. albicans, C. tropicalis, C. stellatoidea, C. glabrata, C. krusei, C. parapsilosis, C. guilliermondii, C. viswanathii, C. lusitaniae and Rhodotorula mucilaginosa.^[19] Candida glabrata is the second most common Candida pathogen after C. albicans, causing infections of the urogenital tract, and of the bloodstream (Candidemia).^[20]

See also

• Ethanol fermentation
• Fermentation (food)
• Fungi

References

External links

• Yeast growth and the cell cycle
• Science of Breadmaking
• History of Bread
• Ancient Egyptian Bread Making
• Nutritional Information about brewers yeast
• Brewing Yeast Propagation and Maintenance: Principles and Practices

Yeast brands

• Lesaffre : baker's yeast
• Fleischmann's Yeast
• Red Star Yeast
• SAF Yeast
• Lallemand Yeast