Yogurt Fermentation Yogurt is made by lactic acid fermentation. The main (starter) cultures in yogurt are Lactobacillus bulgaricus and Streptococcus thermophilus. The function of the starter cultures is to ferment lactose (milk sugar) to produce lactic acid. The increase in lactic acid decreases pH and causes the milk to clot, or form the soft gel that is characteristic of yogurt. The fermentation of lactose also produces the flavor compounds that are characteristic of yogurt. Lactobacillus bulgaricus and Streptococcus thermophilus are the only 2 cultures required by law (CFR) to be present in yogurt.
Other bacterial cultures, such as Lactobacillus acidophilus, Lactobacillus subsp. casei, and Bifido-bacteria may be added to yogurt as probiotic cultures. Probiotic cultures benefit human health by improving lactose digestion, gastrointestinal function, and stimulating the immune system. Lactic acid fermentation is the simplest type of fermentation. Basically, it is a redox reaction. In anaerobic conditions, the cell’s primary mechanism of ATP production is glycolysis. Glycolysis reduces – that is, transfers electrons to – NAD+, forming NADH.
However, there is only a limited supply of NAD+ available in a cell. For glycolysis to continue, NADH must be oxidized – that is, have electrons taken away – to regenerate the NAD+. This is usually done through an electron transport chain in a process called oxidative phosphorylation. However, this mechanism is not available without oxygen. Instead, the NADH donates its extra electrons to the pyruvate molecules formed during glycolysis. Since the NADH has lost electrons, NAD+ regenerates and is again available for glycolysis.
Lactic acid, for which this process is named, is formed by the reduction of pyruvate. The total fermentation process to make yogurt is fairly simply. The milk mixture is pasteurized at 185°F (85°C) for 30 minutes or at 203°F (95°C) for 10 minutes. A high heat treatment is used to denature the whey (serum) proteins. This allows the proteins to form a more stable gel, which prevents separation of the water during storage. The high heat treatment also further reduces the number of spoilage organisms in the milk to provide a better environment for the starter cultures to grow.
Yogurt is pasteurized before the starter cultures are added to ensure that the cultures remain active in the yogurt after fermentation to act as probiotics; if the yogurt is pasteurized after fermentation the cultures will be inactivated. Next, the blend is homogenized (2000 to 2500 psi) to mix all ingredients thoroughly and improve yogurt consistency. Then, the milk is cooled to 108°F (42°C) to bring the yogurt to the ideal growth temperature for the starter culture. Following this, the starter cultures are mixed into the cooled milk.
Next, the milk is held at 108°F (42°C) until a pH 4. 5 is reached. This allows the fermentation to progress to form a soft gel and the characteristic flavor of yogurt. This process can take several hours. The yogurt is then cooled to 7°C to stop the fermentation process. Fruit and flavors are added at different steps depending on the type of yogurt. Finally, the yogurt is pumped from the fermentation vat and packaged as desired. Primary Source: “Yogurt Production. ” Milk Facts. Cornell University, n. d. Web. 8 Oct 2012.