Introduction to Fermentation
Key concepts, mechanisms, types, and practical applications
1. What Is Fermentation?
| Aspect |
Description |
| Definition |
A metabolic process in which microorganisms (yeasts, bacteria, molds) convert organic substrates (usually sugars) into simpler compounds, releasing energy and producing characteristic by‑products (e.g., ethanol, lactic acid, CO₂). |
| Purpose in Nature |
Allows organisms to generate ATP when oxygen is limited (anaerobic respiration) or to regenerate NAD⁺ for glycolysis. |
| Human Relevance |
Basis for many foods, beverages, pharmaceuticals, and industrial chemicals. |
2. Core Biochemistry
2.1 Glycolysis – The Common Starting Point
- Glucose → 2 Pyruvate
- Net gain: 2 ATP and 2 NADH per glucose molecule.
2.2 Why Fermentation Is Needed
- NAD⁺ regeneration: NADH must be oxidized back to NAD⁺ for glycolysis to continue.
- Energy extraction: In the absence of a functional electron transport chain (e.g., low O₂), cells rely on substrate‑level phosphorylation.
2.3 Main Fermentation Pathways
| Pathway |
Primary Organism(s) |
Main Products |
Key Enzyme(s) |
| Alcoholic Fermentation |
Saccharomyces cerevisiae (yeast) |
Ethanol + CO₂ |
Pyruvate decarboxylase, Alcohol dehydrogenase |
| Lactic Acid Fermentation |
Lactobacillus, Streptococcus spp. |
L‑lactic acid (homo‑fermentative) or mixture of lactate, ethanol, CO₂ (hetero‑fermentative) |
Lactate dehydrogenase |
| Propionic Fermentation |
Propionibacterium spp. |
Propionate + CO₂ |
Propionyl‑CoA carboxylase |
| Butyric Fermentation |
Clostridium spp. |
Butyrate + Acetate + CO₂ + H₂ |
Butyrate kinase, Acetyl‑CoA pathways |
| Acetone‑Butanol‑Ethanol (ABE) Fermentation |
Clostridium acetobutylicum |
Acetone, butanol, ethanol |
Acetyl‑CoA acetyltransferase, butyraldehyde dehydrogenase |
3. Types of Fermentation (Industrial & Food Context)
| Category |
Examples |
Typical Substrate |
End‑Products |
Notable Uses |
| Alcoholic |
Beer, wine, spirits |
Sugars (glucose, fructose, maltose) |
Ethanol, CO₂ |
Beverage production, bio‑fuel |
| Lactic |
Yogurt, kimchi, sauerkraut, sourdough |
Milk lactose, vegetable sugars |
Lactic acid, sometimes CO₂ |
Food preservation, flavor development |
| Acetic |
Vinegar |
Ethanol (from prior alcoholic fermentation) |
Acetic acid |
Food condiment, biocontrol |
| Mixed‑Culture (e.g., Kombucha) |
Kombucha tea |
Sweetened tea (sucrose) |
Acetate, ethanol, glucuronic acid |
Functional drinks |
| Industrial Bulk |
Bio‑ethanol, biobutanol, lactic acid polymers |
Corn starch, sugarcane, lignocellulose |
Ethanol, butanol, PLA monomers |
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