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In this article we will discuss about the family saccharomycetaceae and endomycetales.
The Saccharomycetaceae are commonly known as the yeasts. Yeasts were first detected by Anton van Leeuwenhoek as early as 1680.
They were compared with globular, oval bodies of albuminoids. After Pasteur (1875) had demonstrated that alcoholic fermentation results from the activity of yeasts in grape juice, these microorganisms, previously regarded as lifeless, attracted attention of botanists and were the subjects of morphological and systematic investigations.
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What are yeasts? In the botanical sense, yeasts are unicellular fungi of biochemical interest which when placed in sugar solutions decompose them into alcohol and carbon dioxide causing alcoholic fermentation.
Yeasts may be:
(i) Fungi with special morphology, spherical or oval in shape, multiply by fission or budding and produce ascospores at certain stages in their life cycles—these are known as true yeasts; and
(ii) Fungi living normally with a mycelium are able to reproduce by budding of their spores to form cells which have the shapes of yeasts, the basidiospores of some Basidiomycetes (Ustilago, Coprinus lagopus) and ascospores of certain Ascomycetes (Taphrina) multiply by budding, and hyphae of Mucor when submerged in sugar solution dissociate into round structures, but in a similar manner as the yeasts—all these are yeast-like fungi.
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Physiologically true yeasts differ from the yeast-like fungi by their resistance to anaerobic conditions and high fermenting capacity. Again some yeasts may be deprived of fermenting function. Yeasts are typically single-celled. Under certain conditions of the medium some yeasts may develop short hyphae. Yeasts are widespread in their distribution. Mostly the yeasts are saprophytic fungi.
They are commonly found in organic substrates like soil, decaying vegetables, ripe fruits and grains, sugary exudates of trees, and in the nector of flowers. Yeasts are of considerable economic importance. They are capable of fermenting carbohydrate solutions producing ethyl alcohol and carbon dioxide.
The beer, alcohol, and wine yeasts are included in the genus Saccharomyces, which has a number of species. Saccharomyces cerevisiae is by far the most important species being the one employed in beer-brewing and bread-making. Both the biological interest and the economic importance of yeast centre round its metabolism, which is closely connected with its enzyme content.
The yeast cells absorb certain sugar solution which is decomposed within the portoplast forming ethyl alcohol and carbon dioxide. This change is brought about by an enzyme, zymase, which is a product of the protoplasm. The enzyme zymase was discovered by Buchnar (1900).
It is not always necessary to add yeast to the solutions containing the fruit sugars since wild yeasts are present in nature on the surface of the fruits.
Another enzyme present is invertase, which hydrolyses cane sugar into glucose and laevulose. Many other enzymes, too, are present, such as lipases and proteases, which split fats and proteins respectively, so that they may be absorbed in the yeast protoplasm. In bread-making, yeast plants are mixed with flour and water.
The flour contains some sugar in addition to the starch. At a suitable temperature, the yeast cells multiply rapidly. Zymase is formed, which aids in the breaking down of the sugar into akohol and carbon dioxide.
The bubbles of carbon dioxide gas are prevented from escaping by the dough, which, however, is caused to rise by the expansion of the bubbles. At baking temperatures there is further expansion of the gas bubbles, and also a driving off alcohol and water. The carbon dioxide released in fermenting dough raises the bread in making its escape.
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Yeasts are employed to impart flavour to cacao beans. Yeast cells are rich in protein and certain strains afford considerable fat. They are, therefore, mingled with other foodstuffs to increase their food value. Yeasts are also important as a commercial source of vitamins. Fresh yeast cells are excellent source of vitamins B and G.
A few species of yeasts are parasites of higher plants. They occasionally induce diseases of tomato fruits and pods of beans. Several species of yeast are pathogenic to man, causing diseases, of which blastomycosis, or torulosis is common. They attack the central nervous system and skin of man. Often they also cause diseases of ears, liver, spleen, and lymph glands.
