The term “unpasteurized yogurt” can refer to yogurt that is made with unpasteurized milk or yogurt that has not been pasteurized itself. Which definition applies to a given set of yogurt products is usually a factor of location, custom and interpretation. In most parts of the world, milk is routinely pasteurized before it enters the marketplace, and yogurt typically is made with this milk. Completed yogurt can itself then be pasteurized again, though this is not as common. So long as a yogurt is made with pasteurized milk, the need for pasteurization is largely fulfilled.
Pasteurization is a process through which milk and other dairy products are heated to very high temperatures and held there for a certain length of time. This kills pathogens and other bacteria by creating an environment in which they cannot thrive or reproduce. The milk is then cooled, packaged and sold. It typically has a much longer shelf life than unpasteurized milk, and it presents fewer health risks as well.
One understanding of unpasteurized yogurt is yogurt made with unpasteurized milk. Milk is yogurt’s primary ingredient. Using unpasteurized milk in cooking, baking or fermentation raises the same health concerns as would drinking it. People in rural farming communities often make yogurt this way — generally with milk straight from the cows — but it is uncommon in more metropolitan areas. Most governments’ health agencies place restrictions on the sale and distribution of this kind of unpasteurized yogurt as well.
More often, the term "unpasteurized yogurt" relates to yogurt made with pasteurized milk, but that has not undergone a pasteurization process of its own. The majority of yogurt-based foods that are available are unpasteurized yogurt in this sense. Pasteurizing the finished product is usually not necessary, and it typically kills the active cultures that make eating yogurt beneficial.
Yogurt is made by introducing certain bacteria to warmed milk, with Lactobacillus bulgaricus , Streptococcus thermophilus and Lactobacillus acidophilus being the most common. These bacteria feed on the natural sugars in the milk and secrete an acid byproduct. With time — usually at least four hours — these secretions transform the milk from a liquid to a tangy, creamy solid.
This final product, which is itself yogurt, must be refrigerated to keep from spoiling. It will stay fresh for several weeks, however, and it is safe to eat immediately. The bacterial cultures stop fermenting the milk after they are chilled, but they stay alive and are well regarded in the medical community as healthful and beneficial, particularly when it comes to aiding in digestion. Manufacturers often label yogurts that contain these bacteria with the phrase “live and active cultures.”
These live and active cultures will not survive pasteurization, just as they will not survive baking or cooking. As such, the majority of the yogurts that are familiar to most consumers are technically unpasteurized yogurt. Some dairies do re-pasteurize their yogurt products, but this is relatively uncommon because it kills the helpful bacteria. The taste is the same, and the shelf-life is longer because pasteurized yogurt does not need refrigeration. Just the same, it is less popular with consumers because it imparts so few benefits.