What is the composition of yogurt? Yogurt, a fermented dairy product whereby milk is inoculated with bacteria cultures, is an example of a mixed pure culture fermentation. That is, a controlled mixture of known cultures of bacteria used in the fermentation process. A. Yogurt is the product of fermentation, a slow decomposition process of organic substances induced by microorganisms or enzymes. Food fermentation is the study of microbial activity, usually anaerobic, on suitable substrates under controlled or uncontrolled conditions. To produce yogurt, lactose, a compound sugar found in milk known as lactin or milk sugar, is fermented by two different species of bacteria: Lactobacillus and Streptococcus.
B. Yogurt is commercially produced by adding to milk a 2–4% nonfat dry milk powder that has been inoculated with a 5% combination of Lactobacillus bulgaricus and Streptococcus thermophilus (1:1 ratio). The milk mixture is then incubated at 45 degrees C for 3 to 6 hours. The product must be chilled immediately. C. The fat content of yogurt varies from 0-3.5%; most yogurt is low fat and contains 1–1.5% fat. D. Lactic acid, the end product of anaerobic metabolism of glucose, provides the tart flavor of yogurt, as well as the formation of a gel structure. The major flavor components of yogurt are carbonyl compounds; among these, acetaldehyde is the most important and gives the yogurt its green apple or nutty flavor. Yogurt quality is based on color, appearance, body, texture, and flavor.
E. Ropy (slime-producing) lactic acid bacteria produce polysaccharides that are released into the yogurt where they increase viscosity and improve water retention. Viscosity is the resistance of a fluid to flow. Yogurt has a high moisture content of 82–86%. F. Milk SNF (solids-not-fat) content of yogurt varies from 9–16%. SNF can be increased by adding milk powder, and by other means. Increased SNF levels are needed to increase protein content, which helps to increase product viscosity to desired levels. G. Frozen yogurt is manufactured by mixing varied amounts of fermented yogurt with ice milk containing sweeteners, stabilizers, etc.
How are fermentation and anaerobic respiration needed to create the yogurt product? A. Aerobic respiration is the total oxidative degradation of glucose that must have oxygen to take place. Fermentation is an example of anaerobic respiration, which takes place in the absence of oxygen and consists essentially of the early stages of aerobic respiration. During this type of respiration, glucose is converted to a variety of end products, such as lactic acid. Since acids produce a sour taste, the result is a sour- tasting dairy product. The increased acidity (lowered pH) of the milk causes milk protein to coagulate and become more viscous. The bacteria Streptococcus also produces other compounds that have an effect on final flavor.
B. Lactic acid bacteria are gram-positive, non-spore forming bacteria that produce lactic acid as the major product of fermentation. These bacteria are very important in pickling, cheese making, fermented dairy products, and other technologies. C. Milk proteins have the unique ability to curdle (form a gel). Curdling is induced by proteolytic enzymes, lactic acid, heat, and other means. Each milk gel has a solid structure consisting of a protein matrix and other components. This gel matrix has the ability to immobilize the liquid phase of milk. By modifying this ability, it is possible to manufacture stable milk products with a high water content (i.e. yogurt).
D. Milk to be used in yogurt production is usually heated to increase the total solids content in order to make a firm end product. An end product is the final compound or substance resulting from a chemical reaction. Heating causes the micelles in the milk to interconnect in chains to form the gel matrix. E. Cooling of the yogurt after incubation (fermentation) stops bacterial fermentation. F. If cows are receiving antibiotic treatments, their milk is usually discarded for a specific period of time. Otherwise, residual effects from the antibiotics in the milk could inhibit the growth of desirable bacteria necessary for yogurt production.
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