Aerobic Acidogenesis
The Core Reaction
Acetic acid bacteria (AAB) oxidise residual ethanol to acetic acid in the presence of oxygen, driving titratable acidity up while pH falls.
C₂H₅OH + O₂ → CH₃COOH + H₂O
This oxidation is the primary driver of TA accumulation throughout the batch.
Oxygen Transfer
The volumetric O₂ transfer coefficient kLa governs how much dissolved oxygen reaches the bacteria. Aeration level and impeller speed together raise kLa.
OTR = kLa · (C* − CL)
The simulation maps aeration (OFF→MED) to kLa values 0.18–1.0, consistent with the 16-batch historical dataset. When OTR < OUR, oxygen shortfall limits the acidification rate.
Temperature Optimum
AAB activity follows an Arrhenius-bell response. The model uses a Gaussian factor centred at 31 °C (σ ≈ 7.2 °C).
f_temp = exp(−((T − 31) / 7.2)²)
Jacket setpoint and pomace thermal mass blend to yield the effective process temperature. Below 24 °C or above 36 °C the rate drops substantially.
Substrate Factors
Residual ethanol (electron donor) and pomace Brix (co-substrate) together amplify the acidification rate. Pomace charge modulates available biomass; nutrient dose extends viable microbial density.
f_substrate = (0.36 + EtOH·0.11 + Brix·0.052) · f_charge · f_nutrient
Endpoint Criteria
The batch is complete when both conditions are met simultaneously — validated across all 16 historical batches.
| Parameter | Start | Target |
|---|
| Titratable Acidity | 3.2 g/L | ≥ 30 g/L |
| pH | 4.42 | ≤ 4.4 |
| Mean TA rate | — | ~0.57 g/L·day |
| Mean batch time | — | ~10.1 days |
Dilution & Volume
Vat volume dilutes acid concentration. A larger charge at the same production rate yields lower final TA — reflected in the 820 L baseline with a ±16 % operating window.
Visual Cues in the 3D Scene
Amber broth deepens as TA climbs toward target.
Blue bubble density tracks oxygen transfer rate.
Green endpoint ring glows when pH ≤ 4.4.
Red sparger indicates oxygen shortfall.