Under-pasteurised milk after a flow rate increase and a home-cut gasket
A cooperative dairy in Alwar runs a plate pasteuriser rated at 1,000 L/h with a holding tube of 4.2 L, a flow diversion valve set at 72 °C and a chart temperature recorder. To meet a new school milk order, the manager raises the pump stroke so the plant runs at 1,400 L/h and replaces a leaking regenerator gasket with a locally cut rubber sheet. The recorder chart for that day shows the pen sticking at 74 °C for two hours. The milk is filled into pouches, held in a cold room at 6 °C and delivered the next morning. The plant's phosphatase test on a sample from the first hour is negative, so the manager concludes the milk was properly pasteurised. Two days later two schools report sour milk with a high coliform count, and the dairy's retained sample shows a plate count of 4 × 10⁵ CFU/mL.
1. Calculate the actual holding time at 1,400 L/h and decide whether the milk was properly pasteurised. Explain why the negative phosphatase test did not prove safety.
Read the worked answer
Holding time: 1,400 L/h = 1,400 / 3,600 = 0.3889 L/s. Holding time = volume / flow = 4.2 / 0.3889 = 10.8 seconds.
At the rated 1,000 L/h: 0.2778 L/s, so 4.2 / 0.2778 = 15.1 s - just meeting the 72 °C/15 s requirement.
Decision: 10.8 s at 72 °C is well short of the legal minimum of 15 s. The milk was not legally pasteurised; it was only heat-treated, whatever the recorder showed.
Why the phosphatase test alone is not proof:
- Alkaline phosphatase is more heat-labile than the pathogens of concern, so a negative result is a necessary but not a sufficient check - it can be negative even where flow or temperature control has drifted.
- The sample came from the first hour, not from the whole two-hour run, and the sticking recorder pen means the temperature record itself is unreliable; the process must be judged on validated flow, hold time and a working FDV.
- The gasket and the 6 °C cold room allow post-pasteurisation contamination, which phosphatase cannot detect at all; coliforms at high count point strongly to post-process contamination.
- A plate count of 4 × 10⁵ CFU/mL is far beyond the limit for pasteurised milk in the FSSAI microbiological standards, so the product is non-compliant on its own evidence.
2. Identify the equipment and hygiene failures in this incident and specify the corrective actions, with numbers.
Read the worked answer
Failures and corrections:
- Flow/hold time. Required hold volume at 1,400 L/h for 15 s = 0.3889 × 15 = 5.83 L. Either restrict the pump to 1,000 L/h (giving 15.1 s) or fit a holding tube of at least 6 L; fit a mechanical flow-control or a flow transmitter interlocked with the FDV so that flow above set point diverts the product.
- Temperature control. Recalibrate the recorder, replace the sticking pen, check chart speed, and verify the FDV opens within 1 s if temperature falls below 72 °C. Use a calibrated mercury-in-glass thermometer and an independent recorder.
- Gasket. A locally cut rubber sheet has the wrong profile, no food-contact approval and cannot seal under pressure. Replace with a food-grade EPDM/silicone gasket of the correct profile from the plate manufacturer. Check pressure differential: pasteurised-side pressure must exceed raw-side pressure by about 0.5–1 bar in the regenerator, otherwise raw milk leaks into pasteurised milk.
- Hygiene and storage. Verify CIP (alkali 1–2% at 75–80 °C for about 20 minutes, rinse, acid, sanitise) with ATP swabs and rinse-water tests. Cold room at 6 °C is too warm for stored pasteurised milk - set it at 4 °C or below and monitor it with a data logger.
- Records. Create a pasteuriser log per run: flow, inlet/outlet temperature, holding time, diversion events, CIP record and operator signature.
3. Draft a verification plan before the dairy is allowed to supply schools again: what you would test, where the samples come from, and what you would require before release.
Read the worked answer
Before restart (engineering verification):
- Hold-tube test by the dye or conductivity method at 1,000 and 1,400 L/h to confirm actual residence time.
- FDV test: force the temperature below 72 °C and confirm diversion within 1 s and that no diverted milk reaches the filler.
- Recorder calibration certificate and a 24-hour chart run.
- Pressure differential check across the regenerator in both directions; gasket inspection and replacement record.
- CIP validation: ATP swabs on plates, gaskets and filler heads, plus a rinse-water count.
Product testing (raw, pasteurised, and finished pack):
- Alkaline phosphatase on pasteurised milk, every run, with the result written on the pasteuriser log.
- Total plate count and coliform count on pasteurised milk, and on the finished pouch after cold storage, against the FSSAI microbiological standards for pasteurised milk (coliforms must be absent).
- Methylene blue reduction or a similar keeping-quality test on the finished product; storage trial at 7 °C for the declared shelf life.
- Swabs from the filler and the cold room.
Release criteria: every parameter within the current FSSAI standard; a signed pasteuriser log with no diversion events and a valid chart; three consecutive compliant production days before schools are resupplied.
Also: withdraw and recall all pouches from the affected run, inform the schools and the food safety authority as required, re-train operators, and re-audit the plant before resuming supply.