The Overdue Flow-Meter Calibration on the HTST Line
A dairy's HTST pasteuriser is rated at 10,000 L/h and is designed for 72 °C for 15 s. The holding tube has a measured volume of 42 L. After a pump seal was replaced, the night shift ran the plant at 11,200 L/h to clear a tanker queue. The chart recorder shows 73 °C throughout the run, and the flow-diversion valve did not trip. The plant packed 20,000 L of toned milk from this run. Next morning, the laboratory reports a positive alkaline phosphatase test on a retained sample from the first hour. The operator says, 'The temperature was above 72 °C, so the milk is pasteurised.' The flow meter was calibrated six months ago and its calibration due date has passed.
1. Calculate the holding time at 11,200 L/h and decide whether the run complied with the 72 °C/15 s requirement.
Read the worked answer
Holding time = holding-tube volume ÷ flow rate. At 11,200 L/h: 42 L ÷ 11,200 L/h = 0.00375 h = 13.5 s. At the design flow of 10,000 L/h it would be 42 ÷ 10,000 × 3,600 = 15.1 s. Therefore the run at 11,200 L/h did not meet 72 °C/15 s, even though the recorder showed 73 °C. Temperature alone is not the critical control point; holding time is set by flow rate. The positive phosphatase test confirms inadequate heat treatment, because alkaline phosphatase is inactivated by correct pasteurisation. The operator's statement is therefore wrong.
2. Decide the disposition of the 20,000 L and state the immediate actions.
Read the worked answer
Hold all 20,000 L and do not release it as pasteurised market milk. Trace the batch code; if any has left the plant, initiate a withdrawal or recall. Immediate options are to re-pasteurise the whole batch at a validated flow of not more than 10,000 L/h with a functioning diversion valve, or to divert it to a product process that includes a verified heat treatment. Do not blend it with properly pasteurised milk. Check the flow-diversion valve: it should divert when temperature or flow is outside the set limits; if it did not trip, the valve or control interlock is faulty. Calibrate the overdue flow meter and verify the holding-tube volume before restarting.
3. Recommend corrective and preventive actions, including verification of the critical control point.
Read the worked answer
Set a physical or electronic flow limit at 10,000 L/h and lock it. Calibrate the flow meter and verify the holding-tube volume. Test the diversion valve by deliberately lowering temperature and by raising flow above the set point. Retrain operators that a temperature chart does not prove holding time. Add a defined verification schedule, such as daily phosphatase testing or periodic challenge testing. Release the reprocessed batch only after re-pasteurisation, a negative phosphatase result and satisfactory microbiological checks. Record the CCP monitoring and the corrective action; this is what an auditor or food safety officer will ask for.