Yogurt is made when specific bacteria are added to milk in a controlled environment and allowed to ferment. The majority of yogurt made in the United States is produced with cow's milk, but the milk of other species, including sheep, goat, and camel, is also used to make yogurt in other parts of the world. Yogurt is much thicker than plain milk and often has chunks of material in it. It also has a distinctive tangy flavor, caused by the bacterial fermentation.
For a dairy product to be called yogurt, it must contain two bacteria: Streptococcus thermophilus and Lactobacillus bulgaricus. Many types of yogurt incorporate other species as well, including Lactobacillus acidophilus and Lactobacillus casei. In many countries, yogurt must also contain live bacteria and remain unpasteurized, with pasteurized yogurts being specially labeled. Pasteurized yogurt has a long shelf life and does not need to be kept refrigerated, but it also doesn't have the health benefits of live yogurt.
When yogurt is made, milk is heated to approximately 200° Fahrenheit (93° Celsius) and kept at that temperature for 10-30 minutes, depending upon the thickness desired. For thicker yogurt, the milk is heated longer. Next, the milk is rapidly cooled to approximately 112° Fahrenheit (44° Celsius) and mixed with a yogurt starter, which contains the necessary bacteria. This dairy mixture is placed in clean containers and incubated for a minimum of four hours at 100° Fahrenheit (37° Celsius). The longer the incubation, the more tart the yogurt, because more acids will develop.
To stop the incubation, the yogurt is placed in a cool environment such as a fridge. Kept cold, it will keep for approximately ten days. Yogurt can be made at home using commercial yogurt as a starter.
The most important step in the yogurt making process, and what creates the characteristics consumers think of when they imagine yogurt, is the introduction of the bacteria. The bacteria consume natural milk sugars and excrete lactic acid, which causes the milk proteins to begin to curdle and create a more solid mass. At the same time, the increased acidity of the dairy is too high for most harmful bacteria, so the yogurt keeps itself clean. Other bacteria can be added for flavor or health benefits, especially Lactobacillus acidophilus.
Yogurt is rich in protein and minerals and can in theory be drunk by people suffering from lactose intolerance, because it contains an enzyme that breaks down lactose in the intestines. Yogurt containing Lactobacillus acidophilus is often ingested by women attempting to avoid yeast infections, because it creates an environment too acidic for Candida albicans to thrive in.