Yeasts are single-celled organisms of the kingdom, Fungi. Yeasts constitute about 1% of the fungi species and currently, around 1500 species of yeasts are known. Yeast fungus is eukaryotic in nature which means they have a nucleus enclosed within a nuclear membrane. The most common question asked about yeasts is, is yeast a fungus? The answer is yes. Yeasts are a type of fungi.
Yeast is a very useful microorganism and is used to cause fermentation. Yeast grows by fermentation, feeding upon sugars. During fermentation, yeast converts glucose into ethanol and carbon dioxide and is therefore used in beer and wine manufacturing, and baking. In beer and wine manufacturing, ethanol is the desired product and carbon dioxide is useful in baking.
Yeasts are single-celled organisms that evolved from multicellular organisms. The size of yeast mainly depends upon its environment. Typically yeasts are 3-4min in diameter but can grow up to 40m.
As the cell organization in yeast resembles that of higher organisms. Due to a high degree of similarity between yeast and human genes, yeast has become a popular model system for the study of human diseases. Yeast is increasingly becoming the organism of choice in the study of drug targeting and mode of action.
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Yeast fungus can reproduce both sexually and asexually. The most common form of yeast reproduction is by the method of budding. It is an asexual reproduction process in which a small bud is formed on the parent cell. This bud is called a bleb or a daughter cell. Further, the nucleus of the parent cell splits into two and migrates towards the daughter cell. The growth of the bud continues and then is separated from its parent cell forming a new cell. Yeasts can also reproduce by the method of fission in which two identical daughter cells are formed.
Under conditions such as nutrient starvation, the haploid cells can’t survive. Under these high-stress conditions, the diploid cells undergo sporulation and perform meiosis, which is a mode of sexual reproduction to produce haploid cells. The haploid cells then can further conjugate to form diploid cells. However, this mode of yeast reproduction occurs only when the nutrients are limited as in a nutrient abundance situation, yeast cells reproduce using asexual modes of reproduction.
The various characteristics of Yeast are as follows:
Is Yeast a Fungus? -
Yeasts are single-celled microorganisms and are members of the Kingdom Fungi. Yeasts are then further classified based on the characteristics of their cell, ascospore and cellular physiology.
Single-Celled Organisms -
Even though yeasts are single-celled, their cellular organization resembles that of higher organisms including humans. Their genetic content is contained within their nucleus and are therefore considered eukaryotic in nature.
The natural habitat of yeast is widely dispersed in nature. Yeasts are commonly found on plant leaves, flowers, fruits and soil. They are also present on the skin surface and in the intestinal tract of warm-blooded animals. The common yeast infection such as vaginal yeast infections, diaper rash, and thrush of the mouth and throat are caused by Candida albicans.
Growth and Metabolism -
Yeast grows by fermentation by converting glucose into carbon dioxide and ethanol. Fermentation is an anaerobic process as it generally occurs in the absence of oxygen. Yeast cells prefer to grow by fermentation when they are grown in rich carbon sources and even in the presence of oxygen due to the crabtree effect.
Yeasts normally reproduce through asexual modes of reproduction such as budding and fission. But under nutrient starvation, can undergo sexual mode of reproduction as well.
Yeasts are used to ferment glucose into ethanol and carbon dioxide and find their use in the production of alcoholic beverages, bread, lubricants, detergents, food additives, enzymes, chemicals etc.
Yeasts are single-celled microorganisms that are commonly used for a variety of purposes. Yeasts are a type of fungus and belong to the kingdom fungi. The most common use of yeast is for the fermentation of sugars in the production of alcoholic beverages and bread. Also, yeasts are being used in the study of human diseases and drugs research.
1. What is yeast used for?
Yeast is commonly used in various commercial applications due to its distinct properties. The metabolism of yeast can be manipulated by using genetic techniques for the production of alcoholic beverages, bread and industrial products. The various uses of yeast are-
Yeast has long been used to ferment the sugars of rice, barley and corn to produce beer and wine. Although yeast is present in the air and can begin the process of fermentation. Commercially, many vineries choose to add pure yeast to control and dominate the fermentation.
Yeast is used in baking as a leavening agent. The sugars present in the dough are fermented to produce carbon dioxide and ethanol. The carbon dioxide thus produced gets trapped in small bubbles which causes the dough to rise.
Further yeast is used in the petrochemical industry, food industry, in the production of industrial enzymes, chemicals, lubricants and detergents.
