Biochemically, homofermentative lactic acid bacteria split one 12-carbon lactose molecule into four 3-carbon lactic acid molecules:
\[ \text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O} \rightarrow 4 \ \text{C}_3\text{H}_6\text{O}_3 \]
Thus, the molecular weight of lactose is nearly 4 times that of lactic acid.