2.5.1 What is lactic acid fermentation?

Lactic acid has alcohol (OH) and carboxylic (COOH) sites inside the molecule. Since it includes chiral carbon, it has two chiral isomers, Ddactic acid and L-lactic acid. Recently, demand of poly-lactate, a biomass plastic, is increasing, and demand of lactic acid is also increasing as a raw material of poly-lactate. Then the lactic acid with almost 100% of the optical purity is strongly requested. Generally, lactic acid is produced by chemical syntheses or by microbial fermentations. In the chemical syntheses, a method using hydrolysis of lacto-nitrile is usually adopted, yielding D-lactic acid and L-lactic acid half by half of which the optical purity is nul. Thus lactic acid for the production of poly-lactate is always produced by fermentation. Lactic acid can be produced by either bacteria or fungi. Here the lactic acid fermentation with bacteria is focused on.