BIOLOGY OF YEAST

This is a degenerate fungus consisting of very small single cells. It occurs naturally in warmer climates, where it flourishes in fruit juices of fallen over-ripe fruit. In this country and in other temperate climates it is cultivated by brewers or grown for the production of baker ’s yeast. It is usually grown in warm solutions of sugar obtained by steeping malt in hot water. Malt consists of barley grains which have been allowed to germinate in order that the starch in them shall be converted to sugar. The grains are then killed and dried by placing them in hot ovens, technically known as kilns. Sugar obtained from rye, maize, potato starch and molasses is also used.

Yeast reproduces very rapidly in favourable conditions by a process of budding. Each cell forms a bud, its nucleus divides and one of the two new nuclei passes into the bud, which then separates. Often bud formation occurs so rapidly that the first bud has already started to form another before it separates from its parent cell.

In unfavourable conditions each yeast cell forms four internal spores with thick walls. These on liberation are dispersed by air currents in a dormant state to germinate when favourable conditions again appear, I.e. when blown into a fresh sugar solution, e.g. the juice of an over-ripe fruit. This form of reproduction normally only occurs in the wild state. In some species of yeast, spore production is preceded by conjugation between neighbouring cells, when a process resembling that of Spirogyra occurs. A tube connection is formed between the two cells, the contents of one pass over into the other, and the fused contents then form four spores. This is obviously a form of sexual reproduction.

Economic Importance of Yeast

Yeast is used for the production of alcoholic beverages and of industrial spirit, which is used in many commercial processes and also as a fuel. Bakers use yeast to make bread ’rise ’and to acquire a spongy character which renders it more easily digested. The ’holes ’in bread are due to bubbles of carbon dioxide produced by the yeast. Yeast is also used as a source of vitamin B.

Practical Work on Fungi

Mucor

A slice of bread or of cooked beetroot should be placed on a small plate and well moistened. The bread is covered with an inverted glass crystallizing dish or bell jar and left in a warm place until the fluffy white mycelium with the characteristic black dots appears. Examine a portion of the mycelium with sporangia mounted in water under the microscope. Notice stages in formation of sporangia and also the burst ones showing the columellas.

Mushroom

Examine and draw various stages in the developing mushroom. Cut vertical sections to find the developing gills. Finally draw a fully opened mushroom. Detach a ripe head and place it on a sheet of white paper, gills downwards. Leave overnight, and next day remove the head and see the spore ’print ’of the gills produced by the shed spores.

Yeast

Suspend some baker ’s yeast in a warm dilute sugar solution for two hours. Examine a drop of the milky fluid under the microscope and observe the oval cells, many of them budding.

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