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Some Microbiologically Important Yeasts
(i) Saccharomyces
Most of the industrially important yeasts belong to this genus. They occur mostly on media rich in sugar or on organic matter chiefly of vegetative origin.
About 40 species of Saccharomyce are known, of which the leading one is S. cerevisiae which is commonly called Baker's and Brewer's yeast. It is employed in many food industries, special strains being used for the leavening of bread, as 'top yeasts' for ale, for wines, and for the production of alcohol, glycerol and invertase.
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Top yeasts are very active fermenters and grow rapidly at 20°C. The clumping of the cells and the rapid evolution of CO2 sweep the cells to the surface, hence the term "top yeast". Besides top yeasts, some Brewer's yeasts are 'bottom yeasts'.
The latter do not clump, grow more slowly, and are best fermenters at lower temperature like 10-15°C.
The absence of clumping, slower growth and lack of CO2 evolution permit the yeast to settle to the bottom, hence the term "bottom yeast",
S. cerevisiae var. ellipsoideus is a high alcohol yielding variety used to produce industrial alcohol, wines, and distilled liquors. S. carlsbergensis, a bottom yeast, is used in making bear.
S. fragilis and S. lactis, because of their ability to ferment lactose, may be important in milk product industry.
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(ii) Zygosaccharomyces
This yeast is considered by some as a subgenus of Saccharomyces. However, Zygosaccharomyces are notable for their ability to grow in high sugar concentrations and hence are termed 'osmophilic'. (Saccharomyces rouxii and S. mellis also grow in high concentrations).
These yeasts are involved in the spoilage of honey, syrups and molasses and in the fermentation of soya-sauce and some wines. It is the Z. nussbaumeri species of this genus which grows luxuriently in honey.
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(iii) Candida
Van Uden and Buckley (1970) have recognised eleven species of this genus on the basis of physiological characteristics. Candida, although normally encountered in 'yeast' phase, can usually be induced to produce pseudo-, sometimes, true mycelia under cultural conditions.
This yeast forms white or creamy colonies which consist of subglobose to oval budding cells (3.6 µm x 3.5 - 12.5. µ m). The cells frequently show bipolar or tetrapolar budding. As the colony becomes older, there develops branched pseudomycelia or the true mycelia that produce spores.
The spores may be either blastospores or chlamydospores or, more frequently, both. Sporulation is characteristic feature of C. albicans, the best studied species.
C. utilis is used commercially in the production of fodder; being rich in proteins, fats and minerals. It is also used for food, and in the production of invertase, lipid. ribonucleic acid and urease. C. tropicalis is used for food, protein (hydrocarbons) and ribonucleic acid production.
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C. krusei has been grown with dairy starter cultures to maintain activity and increase longevity of the lactic acid bacteria.
This fungus may become pathogenic causing 'candidiasis'. a disease that may take a number pf forms in humans. About 20 species of genus Candida are pathogenic for man. Examples of candidiasis listed by Emmons et al. (1977) include cutaneous candidiasis, oral candidiasis (thrush-disease). bronco-candidiasis. pulmonary candidiasis and vulvovaginal candidiasis.
It appears that predisposition factors such as other disease, physiological disorders, obesity, alchoholism and the use of broad spectrum antibiotics and steroids contribute to the conditions under which C. albicans becomes pathogenic.
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(iv) Rhodotorula
The cells of this yeast are round, oval, or elongated. Primitive pseudomycelium may be formed but no true mycelium. These red, pink or yellow yeasts may cause discolourations on foods e.g., coloured spots on meats or pink areas in sauerkraut.
R. gracilis is the type species and at present is industrially important because of being used in the production of cystine, lipid, and methionine.
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