Heterolactic fermentation

Heterolactic fermentation begins with the phosphorylation of glucose to glucose-6-phosphate, which is then oxidized to 6-phosphogluconate and decarboxylated to ribulose-5-phosphate. The key enzyme phosphoketolase cleaves xylulose-5-phosphate (formed by epimerization) into glyceraldehyde-3-phosphate and acetyl phosphate. The glyceraldehyde-3-phosphate enters the glycolytic segment to produce lactate, while acetyl phosphate is reduced to ethanol via acetaldehyde, regenerating NAD⁺. The decarboxylation step releases CO₂, and the overall net yield is one ATP per glucose, half that of homolactic fermentation.¹

Heterofermentative lactic acid bacteria are found in diverse niches, including plant surfaces, dairy products, and the gastrointestinal tracts of animals. Notable genera include Leuconostoc, Oenococcus, and Weissella, as well as heterofermentative lactobacilli such as Lactobacillus brevis and L. fermentum. These organisms often coexist with homofermenters in mixed fermentations, where their production of ethanol and CO₂ influences the final product's texture and aroma. Some species, like Oenococcus oeni, are adapted to harsh environments such as wine, where they perform malolactic fermentation alongside heterolactic metabolism.²

Heterolactic fermentation is central to the production of many fermented foods. In sourdough, heterofermentative lactobacilli generate the characteristic tang and the open crumb structure via CO₂ production. In vegetable fermentations, such as sauerkraut and kimchi, Leuconostoc mesenteroides initiates the process, producing a complex flavor profile. In winemaking, Oenococcus oeni contributes to the buttery aroma of certain wines through diacetyl production, a byproduct of citrate metabolism. The ethanol produced can also act as a preservative, and the CO₂ contributes to the effervescence of some beverages.³

Heterolactic fermentation is not limited to bacteria; some fungi, such as certain yeasts, can perform a similar mixed-acid fermentation, though the term is typically reserved for bacteria. The pathway's flexibility allows some species to shift to homolactic fermentation under specific conditions, such as when fructose is available as an alternative electron acceptor, producing mannitol instead of ethanol. This metabolic versatility has biotechnological implications, including the production of polyols and the use of heterofermentative strains as probiotics. Additionally, the phosphoketolase enzyme is a target for antimicrobial agents, as it is absent in humans.⁴

Phosphoketolase

An enzyme that cleaves xylulose-5-phosphate into glyceraldehyde-3-phosphate and acetyl phosphate, a key step in heterolactic fermentation.

Homolactic fermentation

A fermentation process that produces lactic acid as the sole or major end product, via the Embden-Meyerhof pathway.

Malolactic fermentation

A secondary fermentation in wine where malic acid is converted to lactic acid, often performed by Oenococcus oeni.