L-lactic acid at industrial scale

Lactic acid is a three-carbon hydroxy acid, fermented for longer and in greater volume than anything else in this cluster. It is worth reading not as an exception but as a template: three things came together, and each of them, missing, is the reason a neighbouring page describes a failure.

First: no carbon is lost

Homofermentative lactic acid bacteria run glucose through glycolysis to two pyruvate and reduce both with lactate dehydrogenase to two lactate. The balance closes: the NADH made at the glyceraldehyde-phosphate step is spent in the same reaction, and no carbon dioxide leaves at all. The theoretical mass yield is one — 180 grams of glucose give 180 grams of lactic acid. No other monomer in this cluster can say that. Real performance sits close to the ceiling: titres of 120-180 g/L and yields above 0.9 g/g are routine, because these bacteria divert very little carbon into biomass.

Second: optical purity is the specification

Lactic acid is chiral, and the commercial specification is not concentration but enantiomeric excess. The reason lives in the polymer. Polylactide made from pure L crystallises and melts near 175 °C; every D unit built into the chain breaks its regularity, and at a few percent D the crystallinity collapses and the material becomes effectively amorphous with a glass transition around 55-60 °C. The heat resistance of the finished part is therefore set by monomer purity, not by processing.

The enzyme decides it: lactate dehydrogenases come in L- and D-specific forms, and the choice of organism fixes the configuration. Industrial strains deliver above 99 percent excess. And the D isomer is not only a contaminant — blending poly-L- with poly-D-lactide gives a stereocomplex melting near 230 °C, some fifty degrees above either component alone. Optical purity here is not process hygiene; it is a handle on material properties.

Third: a purification route that exists

Lactic acid is non-volatile and oligomerises with itself on heating, so it cannot simply be distilled. The industry solves this by esterification: the acid is converted to methyl lactate, which distils cleanly, and hydrolysed back afterwards. That established route is precisely what succinic acid never had.

Neutralisation remains the open problem. With a pKa near 3.9, the undissociated acid inhibits its own producer, so the fermenter is held near pH 6 by adding base. Traditionally that base is calcium hydroxide, making calcium lactate, from which the acid is liberated with sulfuric acid. The by-product is calcium sulfate: roughly a tonne of gypsum per tonne of acid, a waste stream that has to go somewhere. The answers are known — ammonia neutralisation with ammonia recycle, electrodialysis, and low-pH-tolerant yeast hosts that let the fermentation run below the pKa in the first place — and each moves the cost from disposal into energy or into strain development.

The cluster’s conclusion reads off this page: lactic acid won not because it is easier to ferment, but because it has a buyer who needs a measurable property and a way of reaching that property at an acceptable price.