If a weak sugar solution is exposed to the air, after several days a light, frothy scum appears on the surface and the liquid begins to smell of alcohol. This change takes place because tiny plant cells called yeast have settled from the air into the liquid. They have found conditions 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 has 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 side by side to make a centimeter.
Yeast is a fungus belonging 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 a cell wall; inside the cell, the cytoplasm is present which consists of two parts.
- Ectoplasm: It is the outer part of the cytoplasm.
- Endoplasm: It is the inner part of the cytoplasm.
In the cell large vacuole is present. The nucleus is situated on one side of the vacuole. It has nucleolus and it produces chromosomes.
Reproduction in Yeast
There are 2 types of reproduction in 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 another nucleus migrates into the bud. This bud is separated from yeast cells and can develop into new cells. Sometimes there is a continuous process of bud formation and many buds may be formed in chain-like structures.
In This process, the nucleus of a yeast cell divides into two nuclei, and 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 the 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 changed into the reproductive organ, called an ascus.
The Zygote nucleus divides by meiosis and forms ascopores. After maturation, the ascospores come out from the 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 that 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 takes 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, carbon dioxide collects in bubbles in the dough, which makes it rise. Heat later drives 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 preparing different bakery items through the fermentation process.
- It is also used in the manufacturing of butter and cheese.
- It is 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 hair in men known as moniliasis.
- It may cause lung disease.