Stuck Molasses Fermentation
A cooperative distillery in western Odisha ferments molasses to ethanol in a 40,000 L batch fermenter. The current batch, charged with 18% w/v total sugar molasses, is stuck at 6.2% v/v ethanol after 30 hours; the normal endpoint is 8.5% v/v. Records show: temperature 37°C (set point 32°C), viable yeast 35%, bacterial count 1.2×10^7 CFU/mL, residual sugar 11% w/v, pH fell from 5.0 to 4.2, and lactic acid 1.8 g/L. The yeast inoculum was propagated from a fresh slant but the propagation tank was cleaned only with water. The plant has no antibiotic permitted for its export buyer. The manager proposes adding 500 kg of fresh yeast and 2 kg of ammonium sulphate, then restarting. The next tank is scheduled in 12 hours, and the buyer requires a batch record with a mass balance.
1. Identify the most likely cause of the stuck fermentation and explain which pieces of evidence support or weaken that diagnosis.
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The most likely cause is lactic acid bacterial contamination, not simple yeast failure. The evidence fits: bacterial count is high at 1.2×10^7 CFU/mL, lactic acid has accumulated to 1.8 g/L, pH has dropped from 5.0 to 4.2, and 11% w/v sugar remains while ethanol is only 6.2% v/v. Lactic acid bacteria compete for sugar, produce acid that inhibits yeast, and thrive at 37°C, which is above the 32°C set point. The water-only cleaning of the propagation tank is a credible source. The low viable yeast count of 35% is more likely a consequence of acid and temperature stress than the primary cause.
Osmotic stress from 18% w/v sugar could also slow fermentation, but it would not explain the lactic acid, the pH drop, or the high bacterial count. A phage infection is unlikely in a bacterial contamination pattern. The diagnosis should be confirmed by plating on MRS or de Man–Rogosa–Sharpe agar and checking for Gram-positive rods or cocci, but the corrective decision should not wait for a full identification.
2. Evaluate the manager's plan of adding fresh yeast and ammonium sulphate. What would you do with this batch, given the buyer's no-antibiotic rule and the 12-hour schedule?
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The plan treats a contamination problem as a yeast-nutrient problem. Adding 500 kg of fresh yeast may not help because the fresh cells will face the same lactic acid, low pH and high temperature; ammonium sulphate may feed the bacteria and is not a cure. Antibiotics are excluded by the buyer, so the usual industrial shortcut is unavailable.
For this batch, cool to 30–32°C, check and adjust pH only if it is below 4.0, and consider partial dilution with clean water to reduce sugar and lactic acid concentrations. Re-inoculate with an actively growing, acid-washed yeast culture at a high pitching rate, add a balanced nutrient including yeast extract and diammonium phosphate at a controlled dose, and monitor CO2 evolution and sugar consumption hourly. Accept a lower final ethanol titre and record the deviation with a full mass balance for the buyer. If fermentation has not restarted within 8 hours, divert the batch as low-wine to distillation, pasteurise it, and clean the fermenter thoroughly. Do not blend it silently into a normal batch. The next tank should be delayed until the fermenter and propagation line are sanitised.
3. What preventive changes would you make before the next batch?
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First, validate cleaning. Use a caustic wash at 1–2% NaOH and 70–80°C, followed by a food-grade sanitiser such as peracetic acid, and verify with ATP swabs and microbial counts. Separate yeast propagation from the main fermentation area and use a validated pure-culture build-up. Cover molasses storage and specify bacterial loads in the incoming molasses. Control temperature to 30–32°C rather than 37°C, because the higher temperature selects for lactic acid bacteria and stresses yeast. Rehydrate or propagate yeast to a high viable count and check vitality before pitching. Add a rapid lactic acid or CFU test at 12 hours so contamination is detected before the batch is stuck. Keep a batch record that includes cleaning verification, inoculum size, temperature, pH, lactic acid and mass balance. Do not use antibiotics as a routine fix, because it would breach the buyer's specification and select resistant bacteria.