The main difference between homolactic and heterolactic fermentation is that homolactic fermentation involves the conversion of one glucose molecule into two lactic acid molecules, while heterolactic fermentation involves the conversion of one glucose molecule into a lactic acid molecule, carbon dioxide, and ethanol.
Fermentation is a metabolic process where organic molecules are converted to acids, gases, and alcohols. This process happens in the absence of oxygen or any other electronic transport chain. Moreover, the two most common types of fermentation are lactic acid fermentation and ethanol fermentation. Lactic acid fermentation can be further divided into two groups; they are homolactic fermentation and heterolactic fermentation.
Key Areas Covered
1. What is Homolactic Fermentation
– Definition, Mechanism, Features
2. What is Heterolactic Fermentation
– Definition, Mechanism, Features
3. Difference Between Homolactic and Heterolactic Fermentation
– Comparison of Key Differences
Key Terms
Homolactic Fermentation, Heterolactic Fermentation
What is Homolactic Fermentation
Homolactic fermentation is a fermentation that involves the conversion of one glucose molecule into two lactic acid molecules (conversion of a molecule into similar products). This is a relatively simple process and is the opposite of heterolactic fermentation. In homolactic fermentation, sugars are metabolized via the Embden-Meyerhof glycolytic pathway to pyruvate. This is then converted into lactate.
Furthermore, the process of homolactic fermentation involves homofermentative bacteria or homofermenters. Homofermenters are the lactic acid bacteria species that can carry out homolactic fermentation. Moreover, these bacteria predominantly produce lactic acid via the glycolytic pathway. Examples of such bacteria are thermo-bacteria species, Streptococcus species, and Lactococcus lactis.
These bacteria can convert glucose into two molecules of lactate. In fact, in many homofermentative strains, a sugar phosphotransferase system (PTS) transports sugars. This sugar phosphotransferase system carries out the sugar phosphorylation during transport. The chemical reaction for the process is as follows:
Glucose +2ADP+2Pi → 2 Lactate + 2 ATP
Pyruvate is reduced to lactic acid or lactate during the homolactic fermentation process. This reaction happens in the presence of lactate dehydrogenase enzymes.
Figure 1: Lactic Acid Fermentation
Homolactic fermentation is helpful in the food industry to produce a wide range of food products, including yogurt, bread, sourdough, and cheese. This process is also useful in the production of chemicals, pharmaceuticals, and biofuels.
What is Heterolactic Fermentation
Heterolactic fermentation involves the conversion of one glucose molecule into a lactic acid molecule, carbon dioxide, and ethanol. In fact, this is the opposite of homolactic fermentation. The pentose phosphate pathway first metabolizes glucose into pyruvate, acetic acid, and carbon dioxide in this reaction. Then the pyruvate is reduced to lactic acid, and the acetic acid is reduced to ethanol and carbon dioxide.
The chemical reaction for the process is as follows:
Glucose +ADP+2 Pi → Lactate +ethanol+ CO2 +ATP
Moreover, this process involves heterofermentative bacteria, which produce lower amounts of lactate and ATP. However, these bacteria can produce several other products, including carbon dioxide and ethanol. In addition, some examples of bacteria that perform heterolactic fermentation are Leuconostoc mesenteroides, Lactobacillus bifermentans, and Leuconostoc lactis.
Difference Between Homolactic and Heterolactic Fermentation
Definition
Homolactic fermentation is a fermentation that involves the conversion of one glucose molecule into two lactic acid molecules, while heterolactic fermentation is a fermentation that involves the conversion of one glucose molecule into a lactic acid molecule, carbon dioxide, and ethanol.
End Product
The end products of homolactic fermentation are lactic acid and ATP, whereas the end products of heterolactic fermentation are lactic acid, carbon dioxide, ethanol, and ATP.
ATP Production
In homolactic fermentation, the ATP production is high, whereas, in heterolactic fermentation, the ATP production is comparatively lower.
Bacteria Involved
The homolactic fermentation process involves homofermenters like Lactococcus lactis, Streptococcus species, and thermobacteria species, whereas the heterolactic fermentation process involves heterofermenters like Leuconostoc mesenteroides, Lactobacillus bifermentans, and Leuconostoc lactis.
Conclusion
In brief, fermentation is an important biological process that converts organic molecules into acids, gases, and alcohols. There are two types of fermentation processes in lactic acid fermentation, namely homolactic fermentation and heterolactic fermentation. The main difference between homolactic and heterolactic fermentation is that homolactic fermentation involves the conversion of one glucose molecule into two lactic acid molecules, while heterolactic fermentation involves the conversion of one glucose molecule into a lactic acid molecule, carbon dioxide, and ethanol.
Reference:
1. “Homolactic Fermentation.” PubChem. National Library of Medicine.
Image Courtesy:
1. “Lactic acid fermentation” By Sjantoni – Own work (CC BY-SA 3.0) via Commons Wikimedia
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