What process converts lactose into lactic acid in bacteria?

Lactic acid fermentation converts lactose into lactic acid in bacteria.

Lactic acid bacteria use enzymes to break lactose, the principal sugar in milk, into smaller sugars that enter central metabolism. The resulting pyruvate is reduced to lactate, commonly called lactic acid in food and general biology contexts. This process regenerates NAD+, allowing glycolysis to continue and supplying the cells with a modest amount of ATP without relying on oxygen-based respiration.

The chemistry behind fermented milk has been used since prehistoric times, but Louis Pasteur identified microbial lactic fermentation in 1857. Today, the process is central to yogurt, cheese, cultured milk, and some pickled vegetables. Yogurt cultures commonly include Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus; their acid production lowers pH, thickens milk proteins, and inhibits many spoilage organisms.

Not every lactic acid fermentation is identical. Homofermentative bacteria produce mostly lactate, while heterofermentative pathways also produce compounds such as carbon dioxide and ethanol. Alcoholic and acetic acid fermentation are different metabolic processes.