Yeast is one of the most important fermenters for various fermented products and foods. The role of yeast in fermentation is that it reduces the pH level during fermentation. It also gives good quality such as texture, taste, nutritive values, odor, and functional properties of fermented products. It is used in fermented food either alone or with the association of other bacteria. Yeast also play a role in beverages, which ranges from alcoholic beverages (e.g., fruit, palm and rice wines), non-alcoholic beverages (fermented tea), cereal-based leavened products, fermented legume products, milk products, fermented meat, and fermented fish products.
Role of yeast in fermentation:
The role of yeast in fermentation is the conversion of carbohydrate in alcohol and carbon-di-oxide. The yeast mostly used in fermentation in Saccharomyces cerevisiae. Besides alcohol and carbon dioxide, yeast also converts the sugar into glycerol, acetaldehyde, acetic acid, lactic acid, and pyruvate.
Organic Acids Production and Utilisation:
Yeast fermentation produces organic acids as secondary metabolites. It both reduces the pH of the products and affects the aroma and flavor of the food and beverages.
Besides the production of organic acids, yeast also plays a role in the utilization of selected organic acids which affect the quality of the final fermented product.
The organic acids such as acetate, malate, citrate, pyruvate, and succinate are produced from yeast fermentation
by carbohydrate metabolism.
Yeast also causes the production of lactic acid, pyruvic acid, propionic acid, acetic acid, and butyric acid during milk fermentation.
Yeast is commonly used as leavening agents in bakery and pastry products. During the dough fermentation process, yeast converts sugar into alcohol and carbon dioxide, which affects the textural property of the baked product. The most widely used yeast in bakery products as a leavening agent is Saccharomyces cerevisiae or baker’s yeast. Using the yeast as leavening agent is an important role of yeast in fermentation process.
Hydrolytic Enzyme Production:
Yeast enzymes are very important in the fermentation process as they improve the quality of fermented products.
Yeast can produce several intracellular and extracellular hydrolytic enzymes during fermentation. These enzymes include proteases, amylase, invertase, xylanase, cellulases, lipases, phytases, b-glucosidases, pectinases.
Yeast enzymes like carboxypeptidases and aminopeptidases cause the hydrolysis of milk protein. Proteolytic and lipolytic enzymes also play an important role in improving the quality of the wine, beer, and cheese.
Beta-Glucosidases degrades polyphenols to increases the antioxidant activity in the fermented food. Phytases cause a reduction of antinutritional phytate.
Flavour, Aroma, and Texture:
Yeast fermentation improves flavor, aroma, and texture in either of two ways.
- Biotransformation of fruit components into flavor and aroma impacting compounds or
- By the synthesis of compounds such as ethanol, glycerol, acetaldehyde, organic acids, esters, fatty acids, and higher alcohols, which are responsible for aroma and flavor development.
Yeast converts the carbohydrates into several metabolic products like organic acids, esters, and carbonyl compounds during the fermentation process. These products add flavor, aroma, and texture to the fermented products.
Several other metabolic compounds are produced by yeast. These compounds add flavor to fermented products.
Improvement of Nutritive Values:
As fermentation breaks down the larger molecules into the smaller utilizable molecules, it improves the nutritive values and digestibility of fermented products.
Yeast produces hydrolytic enzymes and improves the digestibility of food through these enzymes. It also enhances the production of vitamins during the fermentation process.
Yeast improves the nutritional quality of bread during sourdough fermentation. It improves the production of phenols, sterols, vitamins, and folate. It helps solubilize the fiber, enhance the bioaccessibility of minerals, and reduces starch digestibility.
Reduction of Toxins and Anti-nutritional Factors:
The raw materials of fermented products toxins and antinutritional factors. But the fermentation process reduces these toxins and antinutritional factors degrading them. A study shows that Saccharomyces cerevisiae can reduce mycotoxins during alcohol fermentation. The other two yeast Phaffia rhodozyma and Xanthophyllomyces dendrorhous also reduce mycotoxin during alcohol fermentation.
Yeast also produces phytase enzymes that degrade the phytic acid, an antinutritional factor. Degradation of phytic acid by yeast during food fermentation enhances the bioavailability of divalent metals such as iron, calcium, magnesium, and zinc.
Yeast also reduces oxalic acid during the fermentation of wine.
There are also several role of yeast in fermentation which will be discussed later. Each role of yeast in fermentation will also be discussed.
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