How Bacteria Make Vinegar — Acetobacter & Acetic Acid
Helpful Bacteria #14: Acetobacter converts ethanol → acetaldehyde → acetic acid. Clear animation of the PQQ-dependent pathway described above.
Chapter 01
What Is Vinegar?
Definition, legal thresholds, and the core chemistry of double fermentation.
Stage 1
Alcoholic fermentation
Yeasts (chiefly Saccharomyces cerevisiae) convert sugars into ethanol.
C6H12O6 → 2 C2H5OH + 2 CO2
Stage 2
Acetic fermentation
Acetic acid bacteria (Acetobacteraceae) oxidize the ethanol into acetic acid in the presence of oxygen.
CH3CH2OH + O2 → CH3COOH + H2O
Legal definitions around the world
| Jurisdiction | Minimum acetic acid | Notes |
|---|---|---|
| USA (FDA) | 4 g / 100 mL (4%) | CPG Sec. 525.825; diluted vinegars must state final strength; must derive from the named substrate |
| Codex Alimentarius (global) | 4% | CXS 162-1987; product of double fermentation from suitable agricultural raw materials |
| EU | Varies by type | “Spirit vinegar” regulated under Regulation (EU) 2019/304; many member states require ≥5%; Italian national rules require ≥6% for wine vinegar |
| Japan (JAS) | ≈4% | Distinction between brewed rice vinegar (jōzō-su) and seasoned blends; synthetic acid cannot be labeled as brewed vinegar |
| China (GB standards) | ≈3.5–6% by style | Famous regional types carry much higher total-acid requirements in practice |
| Thailand (MOPH) | 4 g / 100 mL fermented or distilled | Artificial (synthetic-acid) vinegar permitted but must be labeled; capped at 7% |
The core chemistry
CH3CH2OH + O2 → CH3COOH + H2O ΔH ≈ −494 kJ/mol
- One mole of ethanol (46 g) yields one mole of acetic acid (60 g). Because the molecule gains an oxygen atom, the finished product weighs more than the alcohol consumed.
- 1 liter of pure ethanol theoretically yields ~1.03 kg of acetic acid — enough for roughly 20 liters of 5% vinegar. Practical plant efficiency runs 85–95%.
- The reaction is strongly exothermic: every liter of 5% vinegar produced releases roughly 400 kJ of heat — trivial on your counter, a serious cooling problem in a 50,000 L acetator.
- Oxygen demand is enormous: converting 1 L of ethanol requires about 385 L of pure oxygen, i.e., nearly 2,000 liters of air. This single fact drives most industrial reactor design.
- Finished vinegar pH typically sits between 2.0 and 3.5. pH and titratable acidity are different things: dark cereal vinegars are heavily buffered by amino acids and minerals, so they taste softer than their total acidity suggests.
What vinegar is not
- Not industrial glacial acetic acid diluted with water (“artificial vinegar,” banned or restricted for food use in many countries). Chemically similar, gastronomically dead.
- Not merely “sour liquid”: the double-fermentation origin is what most legal regimes require, and it carries the hundreds of secondary compounds that give real vinegar its aroma.