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name applied specifically to a certain group of microscopic fungiFungi
, kingdom of heterotrophic single-celled, multinucleated, or multicellular organisms, including yeasts, molds, and mushrooms. The organisms live as parasites, symbionts, or saprobes (see saprophyte).
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 and to commercial products consisting of masses of dried yeast cells or of yeast mixed with a starchy material and pressed into yeast cakes. Although a number of fungi are sometimes called yeasts, the true yeasts are unicellular, consist of oval or round cells, and reproduce chiefly by budding. Under certain conditions some yeast cells secrete a thickened wall, and the cytoplasm of the single cell within divides to form four or eight cells, or spores, known as ascospores, which emerge when the wall ruptures. In a few species two cells fuse before undergoing spore formation. There are about 500 species in all.

Yeasts, especially those of the genus Saccharomyces, have long been of commercial importance because they are the chief agents in alcoholic fermentationfermentation,
process by which the living cell is able to obtain energy through the breakdown of glucose and other simple sugar molecules without requiring oxygen. Fermentation is achieved by somewhat different chemical sequences in different species of organisms.
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. Because of this they are essential to the making of beerbeer,
alcoholic beverage made by brewing and fermenting cereals, especially malted barley, usually with the addition of hops as a flavoring agent and stabilizer. One of the oldest of alcoholic beverages (there is archaeological evidence dating to c.3000 B.C.
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, winewine,
alcoholic beverage made by the fermentation of the juice of the grape. So ancient that its origin is unknown, wine is mentioned in early Egyptian inscriptions and in the literature of many lands.
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, and other alcoholic beverages and industrial alcohol. Wild yeasts, those found in nature and probably carried by insects from the soil to fruits, are frequently active in the fermentation process. In breadmaking the yeasts act upon the carbohydrates in the dough, forming carbon dioxide and ethyl alcohol, which are driven off in the baking process; the escaping carbon dioxide causes the bread to rise. Since early times yeast has been used in treating various ailments; brewer's yeast has a high content of thiamine and other vitamins of the B-complex group. Yeasts are classified in the kingdom Fungi, phyla (divisions) Ascomycota and Basidiomycota.


A collective name for those fungi which possess, under normal conditions of growth, a vegetative body (thallus) consisting, at least in part, of simple, single cells. The cells making up the thallus occur in pairs, in groups of three, or in straight or branched chains consisting of as many as 12 or more cells. Vegetative reproduction is characterized by budding or fission. Sexual reproduction also occurs in yeast, and is differentiated from that of other fungi by sexual states that are not enclosed in a fruiting body. Yeasts are a phylogenetically diverse group of organisms that occur in two divisions of fungi (Ascomycotina and Basidiomycotina) and 100 genera. The 700 or more species that have been described possibly represent only 1% of the species in nature, so the majority of the yeasts have yet to be discovered. Yeast plays a large part in industrial fermentation processes such as the production of industrial enzymes and chemicals, food products, industrial ethanol, and malt beverage and wine; in diseases of humans, animals and plants; in food spoilage; and as a model of molecular genetics. See Genetic engineering, Medical mycology

The shape and size of the individual cells of some species vary slightly, but in other species the cell morphology is extremely heterogeneous. The shape of yeast cells may be spherical, globose, ellipsoidal, elongate to cylindrical with rounded ends, more or less rectangular, pear-shaped, apiculate or lemon-shaped, ogival or pointed at one end, or tetrahedral. The diameter of a spherical cell may vary from 2 to 10 micrometers. The length of cylindrical cells is often 20–30 μm and, in some cases, even greater.

The asexual multiplication of yeast cells occurs by a budding process, by the formation of cross walls or fission, and sometimes by a combination of these two processes. Yeast buds are sometimes called blastospores or blastoconidia. When yeast reproduces by a fission mechanism, the resulting cells are termed arthrospores or arthroconidia.

Yeasts are categorized into two groups, based on their methods of sexual reproduction: the ascomycetous (Division Ascomycotina) and basidiomycetous (Division Basidiomycotina) yeasts.

The sexual spores of the ascomycetous yeasts are termed ascospores, which are formed in simple structures, often a vegetative cell. Such asci are called naked asci because of the absence of an ascocarp, which is a more complex fruiting body found in the higher Ascomycetes. If the vegetative cells are diploid, a cell may transform directly into an ascus after the 2n nucleus undergoes a reduction or meiotic division. See Ascomycota

Certain yeasts have been shown to be heterothallic; that is, sporulation occurs when strains of opposite mating type (usually indicated by “a” and α) are mixed on sporulation media. However, some strains may be homothallic (self-fertile), and reduction division and karyogamy (fusion of two haploid nuclei) take place during formation of the sexual spore. Yeasts that produce sporogenous cells represent the teleomorphic form of the life cycle. In cases, in which sexual cycles are unknown, the yeast represents the asexual or anamorphic form. A species of yeast may be originally discovered in the anamorphic form and named accordingly; subsequently, the sexual state may be found and a name applied to represent the teleomorph. Consequently, the anamorphic and teleomorphic names will differ.

