Yogurt, a popular food made from fermented milk, owes its characteristic tangy flavor to a combination of natural acids. Probiotics, which are the "friendly" bacteria involved in yogurt production, produce these acids naturally during the fermentation process. As a result, yogurt generally has a slightly acidic pH between 4.5 and 5.0, depending on the cultures and recipe involved in its production.
Yogurt-makers use a combination of natural bacteria to create the food. The most common bacteria used in yogurt-making include nonpathogenic strains of streptococcus, lactobacillus and bifidobacterium. These are different bacteria strains and species than those responsible for human diseases. When added to milk, they begin reproducing rapidly to colonize the product. This causes the milk to thicken and gain an acidic flavor. Unlike milk that has simply spoiled because of colonization with unhealthy bacteria, yogurt is both edible and nutritionally beneficial.
As friendly bacteria populate a culture of yogurt, they create several acids as a by-product of respiration and metabolism. According to the University of Guelph four major acids contribute to the taste and acidity of yogurt. These include lactic acid, acetaldehyde, acetic acid and diacetyl acid. All of these are nontoxic and contribute to yogurt's acidity to varying degrees. Depending on the exact culture used and the environment in which the yogurt was prepared, its exact amount and ratio of acids may vary.
The acidity of yogurt can range from surprisingly acidic to relatively neutral. While some mild yogurts have a pH as high as 5.5, which is only slightly acidic, others have a low pH of about 3.0, which is extremely acidic compared to most other foods. Ordinary yogurts generally have a pH of about 4.0 to 4.5. Aside from cultures and environment, several other factors can affect the product's acidity. Alkaline ingredients such as banana and blueberry may raise yogurt's pH, while acidic ingredients such as orange and sugar may lower it.
Yogurt's acidity contributes to many of its culinary and health effects. The acids in yogurt give it the sharp flavor that separates it from other dairy products. The acidic compounds produced by "friendly" bacteria can also contribute to their health benefits, which, according to the National Institutes of Health, may include digestive health, yeast defense and immune support. More studies are needed to evaluate the exact benefits associated with acidic yogurts compared to those with a more neutral pH.
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