Saccharomyces cerevisiae (S. cerevisiae) is a unicellular fungus widely used for producing yeast fermentation in bread and in the production of wine and beer, belonging to the Saccharomyces.
Its morphology is influenced by its mode of reproduction, which is by budding, meaning it reproduces asexually from a bud or protuberance of a Saccharomyces.
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Characteristics of Saccharomyces cerevisiae
It has a yellow color with green hues, being a unicellular eukaryotic microorganism. It is present in many types of habitats. It can be found naturally in soil, water, as well as on various plants. But undoubtedly, S. cerevisiae currently has widespread uses in the food industry due to its ability to ferment certain foods.
The characteristics of Saccharomyces cerevisiae are:
- Unicellular eukaryotic organism.
- Yellow-green color.
- Belongs to the chemorganotroph family.
- It is a facultative anaerobic bacterium.
- Feeds on glucose and other carbohydrates.
- Produces a protein toxin that inhibits the growth of other yeasts (killer effect).
Chemorganotrophs obtain their energy from the oxidation of organic compounds. Thus, Saccharomyces cerevisiae does not need sunlight to grow and develop.
As a facultative anaerobic microbe, S. cerevisiae can live in low oxygen conditions. To do this, it can ferment organic matter and use it to breathe, producing CO2, ethanol, and glycerol through the chemical reaction called alcoholic fermentation.
It should be noted that in situations where oxygen is scarce, its growth and reproduction are deficient. However, this is the ideal situation to use the microorganism as a biological agent to produce fermentation of cereals such as barley, wheat, or corn.
Although it uses different substrates for nutrition, the most common carbon source is glucose, but it can also feed on other sugars.
Genome
Cerevisiae has 5,800 genes. Its genome has been completed in recent years. When the race to understand genes began, this eukaryotic microorganism was one of the first to be completed.
Of the 5,800 genes studied, about 708 have been identified as functional in its metabolism.
Morphology
It is formed by a cell membrane that isolates the cell nucleus from the exterior, just like other eukaryotic organisms.
Inside this membrane, we can find the nuclear membrane, which functions to protect the DNA it contains.
The mitochondrial membrane is responsible for producing energy, providing the vital energy that Saccharomyces needs.
The endoplasmic reticulum (ER) produces the synthesis of proteins, lipids, and facilitates the detoxification of the cell.
Its cytoskeleton is formed by the vacuole and peroxisomes, organelles present in all plant cells. The vacuole is a closed compartment isolated by the plasma membrane. Peroxisomes contain oxidases and catalases, serving as a digestive tube while also functioning as a protein cytoskeleton.
A common characteristic in many eukaryotic organisms is turgor pressure. It can swell when there are many fluids inside, something that gives Saccharomyces cerevisiae a peculiar morphology. Additionally, its cell wall is composed of chitin, a nitrogenous carbohydrate that is insoluble in water and protects the microorganism against external aggressions.
Metabolism
There are two types of metabolism:
- Oxidative or respiratory metabolism.
- Fermentative metabolism.
Aerobic metabolism needs oxygen to live, and fermentative metabolism uses fermentation to produce energy. The sources used are carbohydrates, amino acids, and sugars such as fructose, glucose, maltose, galactose, mannose, etc.
Under optimal conditions with high concentrations of sugars, Saccharomyces cerevisiae can perform fermentation through glycolysis to subsequently produce ethanol.
When sugars are scarce, the fungus S. cerevisiae can use ethanol for respiration through the Krebs Cycle.
Habitat
Its presence is very broad. It can be found in many places in nature, such as plants, fruits, and even soil. It is common to detect Saccharomyces cerevisiae habitats in ripe fruits (grapes, pears, apples, etc.) as well as in the sap and bark of trees.
The environment where it is most frequently found is in carbohydrate-rich media where it can produce fermentation, tolerating the presence of alcohol very well.
Another common place to find S. cerevisiae is in humid organic environments. It is often discovered alongside other microorganisms that also feed on the same sources.
This unicellular microorganism is widespread in nature, being one of the main catalysts of the organic matter decomposition process. It accelerates decomposition.
pH
Ideally, the culture medium should be maintained with a pH between 4.5 and 6, commonly ranging between 5.2 and 5.7 to produce higher fermentation.
However, within these values, it is possible to modify the pH depending on the interests, such as making wine with S. cerevisiae or beer. Depending on the sourdough used, the values should be optimized to achieve good results.
Wine
In the wine industry, it is of utmost importance as it is one of the most used yeasts. Saccharomyces cerevisiae in wine acts as an ascomycete fungus and modifies some enzymes.
Cerevisiae enhances the wine's aroma, being essential for achieving the best wines.
Due to its prolific nature in grape juice, it is very suitable for wine production. It produces high amounts of ethanol and decreases the presence of sugars, also lowering the pH, which inhibits the growth of other yeasts and microorganisms that could spoil the wine production.
The killer effect, the secretion of a toxin that promotes the death of other yeast strains, makes cerevisiae ideal for keeping wine in perfect condition, as well as other beverages such as beer and even bread dough. It is a highly appreciated control method by wine producers as it keeps germs away.
When the wine is aged, its presence can be observed as a surface film (flor veil).
The use of one of the Saccharomyces strains will be chosen by the winemaker after crushing. This will take into account the final characteristics desired, considering the production temperature as well as the final taste.
Bread
As in the wine industry, in baking, there is a very useful resource with Saccharomyces cerevisiae yeast to make bread.
Yeasts are essential for fermentation, making the dough rise and the crumb become fluffy.
Bread yeast is marketed dry but rich in cerevisiae. To make it, it is added to the flour, which will be fermented by the microorganisms, resulting in the rise of the bread dough and carbon dioxide. The latter is responsible for the increased volume of the dough and its characteristic holey appearance, like Gruyère cheese.
In baking, Saccharomyces absorbs the sugars from the starch in the flour, preferring glucose. The sugar that will disappear first is glucose, then sucrose, fructose, and at the end of fermentation, maltose will be reduced.
