Fermentation is a natural process where microorganisms convert carbohydrates into various products without oxygen. For lactic acid fermentation, CO2 production depends on the specific metabolic pathways used.
The Direct Answer
Lactic acid fermentation, used by bacteria and animal muscle cells, does not always produce CO2. Homofermentative lactic acid fermentation converts glucose into lactic acid without releasing CO2. In contrast, heterofermentative lactic acid fermentation produces CO2 along with lactic acid and other compounds like ethanol or acetic acid. This distinction arises from different biochemical pathways.
Understanding Lactic Acid Fermentation
Lactic acid fermentation is an anaerobic metabolic process where glucose is converted into lactate and ATP. Organisms generate ATP through this process when oxygen is scarce or absent. Lactic acid bacteria (LAB) are the primary microorganisms carrying out this fermentation, producing lactic acid as their main end product. This regenerates NAD+ from NADH, crucial for glycolysis to continue producing ATP.
The Key Distinction: Homofermentative vs. Heterofermentative
The presence or absence of CO2 production in lactic acid fermentation hinges on whether the process is homofermentative or heterofermentative. Homofermentative lactic acid bacteria convert glucose into two molecules of lactic acid through the Embden-Meyerhof-Parnas (EMP) pathway, also known as glycolysis. This pathway does not release CO2. Examples include Lactococcus species and some Lactobacillus strains.
In contrast, heterofermentative lactic acid bacteria use a different metabolic route, such as the phosphoketolase pathway (also known as the Pentose Phosphate Pathway), to ferment glucose. This pathway yields one molecule of lactic acid, one molecule of ethanol (or acetic acid), and one molecule of CO2. CO2 is released during the oxidative phase of the pentose phosphate pathway. Leuconostoc species are classic examples of heterofermentative lactic acid bacteria.
Real-World Applications and Observations
Distinct outcomes of homofermentative and heterofermentative lactic acid fermentation are evident in real-world applications. Homofermentative processes are desirable in products where gas production is unwanted, like yogurt and most cheeses, for smooth texture and tangy flavor. Muscle cells during intense exercise also undergo homofermentative lactic acid fermentation, producing lactate, which can contribute to muscle fatigue.
Conversely, heterofermentative fermentation plays a significant role in products where CO2 production contributes to desired characteristics. The bubbles in fermented foods like sauerkraut and kimchi are a direct result of CO2 generated by heterofermentative bacteria. Similarly, the airy texture of sourdough bread is partly attributed to CO2 produced by heterofermentative lactic acid bacteria, alongside yeast activity. These examples demonstrate how different fermentation pathways influence the final properties of fermented foods.