Basidiospores and teliospores are the sexual spores that are produced in the three classes of basidiomycetous yeasts: Urediniomycetes, Hymenomycetes, and Ustilaginomycetes. Sexual reproduction and life cycle in these yeasts is typical of other basidiomycetes in that it can include both unifactorial (bipolar) and bifactorial (tetrapolar) mating systems. See Basidiomycota

Some yeasts have the ability to carry out an alcoholic fermentation. Other yeasts lack this property. In addition to the fermentative type of metabolism, fermentative yeasts as a rule have a respiratory type of metabolism, whereas nonfermentative yeasts have only a respiratory, or oxidative, metabolism. Both reactions produce energy, with respiration producing by far the most, which is used in part for synthetic reactions, such as assimilation and growth. Part is lost as heat. In addition, small or sometimes large amounts of by-products are formed, including organic acids, esters, aldehydes, glycerol, and higher alcohols. When a fermenting yeast culture is aerated, fermentation is suppressed and respiration increases. This phenomenon is called the Pasteur effect. See Fermentation

Yeasts are ubiquitous in nature. They exist on plants and animals; in waters, sediments, and soils; and in terrestrial, aquatic, and marine habitats. Yeasts require oxygen for growth and reproduction; therefore they do not inhabit anaerobic environments such as anoxic sediments. Many species have highly specific habitats, whereas others are found on a variety of substrates in nature.


A collective name for those fungi which possess, under normal conditions of growth, a vegetative body (thallus) consisting, at least in part, of simple, individual cells.


1. any of various single-celled ascomycetous fungi of the genus Saccharomyces and related genera, which reproduce by budding and are able to ferment sugars: a rich source of vitamins of the B complex
2. any yeastlike fungus, esp of the genus Candida, which can cause thrush in areas infected with it
3. a preparation containing yeast cells, used to treat diseases caused by vitamin B deficiency
References in periodicals archive ?
The product is made by combining dried yeast with the re-hydrating agent sorbitan monostearate.
ASPARAGUS, RICOTTA & PANCETTA PIZZA FOR THE DOUGH 250ml tepid water 1 tsp caster sugar 1 x 7g sachet easy blend dried yeast 300g strong white bread flour, plus extra for dusting 1 heaped tsp flaked sea salt 1 tbsp extra virgin olive oil for greasing FOR THE TOPPING 1 tbsp olive oil 1 clove of good hard garlic 300ml tinned chopped tomatoes Half tsp salt Half tsp sugar 200g ricotta 2 bundles of British asparagus, trimmed and cut into 4 lengthways 1 large handful of dried oregano leaves Handful of pecorino cheese Olive oil The dough: Pour the water into a bowl and stir in the sugar and yeast.
INGREDIENTS 90g fine cornmeal, plus extra for dusting 370g strong white bread flour 11/2tsp salt 5g (1tsp) fast-action dried yeast 1/2tsp chilli powder (mild or hot to taste) 1/2tsp ground cumin 2tsp paprika 2tsp smoked sweet paprika 1/4tsp garlic powder or fine granules 1/4tsp onion powder 4tsp virgin olive oil 300ml lukewarm water 100g drained sliced jalapeno chillies (from a jar) 1.
INGREDIENTS: 450g (14 1/2oz) strong white bread flour 1 pinch salt 7g (1/4oz) fast-action dried yeast 1tsp honey 1 pinch cayenne pepper 2tsp mustard powder 285ml (9fl oz) water, warm 115g (4oz) cheddar, grated 45g (1 1/2oz) Parmesan, grated To decorate 1 egg, beaten 20g (3/4oz) raisins 2 tbsp poppy seeds METHOD: Preheat the oven to gas 6, 200degC, fan 180degC.
Stage One (make the evening before you want the buns) Half a teaspoon of dried yeast, 8 tablespoons of tepid water, 2oz of wholemeal flour, 4oz of strong white flour.
Makes 20 INGREDIENTS 500g plain strong white flour half teaspoon salt 50g caster sugar 2 sachets dried yeast 1 medium egg, beaten 125mls skimmed milk 150mls hot water 10 sprays calorie controlled cooking spray To finish: 400g strawberry jam 4 tablespoons caster sugar half teaspoon ground cinnamon METHOD?
1/2tsp dried yeast,dissolved in 1/4 cup warm water -- kha- mee-ra
INGREDIENTS (makes 12) 250g whole milk 10g quick-action dried yeast 650g strong white flour, plus some for kneading 30g caster sugar 5g (1tsp) salt 275g unsalted butter, softened 50g sultanas 1 egg, beaten For the glaze: 40g caster sugar 20g water 4 white sugar cubes METHOD Warm the milk to 30degC in a pan and whisk in the yeast.
Tenders are invited for Supply of Item Potassium phosphate dibasic 500G, Dried yeast extract 500G, Tween 80 500ml, Peptone powder 500G, Trichloro acetic acid 100G, midoblack 25G, Acetic acid 500M,L Mueller hinton agar 500G, Dextrose 50 discs per vial Vial, Maltose 50 discs per vial Vial, Sucrose 50 discs per vial Vial, Lactose 50 discs per vial Vial, Galactose 50 discs per vial Vial, Melibiose ,50 discs per vial Vial, Cellobiose 50 discs per vial Vial, Inositol 50 discs per vial Vial, Xylose50 discs per vial Vial, Raffinose 50 discs per vial Vial etc.
INGREDIENTS 400g plain flour 150g self-raising flour Zest of 2 large lemons 2 large eggs 1tsp vanilla extract 1 /2tsp salt 100g melted butter 225ml warm milk (43[bar]C) 150g mixed fruit 50g chopped almonds 200g marzipan 1tbsp rum 50g caster sugar 1 packet dried yeast Icing sugar for dusting METHOD 1.