2. Give a detailed description of a yeast cell.
Yeasts are single-celled organisms which are eukaryotic in nature. Yeast cells can easily be cultured in laboratories for research purposes. Most yeasts usually thrive in warm, dilute, sugary, acidic and aerobic environments. The yeast cell description is as follows:
Yeasts have a cell organization similar to higher organisms.
The cell organelles such as the nucleus, mitochondria, vacuole, Golgi apparatus, endoplasmic reticulum and peroxisomes are bound by a membrane.
The nucleus structure is located centrally surrounded by a double membrane separating the nucleoplasm from the cytoplasm.
The cytoplasm is an acidic, slightly colloidal fluid containing protein, glycogen and other soluble macromolecules.
The cytoskeleton consists of microtubules and microfilaments.
3. What is fermentation?
Fermentation is a chemical process by which molecules like glucose get broken down anaerobically. In simple terms, fermentation refers to the foaming that takes place during the manufacturing process of wine and beer.
The frothing is actually a result of the evolution of carbon dioxide gas.
The chemical process of fermentation is described as glucose getting converted into pyruvic acid during glycolysis. The metabolic pathway, called glycolysis, converts glucose into pyruvate. This is the first major step of fermentation or respiration in cells.
When oxygen is available, pyruvic acid enters a series of chemical reactions, called the tricarboxylic acid cycle, and proceeds to the respiratory chain.
Cells produce 36–38 molecules of ATP for each molecule of glucose oxidized, during this respiration.
When oxygen is absent, pyruvic acid can follow in two different ways, depending on the cell type:
It can be converted into ethanol and carbon dioxide by alcoholic fermentation.
Or it can be converted into lactate through the lactic acid fermentation way.
In industrial fermentation, the process kickstarts with microorganisms that are suitable for it and specific conditions. For example, a specific nutrient concentration.
4. What are the 3 types of yeast in the market?
There are different types of yeast in the market, based mainly on the time it takes to act:
1. Fresh Yeast: This yeast is cream or beige in colour and has a beer-like odor. Fresh yeast can be stored in the refrigerator for approximately 2-3 weeks.
2. Dried Yeast: This yeast is dried so as to remove most of its water. It has brownish grains and must be activated with a warm liquid and a food source before adding to the flour. It can last up to six months.
3. Fast-Action Dried Yeast: This is a mixture of dried yeast and vitamin C. It is used to speed up the fermentation process. It does not require rehydration before cooking.
4. Active Dry Yeast, Rapid-Rise Yeast, Instant Yeast, and Bread Machine Yeast: Though there are some minor differences in shape and nutrients, Rapid-Rise Yeast is pretty much the same as Instant Yeast and Bread Machine Yeast. The advantage of the Rapid-Rise Yeast is the rising time is half that of the Active Dry and it only needs one rising. Also, Active Dry Yeast is slightly less potent than the Rapid-Rise Yeast.
5. How was yeast discovered?
Yeast has been known to mankind for at least 10,000 years and Egyptians were known to use yeast to make their bread some 5000 years ago, believing it to be a miracle.
It was in 1680 that Antoni van Leeuwenhoek, a Dutch tradesman, made some crucial discoveries about yeast.
Leeuwenhoek developed a high-quality lens, which he used to observe beer yeast globules for the first time. He discovered that yeast consists of globules floating in a fluid, but he thought that they were just the starchy particles of the grain from which the wort was made.
So, fermentation was first understood in the year 1857 when French scientist, Louis Pasteur, threw light on the process.
Pasteur believed that the agents that were responsible for fermentation were yeasts. He established the key role of yeast as the microorganisms responsible for alcoholic fermentation. He understood very quickly that yeast was indispensable for forming the bread’s texture, aroma and flavours. He also proved that the yeast cell can live with or without oxygen.
At the time, nobody understood that yeast was indeed a living being. It was viewed as just an organic chemical agent that is needed for fermentation.
Towards the end of the 19th century, Eduard Buchner, a German biochemist, presented that fermentation could occur in yeast extracts that are free of cells. This made it possible to study fermentation biochemistry in vitro.
Upon further studying the cell-free extracts, Buchner found zymase, the active constituent that carries out fermentation. This is when he realised that the chemical reactions responsible for fermentation were actually taking place inside the yeast.
Buchner was awarded the Nobel Prize for Chemistry in 1907 for proving that carbohydrate fermentation is a result of the action by certain enzymes contained in the yeast, and not by the yeast cell itself. He demonstrated that it was possible to extract the enzyme called zymase from yeast cells. Zymase is what causes sugar to break down into carbon dioxide or alcohol.