How do bacteria make yogurt, cheese, and buttermilk?

Bacteria play a central role in transforming milk into yogurt, cheese, and buttermilk through the process of fermentation. In all three cases, specific bacteria convert lactose (milk sugar) into lactic acid.123

The Fermentation Process

• Buttermilk: After heating milk to destroy natural bacteria, starter cultures such as Streptococcus lactis and Leuconostoc citrovorum are added. These bacteria produce lactic acid and flavor compounds like diacetyl, resulting in a thick, sour product.13
• Yogurt: Milk is inoculated with specific cultures, typically Streptococcus thermophilus and Lactobacillus bulgaricus. The mixture is incubated at warm temperatures (43–44 °C), allowing the bacteria to rapidly convert lactose into lactic acid, which thickens the milk into a custard-like curd.34
• Cheese: Milk is inoculated with fermenting microorganisms and often rennet. The resulting lactic acid lowers the pH, which promotes curdling—the clumping of the protein casein. This solid curd traps fat and bacteria, while the liquid whey is drained away. During ripening, enzymes released by the bacteria further develop the cheese's unique flavor.2