Yeast is a saprophyte and grows in a variety of substances containing sugar such as grape, datepalm, toddy, nectar, hoey. It also grows in soil, animal excreta, on vegetable parts and in milk.
Structure- it is unicellular and microscopic. The cells are oval or round and range 8µ to 15µ in size. Each cell is covered by a cell wall and encloses inside the dense granular cytoplasm. The cytoplasm contains reserve food in the form of glycogen. It also contains endoplasmic reticulum, ribosomes, mitochondria, Golgi bodies, fat globules, proteins and volutin. There is a large vacuolated nucleus with small nucleolus at one end and chromatin network in the center. There is a centrosome very close to the anterior end of the nucleus.
Yeast reproduces by vegetative, asexual and sexual methods.
1. Vegetative reproduction:
Yeast reproduces by vegetative methods when there is enough sugar solution and oxygen is freely available. It takes place by budding and fission.
(a) Yeast budding:
Protoplasm of the cell surrounded by cell wall bulges out to form a bud. The nucleus divides by intranuclear mitosis and the nuclear membrane containing a group of chromosomes elongates to enter the bud. Then the bud along with nuclear membrane is pinched off from the mother cell may produce several buds in a similar way.
(b) Yeast fission:
Yeast cell elongates. The nucleus divides mitotically into two daughter nuclei. Cytokinesis takes place by transverse wall at the center of the cell, which divides it into two daughter cells of equal size. Each newly formed cell enlarges into individual yeast cell.
2. Asexual reproduction:
It occurs when there is scarcity of sugar solution for the vegetative growth of yeast. In such unfavourable condition, the vegetative cell functions are ascus. The protoplast of the cell contracts and its nucleus divides to form four nucei. Each nucleus is surrounded by small amount of cytoplasm and the wall. They are now called as ascospores. They are released by the rupture of ascus wall. Under favourable condition, they germinate by budding producing yeast cells.
3. Sexual Reproduction:
When there is scarcity of food, sexual reproduction is not of common occurrence. Sex organs are not formed. Sexual union takes place between two vegetative cells or two ascopores resulting in the formation of a diploid zygote. Zygote divides meiotically and meiotically producing 4 to 8 ascospores depending upon the nuclear division. Three types of sexual life cycles are observed in different species of yeast. Ascospores germinate into new yeast.
(a) Haplobiontic life cycle:
It is found in Schizosaccharomyces octosporus. In this life cycle, the diploid phase is very short and haploid phase is long.
(b) Diplobantic life cycle:
It occurs in Saccharomyces ludwigii. In this life cycle, diploid phase is long and haploid phase is short.
(c) Haplo-Diplobiontic life cycle:
It occurs in Saccharomyces cerevisiae. In this life cycle, haploid and diploid phase are equally represented.
- Yeast contains zymase group of enzymes and has great quality of fermenting carbohydrate into alcohol. So it is used in breweries and bakeries.
- In bakeries, Sec. Cerevisiae is used for bread making. Fermentation releases CO2 which bubbles out during heating that make the bread porous.
- Yeast is rich in vitamin been and protein. It is used in the productions of vitaminised food.
- It is also used in the production of riboflavin.
- It is used in the manufacture of various syrups and confectionary items.
- Various foods like milk and sugar products are spoiled by yeast.