To turn milk into yogurt, you must keep it at a steady 110°F (43°C) for several hours.

The production of yogurt relies on a process called thermophilic fermentation. The two primary bacterial strains used are Lactobacillus bulgaricus and Streptococcus thermophilus. These specific microbes were first identified and linked to yogurt's health benefits by Nobel Prize-winning biologist Stellan Mörner and researcher Ilya Mechnikov in the early 1900s.When milk is held at a constant 110°F (43°C), these bacteria consume lactose, the natural sugar found in dairy. They convert this lactose into lactic acid through metabolic pathways. This increase in acidity causes the milk protein, known as casein, to denature and coagulate. This chemical reaction is what transforms liquid milk into a thick, semi-solid gel.Maintaining the temperature is critical because S. thermophilus grows best between 104°F and 113°F. If the temperature drops below 100°F, the fermentation slows significantly or stops entirely. Conversely, temperatures exceeding 120°F will destroy the bacterial cultures. Most commercial yogurt producers maintain this heat for 4 to 12 hours to reach a pH level of approximately 4.5.