The over-sour yogurt problem
Why yogurt becomes too sour — excess lactic acid
Sour yogurt has produced excess lactic acid — from over-long incubation, incubation at too-high temperature (faster acid production), or refrigerating too late after setting. Like dosa batter, yogurt sourness increases continuously unless fermentation is stopped by refrigeration at the correct moment.
The Fix
How to prevent sour yogurt
- Refrigerate as soon as the yogurt has set — do not leave at room temperature after setting
- Incubate for 4–6 hours at 42°C, not 8–10 hours — check setting after 4 hours
- Test by tilting the container — set yogurt does not slosh. Refrigerate immediately when it holds its shape.
- For yogurt that is already too sour: use for marinades, curry, lassi, or raita where the sourness is a feature
- Next batch: reduce incubation time by 1 hour and refrigerate immediately when set
The Science
Why does yogurt continue souring even after it has set?
Yogurt bacteria continue producing lactic acid as long as temperature and available lactose allow. Setting (gelation) occurs when pH drops to approximately 4.6 — but bacteria remain active well below this pH. At incubation temperature (42°C), bacteria continue producing acid for hours after setting — the yogurt becomes increasingly sour until refrigeration drops the temperature below 10°C where bacterial activity nearly stops. Refrigerating immediately after setting is the only way to prevent over-souring.
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