How do lactic-acid bacteria turn milk into cultured dairy foods?

Lactic-acid bacteria, such as Streptococcus and Lactobacillus, transform milk into cultured dairy foods primarily through a process called fermentation.12

During this process, the bacteria consume lactose (milk sugar) and convert it into lactic acid. This conversion is essential for several reasons:1

• Preservation: The production of lactic acid increases the acidity of the milk, which suppresses the growth of spoilage and pathogenic organisms, allowing the product to be preserved without refrigeration.1
• Coagulation: The resulting acidity can cause the milk proteins to coagulate, a necessary step in the production of items like yogurt, buttermilk, and certain cheeses.13
• Flavor Development: As the bacteria metabolize the milk components, they produce various minute by-products that contribute to the unique flavors and textures of the final product.1

At temperatures around 32 °C (90 °F), these bacteria reproduce rapidly, facilitating the efficient conversion of milk into cultured products. Secondary fermentations may also occur, producing additional compounds like diacetyl, which provides characteristic flavors in products such as buttermilk.1