If a weak sugar solution is exposed to the air, in several days a light, frothy scum appears on the surface and the liquid begins to smell of alcohol. This change take place because tiny plant cells called yeast have settled from the air into the liquid. They have found condition favorable to their growth.
Man has long known that this process takes place and he has used it for thousands of years to make alcoholic beverages of all kinds. Sugar solution made from molasses, potatoes, rye, corn, malt and hops, apples,, and grapes have been exposed to the air to make alcohol, whiskey, beer, ale, cider, wine and other beverages.
Probably through accidents, it was also discovered that if bread dough were allowed to stand for some time before baking, very often a peculiar change took place. The flat lump of dough began mysteriously to swell and rise. It developed a strange but pleasant odor. When this dough was baked, instead of making a flat, heavy slab, it made light porous and soft bread!
In 1857, Louis pasture announced that he had discovered the explanation for these changes. He said that they were due to the presence of tiny, one-celled plants called yeast. Yeast belongs to the fungi family, and is tiny, rounded, colorless bodies. They are larger than most bacteria, but still so small that it would take from 1,200 to 1,600 of them laid said by side to make a centimetre.
Yeast is a fungus belongs to the division ascomycota. It grows on sugary food substances. Its body consists of a single cell. There is no formation of mycelium. The cell is covered by cell wall; inside the cell cytoplasm is present which consists of two parts.
- Ectoplasm: It is the outer part of cytoplasm.
- Endoplasm: It is the inner part of cytoplasm.
In the cell large vacuole is present. Nucleus is situated at one side of the vacuole. It has nucleolus and it produces chromosomes.
Reproduction in Yeast
There are 2 types of reproduction ¡n yeast.
- Asexual reproduction.
- Sexual reproduction.
There are two types of asexual reproduction.
Budding is a common method of reproduction in yeast. In this process a small body is developed from any side of the cell. It is known as Bud. The nucleus of each cell divides into two nuclei. One nucleus remains in the cells and other nucleus migrates into the bud. This bud is separated from yeast cell and can develop into new cell. Sometimes there is continuous process of bud formation and many buds may be formed in chain like structures.
In This process the nucleus of yeast cell divides into two nuclei, the cytoplasm divides into two parts. Each part contains a nucleus. Finally two cells are separated from each other. In this way two independent cells are formed.
In sexual reproduction two take part. When cytoplasm of two cells fuses together, it is called Plasmogamy. And, when two nuclei of cells unite together, it is called karyogamy. Then two cells unite together to form Zygote. The zygote is change into reproductive organ, called ascus.
The Zygote nucleus divides by meiosis and forms ascopores. After maturation the ascospores come out from ascus. The ascospores develop into new yeast by getting favorable conditions.
Production of Yeast
Yeast cells reproduce by budding. This means they send out projections which become cut off from the parent cell by a cell wall. Finally, these projections grow to full size. As they grow, they form substances called zymase and invertase.
These substances are called enzymes, they have the power to ferment starch to sugar, and sugar to alcohol and carbon dioxide. As fermentation take place, carbon dioxide is formed and rises to the top. Then it escapes, leaving the alcohol. Beer, ale, wine, and cider are fermented beverages in which yeast has changed some of the sugar to carbon dioxide and alcohol.
In bread making, the carbon dioxide collects in bubbles in the dough, which makes it rise. Heat later derives off the carbon dioxide, thus making the bread porous and light.
IMPORTANCE OF YEAST
Yeast has positive as well as negative importance.
- Yeast is used to prepare different bakery items by fermentation process.
- It is also used in the manufacturing of butter and cheese.
- It also used in the formation of alcohol.
- It destroys different food substances.
- Yeast causes damage to butter and cheese.
- It may cause diseases of nails, skin, and hairs in man known as moniliasis.
- It may cause lung disease.