What You Need to Know
During fermentation, indigenous lactic acid bacteria such as Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus convert lactose into lactic acid, lowering the pH to 4.5–5.5. The metabolic activity also generates volatile flavor compounds like acetaldehyde and diacetyl, as well as small amounts of ethanol (0.5–1 %) and butyric acid, contributing to the characteristic sour–nutty, slightly alcoholic profile.
The Science
Primary Reaction
lactose → lactic acid (and minor ethanol)