Yogurt and other cultured dairy products are made by adding specific cultures to fluid dairy products in order to convert some lactose (milk sugar) into lactic acid. The aroma, body and flavor of these products can vary, depending on the type of culture and milk, amount of milkfat and nonfat milk solids, fermentation process and temperature used.

Yogurt is a mixture of milk (whole, reduced fat, lowfat or nonfat) and cream fermented by a culture of lactic acid-producing bacteria, Lactobacillus bulgaricus and Streptococcus thermophilus. Other bacteria (e.g., acidophilus) and other strains of the above bacteria may be added to the culture. Sweeteners (e.g., sugar, honey, aspartame), flavorings (e.g., vanilla, coffee) and other ingredients (e.g., fruits, preserves, stabilizers such as gelatin) may also be added. Yogurt contains at least 3.25% milkfat and 8.25% nonfat solids. The mixture of dairy products and optional ingredients, except bulky flavorings, must be pasteurized or ultrapasteurized. The milk in most yogurts is also homogenized.

Varieties of yogurt
Lowfat yogurt is similar in composition to yogurt except that it contains either 0.5%, 1%, 1.5% or 2% milkfat.

Nonfat yogurt is similar in composition to yogurt and lowfat yogurt except that it contains less than 0.5% milkfat.

Yogurt beverages, which may be a combination of yogurt and milk or may be created from different acid-producing bacteria than yogurt, also may also be characterized by fat content.

Other cultured dairy foods
Buttermilk is made by adding lactic acid-producing bacteria, usually Streptococcus lactis, to pasteurized or ultrapasteurized milk (whole, reduced-fat, lowfat, nonfat) with nonfat dry milk solids under controlled conditions. The product is heated until the desired acidity is achieved, then cooled to stop fermentation. Buttermilk flakes or liquid butter may be added to give cold milk the appearance of churned buttermilk. Salt, citric acid or sodium citrate may be added to enhance flavor. Today, depending on the level of milkfat in the product, buttermilk may be called cultured buttermilk, cultured lowfat buttermilk or cultured skim (nonfat) buttermilk. Originally, buttermilk was the lowfat liquid remaining after churning cream into butter.

Acidophilus milk is typically a lowfat or nonfat milk to which active cultures of Lactobacillus acidophilus have been added. The mixture is heated until a curd forms and the desired acidity is reached. The milk is then refrigerated. Adding Lactobacillus acidophilus cultures to cold, lowfat or nonfat milk and then refrigerating the product to prevent further growth of the harmless bacteria produces sweet acidophilus milk. Unlike fermented acidophilus milk, which has a slightly tart taste, this product has a sweet taste.

Nutritional information
The nutritional and caloric contents of yogurt, buttermilk and acidophilus milk are similar to those of the fluid milks from which they are made. Each is an important source of calcium, riboflavin (B2) and protein.

Yogurt by the numbers

Source: USDA Nutrient Database for Standard Reference.