Which sugar in milk do souring bacteria convert into acid?

Souring bacteria, commonly found in milk, specifically convert lactose into lactic acid during the fermentation process. Lactose is a disaccharide composed of glucose and galactose, and it is the primary sugar present in milk. When bacteria such as Lactobacillus species ferment lactose, they metabolize it, leading to the production of lactic acid. This acidification process is what causes milk to sour and can be a crucial factor in the production of various dairy products, such as yogurt and cheese.

In contrast, while galactose, fructose, and glucose are all sugars found in various foods, they are not the main constituents of milk sugar that souring bacteria target for acid production in milk. Therefore, lactose is distinctly recognized as the sugar being converted into acid during the souring of milk.