Yeast

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with the 1,500 species currently described The industrial production of yeast blocks was enhanced by the introduction of the filter press in 1867. In 1872, Baron Max de Springer developed a manufacturing process to create granulated yeast, a technique that was used until the first World War. In the United States, naturally occurring airborne yeasts were used almost exclusively until commercial yeast was marketed at the Centennial Exposition in 1876 in Philadelphia, where Charles L. Fleischmann exhibited the product and a process to use it, as well as serving the resultant baked bread.

Nutrition and growth

Yeasts are chemoorganotrophs as they use organic compounds as a source of energy and do not require sunlight to grow. Carbon is obtained mostly from hexose sugars such as glucose and fructose, or disaccharides such as sucrose and maltose. Some species can metabolize pentose sugars like ribose, The cells can survive freezing under certain conditions, with viability decreasing over time.

Yeasts are generally grown in the laboratory on solid growth media or in liquid broths. Common media used for the cultivation of yeasts include potato dextrose agar (PDA) or potato dextrose broth, Wallerstein Laboratories nutrient (WLN) agar, yeast peptone dextrose agar (YPD), and yeast mould agar or broth (YM). Home brewers who cultivate yeast frequently use dried malt extract (DME) and agar as a solid growth medium. The antibiotic cycloheximide is sometimes added to yeast growth media to inhibit the growth of Saccharomyces yeasts and select for wild/indigenous yeast species. This will change the yeast process.

The appearance of a white thready yeast commonly known as kahm yeast is often a byproduct of the lactofermentation (or pickling) of certain vegetables, usually the result of exposure to air. Although harmless it can give pickled vegetables a bad flavour and so must be removed regularly during fermentation.

Ecology

Yeasts are very common in the environment, but are usually isolated from sugar-rich material. Examples include naturally occurring yeasts on the skins of 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. Yeasts including Candida albicans, Rhodotorula rubra, Torulopsis and Trichosporon cutaneum have been found living in between people's toes as part of their skin flora. Yeasts are also present in the gut flora of mammals and some insects and even deep-sea environments host an array of yeasts. Yeast colonising nectaries of the Stinking Hellebore have been found to raise the temperature of the flower, which may aid in attracting pollinators by increasing the evaporation of volatile organic compounds. A black yeast has been recorded as a partner in a complex relationship between ants, their mutualistic fungus, a fungal parasite of the fungus and a bacterium that kills the parasite. The yeast have a negative effect on the bacteria that normally produce antibiotics to kill the parasite and so may affect the ants' health by allowing the parasite to spread.

Reproduction

Yeasts have asexual and sexual reproductive cycles. The most common mode of vegetative growth in yeast is asexual reproduction by budding. Top cropping yeasts are so called because they form a foam at the top of the wort during fermentation. An example of a top cropping yeast is Saccharomyces cerevisiae, sometimes called an "ale yeast". Bottom cropping yeasts are typically used to produce lager-type beers, though they can also produce ale-type beers. These yeasts ferment more sugars, creating a dryer beer, and grow well at low temperatures. An example of bottom cropping yeast is Saccharomyces pastorianus, formerly known as Saccharomyces carlsbergensis.

The most common top cropping brewer's yeast, Saccharomyces cerevisiae, is the same species as the common baking yeast. However, baking and brewing yeasts typically belong to different strains, cultivated to favour different characteristics: baking yeast strains are more aggressive, in order to carbonate dough in the shortest amount of time possible; brewing yeast strains act slower, but tend to produce fewer off-flavours and tolerate higher alcohol concentrations (with some strains, up to 22%).

Brettanomyces is a genus of wild yeast important in brewing lambic, a beer produced not by the deliberate addition of brewer's yeasts, but by spontaneous fermentation by wild yeasts and bacteria. Brettanomyces lambicus, B. bruxellensis and B. claussenii are native to the Senne Valley region of Belgium, where lambic beer is produced.

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 by bacteria. Nutritional yeast, though it has a similar appearance to brewer's yeast, is very different and has a very different taste.

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. It can also be used in mashed and fried potatoes, as well as putting it into scrambled eggs. 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.

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Cited texts

External links

Yeast Development, Different Forms of Yeast...

Cell cycle and metabolic cycle regulated transcription in yeast

Yeast Resource Center

Yeast growth and the cell cycle

Yeast virtual library

Ancient Egyptian Bread Making

Current research on Yeasts at the Norwich Research Park

Category:Brewing ingredients Category:Food additives Category:Leavening agents Category:Medicinal fungi Category:Vegan cuisine * * Category:Fermentation (food)