Fermentation of Yogurt and the Biotechnology Behind It
Fermentation is one of the biotechnology applications which refers to the catalytic activity of microorganism by using enzymes to transform organic substances such as carbohydrates, proteins, and lipids. It is known that bacteria and fungi are classified as fermentative microorganisms and these microorganisms are widely used in the production of fermented food such as yogurt, kefir, tempeh, natto, and many more .
As food products resulting from controlled microbial growth and enzymatic action, fermented foods are usually made to preserve and reduce the risk of contamination by pathogenic microorganisms, as well as to improve taste, flavor, aroma, and texture . One important fermenting bacteria in the food and beverage industry is lactic acid bacteria such as Lactobacillus. They are used to produce yogurt to increase its nutritional value and sensory properties, as well as to extend the shelf life of milk product .
Chemically, yogurt is made from a complex gel system that combines proteins, polysaccharides, and lipids in its structure. During the fermentation process, the lactic acid bacteria produces enzymes to convert carbohydrates, proteins, and lipids from the milk. This process can affect the pH, texture, taste, aroma, and flavor of the product . Here are some explanations of what’s happening during the fermentation of milk by lactic acid bacteria.
Milk contains lactose as the main carbohydrate and this compound is the major energy and carbon source for the growth of the lactic acid bacteria. These microorganisms convert lactose into lactic acid, and the accumulation of this acid lowers the pH (4.5 or less) of the yogurt, giving yogurt its sour taste. Depending on the species of the lactic acid bacteria used, lactose fermentation can only produce lactic acid as the main product (as in the homofermentative pathways) or produce other byproducts such as ethanol, carbon dioxides, and acetic acid (as in the heterofermentative pathways) .
The flavor of yogurt has played an important role in increasing its consumer demand since flavor is a complex phenomenon . The glycolysis of lactose and citric acid metabolisms of some lactic acid bacteria can produce pyruvate as an important precursor to generate acetaldehyde as the major and typical flavor compound of yogurt that exhibits a green apple or nutty flavor. In addition to acetaldehyde, the major volatile compounds that also affect desirable flavor to yogurt are the C4 carbonyl compound like diacetyl, acetone, acetoin, and 2-butanone .
Lactic acid bacteria can convert protein into several products that comprises of two steps, such as proteolysis of milk casein and amino acid degradation. The degradation of amino acids leads the production of various flavor compounds such as ammonia, amines, aldehydes, phenols, indole, and alcohol. In addition, researchers have found that lyase enzyme produced by lactic acid bacteria during fermentation can convert amino acid threonine to acetaldehyde .
As a fermenting bacteria, lactic acid bacteria can produce lipase enzyme which is widely used in lipid breakdown. The lipid breakdown process can produce free fatty acids, and in examples of accumulated unsaturated fatty acids, they can be oxidized to form hydroperoxides before being decomposed to form hexanal or unsaturated aldehydes that contributes the flavor of yogurt. Not only that, the beta-oxidation process of unsaturated fatty acids can produces gamma- and delta-lactones which gives the strong fruity flavor to yogurt .
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 The Modern Yoghurt: Introduction to Fermentative Processes (researchgate.net)
 nutrients-11-01806.pdf (nih.gov)
 (PDF) Volatile Flavor Compounds in Yogurt: A Review (researchgate.net)
 Role of lactic acid bacteria on the yogurt flavour: A review (tandfonline.com)
 Scientific and Technical Aspects of Yogurt Aroma and Taste: A Review (mohagheghsho.ir)