A Positive Phosphatase Result After an HTST Temperature Dip
A 10,000 L batch of toned milk was pasteurised on a plate HTST plant rated at 5,000 L/h, with a holding tube designed for 18 s at 72°C and an automatically recording thermometer and flow diversion valve. The chart shows that between 06:12 and 06:16 the temperature fell to 69.5°C, although the operator says the diversion valve 'worked' and the batch was held. Packaged milk sampled 24 h later gives a positive alkaline phosphatase test, a standard plate count of 8 × 10^4 cfu/mL and coliforms at 40 cfu/mL; the raw milk count was 6 × 10^5 cfu/mL. The plant was CIP-cleaned the previous evening, but the final rinse water had a faint milk smell. Maintenance records show that a pinhole leak in the regenerator section was repaired a week earlier. The manager wants to release the batch because 'phosphatase can be positive from lab error' and the milk smells normal.
1. Should the batch be released? Give your decision and the reasoning behind it.
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No. Three independent findings point the same way. First, the recorder chart shows a genuine temperature fall to 69.5°C for about four minutes, which is below the 72°C/15 s legal minimum for HTST pasteurisation. Second, alkaline phosphatase is inactivated by correct pasteurisation, so a positive result means either under-heating or contamination of pasteurised milk with raw milk. Third, coliforms at 40 cfu/mL and a standard plate count of 8 × 10^4 cfu/mL after packaging indicate poor post-pasteurisation hygiene or a raw-milk leak, not a laboratory accident. A normal smell is irrelevant, because spoilage odours appear only after microbial growth. The claim that the flow diversion valve worked must be proved from the valve stem position, the recorder trace and a functional test, not from the operator's memory. Until that proof exists, the whole 10,000 L must be treated as potentially unpasteurised. Hold it, condemn it for market sale, and recall any portion already dispatched.
2. How would you distinguish under-pasteurisation from post-pasteurisation contamination using the available evidence?
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Compare samples taken at the pasteuriser outlet with samples from the packaged milk. If the outlet sample is phosphatase-negative and the packaged sample is positive, the problem lies after the pasteuriser: filler, gaskets, pipeline or the repaired regenerator plate. If both are positive, under-pasteurisation or a raw-to-pasteurised leak is more likely. A pressure test and dye test of the plate pack will show whether raw milk can cross into the pasteurised side; a pinhole repaired only a week earlier is a strong suspect, especially if the raw-side pressure exceeded the pasteurised-side pressure during the run. The peroxidase test is also useful: peroxidase survives below about 80°C, so a positive peroxidase result alongside positive phosphatase argues against overheating and is consistent with under-pasteurisation, but it does not by itself separate a leak from a temperature dip. High coliforms and a raised standard plate count support post-pasteurisation contamination, while the chart dip proves at least one under-processing event. On this evidence, both causes must be excluded before release; the batch cannot be saved by arguing for one explanation alone.
3. List the corrective and preventive actions you would require before the plant restarts.
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Quarantine the batch and any related stock; notify the competent authority if distribution has occurred. Strip and inspect the plate pack, replace damaged gaskets, and pressure-test both sides of the regenerator. Prove the flow diversion valve operates at the set point by a challenge test and check that the recorder pen and chart drive are correct. Recalibrate the indicating and recording thermometers against a reference. Review the CIP programme: the milky final rinse suggests soil or milk solids remained, so check caustic concentration, temperature, flow and contact time, then swab filler nozzles, gaskets and lines for ATP and coliforms. Retrain operators to record valve action and to stop production when the chart is outside the legal range. Until the pressure test and two consecutive negative phosphatase and coliform results are obtained, run the plant on a verified pasteurisation test and increase finished-product sampling. Document every action so the society can show that the correction was verified, not merely promised.