Lactic acid bacteria (LAB) have different ways of metabolizing nutrients depending on species. Homo-fermentation produces lactic acid, and hetero-fermentation makes CO2, ethanol, esters and acids.
Leuconostoc mesenteroides, Pediococcus spp. and Lactobacillus brevis are known for hetero-fermentative capabilities. These bacteria use different carbon sources and produce a broad spectrum of compounds.
They're the unseen workers enhancing the flavors and aromas of fermented foods such as kimchi and sauerkraut. They help ferment sourdough and kefir, a popular grain and milk drink.
Phosphoketolase Pathway
The key distinguishing factor for hetero-fermentation is the enzyme phosphoketolase. This enzyme cleaves xylulose-5-phosphate into two different molecules: glyceraldehyde-3-phosphate and acetyl phosphate.
Glyceraldehyde-3-phosphate enters the glycolytic pathway and leads to production of lactic acid, just as in homo-fermentation. Acetyl phosphate takes a different turn.
It's converted to acetyl-CoA, which can be reduced to either ethanol or oxidized to acetic acid. It's contingent on the specific LAB species and environmental conditions.
Carbon dioxide is generated with these products. Airy and mildly acidic, this gas contributes to the characteristic effervescence and acidity in certain fermented foods.
Examples of Hetero-fermentation
Sauerkraut: This traditional fermented cabbage dish relies heavily on Leuconostoc mesenteroides, a prime example of a hetero-fermentative LAB. The carbon dioxide produced contributes to the desirable crunch and the slight acidity that balances the sourness.
Sourdough Bread: Complex flavors of sourdough come from a symbiotic relationship of yeasts and hetero-fermentative LAB, such as Lactobacillus sanfranciscensis. This bacterium produces both lactic and acetic acid, contributing to the zingy sourdough flavor.
Kefir: This fermented milk beverage, originating from the Caucasus region, is a complex ecosystem of bacteria and yeasts. Hetero-fermentative LAB contribute to the slightly fizzy texture and complex flavor profile.
Production of unique organic acids and alcohols not only affects taste but can also extend shelf life and improve safety. In dairy, byproducts from hetero-fermentation can inhibit unwanted microbial growth.
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