As we all know it was characterized by non-oxygen atmosphere in the early Earth's history. Our ancestors developed a skill to get energy from glucose in order to survive in such anoxic conditions. This is called anaerobic glycolysis and inherited by every living being. It should be followed by other metabolic pathways since glycolysis consumes NAD⁺ to make NADH. If NADH doesn't go back into NAD⁺, then there won't be enough NAD⁺ to support glycolysis.
There are many ways to oxidize NADH and make NAD⁺. Some organisms transfer hydrogen via electron transport chain in anaerobic respiratory where the final electron acceptor is usually an inorganic substance. Another method is fermentation that consumes NADH. An organic molecule acts as a final electron acceptor to capture hydrogens and electrons from NADH. The regenerated NAD⁺ is used again by glycolysis. Most energy still remains in organic molecules, and only little ATP generates during fermentation. The most common types in organisms are alcoholic or lactic acid fermentation.
| Alcoholic Fermentation | Lactic Acid Fermentation | |
| Formula | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ | C₆H₁₂O₆ → 2 C₃H₆O₃ |
| Metabolic Pathway | Glucose → Pyruvate → Acetaldehyde → Ethanol | Glucose → Pyruvate → Lactic acid |
| Main Products | Ethanol and carbon dioxide | Lactic acid |
| By-products | Methanol, esters, glycerol, and fusel alcohols | Little ethanol, carbon dioxide, acetic acid |
| Microorganisms | Yeast (e.g., Saccharomyces cerevisiae) | Lactic acid bacteria (e.g., Lactobacillus) |
| Applications | Beer, wine, and bread making | Yogurt, pickles, and fermented vegetables |
Alcoholic Fermentation, Ethanol Fermentation
In the absence of oxygen, yeasts, plants and some bacteria produce ethanol from glucose. This is known as alcoholic or ethanol fermentation where acetaldehyde serves as the final electron acceptor. It consists of three key steps: glycolysis, producing acetaldehyde from pyruvate and ethanol generating.
Glucose → 2 Pyruvate + 2ATP + 2NADH
2 Pyruvate → 2 Acetaldehyde+2CO₂
2 Acetaldehyde + 2NADH → 2 Ethanol+2NAD⁺
Glycolysis occurs in the cytoplasm. One glucose splits into two pyruvates along with two ATP (via substrate-level phosphorylation) and two NADH. This is the most energy-releasing step in alcoholic fermentation and the only step yields ATP. The next step was catalyzed by decarboxylase. Pyruvate loses a carboxyl group to become acetaldehyde and releases carbon dioxide. The third step is the most important because it involves hydrogen transfer and regenerated NAD⁺. The conversion of acetaldehyde to ethanol is accompanied by the NAD⁺ rebirth. Thus, the NADH from glycolysis is eliminated, and the NAD⁺ is reused by glycolysis.
Most carbohydrates are disaccharides or polysaccharides in nature. Organisms hydrolyze sucrose, starch or cellulose into glucose for alcohol production. The ethyl alcohol is main product in alcoholic fermentation but there are also several by-products like methanol, esters, glycerol and fusel oils (higher alcohols with greasy texture). If the fermentation vats are contaminated, the organic acids (lactic acid and acetic acid) will present in alcohol. This is why the wine tastes sour sometimes.
Homolactic Fermentation
The other common type is the fermentation driven by lactic acid bacteria. Pyruvate acts as the final electron acceptor and lactic acid is the final product when O₂ isn’t sufficient. There are two lactic acid fermentations: homolactic and heterolactic.
Homolactic fermentation involves two main steps: glycolysis and lactic acid formation.
Glucose → 2 Pyruvate + 2ATP + 2NADH
2 Pyruvate + 2NADH → 2 Lactic Acid +2NAD⁺
Glycolysis is the most energy-releasing step in lactic acid fermentation and the only step that produces ATP. Then NADH reduces pyruvic acid to lactic acid in anaerobic situation. The regenerated NAD⁺ is reused in glycolysis again.
Similar to ethanol fermentation, although glucose takes center stage, other saccharides can also be used in fermentation. Monosaccharide should be phosphorylated or converted to glucose. The polysaccharides have to be hydrolyzed into glucose by enzyme. Small amounts of ethanol, carbon dioxide and acetic acid are also formed as by-products for this type of fermentation.
Heterolactic Fermentation
Another name for heterolactic fermentation is hetero alcohol fermentation that converts a glucose into one ethanol, one lactic acid and one carbon dioxide. This process also comprises two main steps: the pentose phosphate pathway and production of lactic acid and ethanol.
A glucose is broken down into one carbon dioxide, one ribulose-5-phosphate, and two NADH via the pentose phosphate pathway. Then, the five-carbon sugar is decomposed into one glyceraldehyde-3-phosphate (G3P) and one acetyl phosphate. G3P is converted into pyruvate that will undergoes lactic acid fermentation, while acetyl phosphate will be turned into acetaldehyde before experiencing alcoholic fermentation. The final electron acceptors are acetaldehyde and pyruvate in heterolactic fermentation. The steps are as follows:
Glucose → CO₂ + Ribulose-5-phosphate + 2NADH
Ribulose-5-phosphate → G3P+Acetyl phosphate
G3P → Pyruvate → Lactic Acid + NAD⁺
Acetaldehyde → Aldehyde → Ethanol + CO₂ + NAD⁺
In traditional fermented foods, their flavor is enhanced by ethanol as well as gases. However, the undesirable heterolactic fermentation should be avoided in modern industrial processes requiring high-purity lactic acid.