Phosphatase-positive pasteurised milk after a regenerator leak
At a 50,000 L/day dairy, an HTST pasteuriser rated for 10,000 L/h runs 72°C for 15 s with a flow diversion valve. During the morning shift, the recorder chart shows a six-minute dip to 69°C at 10:15; the operator says the FDD diverted and the alarm sounded. Six pasteurised-milk packs sampled after the cooler are tested 24 h later: two are phosphatase-positive, and the standard plate count is 4.2 × 10^4 cfu/mL. The raw-side pressure gauge at the regeneration section reads 4.2 bar; the pasteurised side reads 3.8 bar. A pinhole is later found in a regenerator plate. Product from the run-18,000 L in 500 mL pouches-is in the cold store. The laboratory uses the phosphatase test as the release criterion. You must decide the root cause, the fate of the product, and what must be corrected before the next run.
1. Explain how the chart dip, the phosphatase result and the pressure readings fit together. Why did the FDD not guarantee a safe product?
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The FDD protects against under-temperature in the holding tube, not against recontamination downstream of the holding tube. The chart dip to 69°C explains why the FDD should have diverted the product during those six minutes, and if it worked correctly that portion should not have reached the filler. The phosphatase-positive packs therefore point to a second, continuous fault.
The pressure readings reveal that fault: the raw side at the regenerator is at 4.2 bar and the pasteurised side at 3.8 bar. In normal operation the pasteurised side must be held at a higher pressure than the raw side-typically by about 0.5–1 bar-so that any pinhole leaks pasteurised milk into raw milk, not raw milk into pasteurised milk. Here the differential is reversed, so raw milk can be drawn through the pinhole into the already-pasteurised stream. That raw milk carries active phosphatase and bacteria, which explains both the phosphatase-positive packs and the elevated standard plate count.
The chart dip and the pinhole are therefore two separate failures: a process deviation that should have been caught by the FDD, and a mechanical integrity failure that the FDD cannot detect. The phosphatase test on the final pack is the integrated release test and it has correctly failed the run.
2. What is the fate of the 18,000 L of packed milk, and what immediate actions would you take?
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Do not release the 18,000 L. Place the entire run on hold, quarantine it in the cold store and stop dispatch. If any pouches have left the dairy, recall them. Phosphatase-positive pasteurised milk has not met the legal and safety release criterion, and the elevated SPC shows that contamination has occurred; the product cannot be sold as pasteurised milk.
Do not simply re-pasteurise the packed product for retail sale. Repasteurisation of finished liquid milk is not an acceptable route to release, and it would not address the root cause or the regulatory breach. The batch should be condemned unless the competent authority permits a specific downgraded use after a documented risk assessment. Keep samples and records for the investigation.
Immediate engineering actions: stop the line, replace the leaking regenerator plate or plate pack, pressure-test both sides, and verify that the pasteurised side can hold a positive differential over the raw side. Inspect gaskets and plate edges for cracks. Recalibrate the FDD and the temperature recorder, and test the FDD with water at the set point. Clean and sanitise the downstream side, then run water and hold the first production until phosphatase and microbiological results are negative. Release only after two consecutive satisfactory runs and a documented maintenance sign-off.
3. What preventive controls would stop this from recurring?
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The key control is pressure differential management. Fit and maintain reliable pressure gauges on both sides of the regenerator and check the differential at every start-up and during the run. The pasteurised side should be the higher-pressure side under all normal flow conditions. If the differential cannot be maintained, the line must not run.
Supporting controls: a preventive maintenance schedule for plate inspection, gasket replacement and pressure testing; a leak-detection routine after CIP; and a check that the FDD and recorder are calibrated and that the FDD actually diverts when tested. The operator's response to the 69°C alarm should be recorded and reviewed, not just acknowledged.
Laboratory controls should include phosphatase on every pasteurised-milk run, with a hold-and-release policy until the result is available. Microbiological trends-SPC, coliforms and keeping-quality tests-should be reviewed weekly. Finally, train operators to understand that the FDD protects the holding tube only; a pinhole in the regenerator or cooler is a separate contamination route. Any unexplained phosphatase-positive result should trigger an immediate pressure and integrity check, not a repeat test alone.