How is yogurt made using Lactobacillus bulgaricus and Streptococcus thermophilus?

Yogurt production begins by fortifying milk with nonfat dry milk or condensed skim milk to increase total solids to 14–16 percent. The mixture is heat-treated and then cooled to a temperature between 45.6 and 46.7 °C (114 to 116 °F).1

At this stage, a culture consisting of equal parts Lactobacillus bulgaricus and Streptococcus thermophilus is added to the warm milk. These lactic-acid bacteria facilitate fermentation, which is essential for the coagulation process. Following inoculation, one of two methods is used:12

• Set (sundae-style) yogurt: The mixture is poured into cups, often containing fruit, and held in a warm room for approximately four hours until it coagulates, after which it is refrigerated.1
• Blended (Swiss- or French-style) yogurt: The milk incubates in large heated tanks. Once coagulation occurs, the mixture is cooled, flavorings are added, and the product is packaged for sale.1