heterolactic

Heterolactic fermentation refers to a form of lactic acid fermentation in which certain lactic acid bacteria convert hexose sugars not only to lactic acid but also to other end products, typically ethanol or acetate and carbon dioxide. This contrasts with homofermentative lactic acid bacteria, which predominantly yield lactic acid from sugars.

Metabolic pathway: Heterolactic fermentation proceeds mainly via the phosphoketolase pathway. Glucose is metabolized through the pentose phosphate pathway to xylulose-5-phosphate, which is cleaved by phosphoketolase to yield acetyl-phosphate and glyceraldehyde-3-phosphate. The latter is reduced to lactate, while acetyl-phosphate is converted to either ethanol or acetate. CO2 is released during decarboxylation steps. The overall stoichiometry for glucose is lactate plus ethanol or acetate plus CO2, with typically lower ATP yield than the homofermentative route.

Organisms: Classic heterofermentative lactic acid bacteria include genera Leuconostoc and Weissella, as well as certain Lactobacillus species such as L. brevis and L. sakei. They are Gram-positive, non-spore-forming, and often microaerophilic or facultatively anaerobic. Their activity is common in mixed-culture fermentations and in the production of certain fermented foods.

Applications and significance: In food fermentation, heterolactic bacteria contribute to flavor, aroma, and leavening through gas production in sourdough, sauerkraut, kimchi, and pickles. They also influence texture and acid balance. The term is used to distinguish from homofermentative pathways and to describe organisms capable of multiple end products depending on conditions.