The Yoghurt Batch That Split
A small school-based food business produces 40 tubs of set yoghurt each week. The recipe heats 10 L of full-cream milk to 85 °C for 30 minutes, cools it to 43 °C, inoculates with 2 % live culture, fills tubs, and incubates at 43 °C until the pH reaches 4.6 (about 4 hours). Last week the incubator thermostat failed overnight and the batch sat at 43 °C for 11 hours. Final pH was 4.1. The yoghurt tasted sharply sour, and clear whey pooled on the surface of every tub. The owner wants to sell it as 'Greek-style' by straining off the whey, but a customer has already complained that the texture is grainy and the flavour is 'too sour'.
1. Explain why the extended incubation produced both the whey separation and the sharp flavour, and evaluate whether straining the whey makes the product acceptable as 'Greek-style' yoghurt.
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Cause of the changes. The culture continued fermenting lactose to lactic acid after the intended endpoint. pH fell from 4.6 to 4.1, which is below the isoelectric point of casein (about pH 4.6). At pH 4.6 the casein micelles aggregate into a gel that holds water; pushing pH lower causes the proteins to contract and expel water - syneresis - which appears as pooled whey. The same acid production explains the sharp flavour, and the grainy texture comes from the contracted, over-firmed protein network. These are linked consequences of one cause: over-fermentation, not a separate fault.
Is straining acceptable? Straining removes some whey and thickens the product, which mimics the concentration step used for Greek-style yoghurt. But it does not fix the root cause. The remaining protein network is already contracted and grainy, so the texture will stay defective. Straining also removes some lactic acid and water-soluble nutrients, so the yield drops and the nutrition claim becomes weaker. More importantly, Greek-style yoghurt is defined by concentration of milk solids, not by salvaging an over-acid gel. Selling it under that label would misrepresent the product.
Judgement. The batch should not be sold as Greek-style. The owner could either discard it or relabel it honestly as a sharp, thin drinking yoghurt, but only after checking the pH and sensory quality against a specification. The real fix is process control: a calibrated thermostat, a pH or titratable-acidity endpoint check, and a time limit on incubation so the batch cannot drift past pH 4.6.
2. Recommend a process-control plan that would prevent this fault recurring, justifying each control in terms of the food chemistry involved.
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1. Define a measurable endpoint. Set the specification at pH 4.5–4.6, checked with a calibrated pH meter on a cooled sample. Justification: this is the pH at which the casein gel is stable and whey is held; it also sits at the acid boundary relevant to pathogen control. A pH endpoint is more reliable than a fixed time because culture activity varies with milk and temperature.
2. Control temperature. Use an incubator with an independent thermostat and a data logger that alarms if temperature drifts outside 42–44 °C. Justification: culture activity roughly doubles per 10 °C rise, so an uncontrolled overnight hold accelerates acid production far beyond the planned 4 hours.
3. Cap the hold time. Set a maximum incubation of 5 hours and require a check at 4 hours. Justification: this builds a barrier against the exact failure that occurred - a stuck thermostat plus no one checking.
4. Standardise the milk. Use a consistent protein content and add skim-milk powder to raise total solids. Justification: more solids means a firmer gel with better water-holding, so the product tolerates small acid variation without syneresis.
5. Verify with sensory and yield checks. Record texture, whey separation and taste against a reference sample each batch. Justification: chemistry data alone does not catch grainy mouthfeel, which is a structural defect.
6. Document and review. Keep batch records and review any deviation. This supports a hazard-control approach and shows the business can trace and correct faults.