Failed CIP, High ATP and Thermoduric Counts
A milk plant's 20,000 L raw milk silo is cleaned by a CIP set delivering 1.5% NaOH at 70 °C for 10 minutes at 1.5 m/s. After a chemical supplier change, the titration log shows 1.1% NaOH, but the operator signed the chart as 'OK'. The final rinse water has pH 7.6 and a faint milk smell. ATP swabs from the outlet valve and manhole give 450 RLU and 380 RLU against a pass limit of 150 RLU. Milk filled into the silo the next day shows a thermoduric count of 1,200 cfu/mL, up from the usual below 100 cfu/mL, and a slight cooked flavour after pasteurisation. The silo has a spray ball; the flow meter is working. The plant wants to release the milk after pasteurisation because 'pasteurisation kills everything'.
1. Interpret the CIP results and decide whether the silo is clean.
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The silo is not clean. The target is 1.5% NaOH at 70 °C for 10 minutes at 1.5 m/s; titration shows only 1.1%, so the chemical strength is below target. Final rinse pH 7.6 and a milk smell indicate milk residue. ATP results of 450 and 380 RLU are far above the 150 RLU pass limit; ATP measures organic soil, not just microbes. The thermoduric count of 1,200 cfu/mL suggests a biofilm or soil layer that protects heat-resistant organisms. The operator's signature is false verification. Cleaning must be repeated and verified before the silo is used again.
2. Decide the disposition of the milk and evaluate the plant's claim that pasteurisation makes it safe.
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Segregate the milk and do not release it as market milk. Pasteurisation does not destroy all thermoduric organisms or heat-stable enzymes; the claim that 'pasteurisation kills everything' is incorrect. If the milk has already been pasteurised, test phosphatase to confirm heat treatment and run microbiological tests, but thermoduric survivors may still spoil the product. Divert it to a process with a validated sterilization or UHT step only if quality permits; otherwise reject or downgrade it. Trace any product made from the silo and hold it. The silo cannot be refilled until it has been re-cleaned and verified.
3. Recommend the corrective CIP actions and the release criteria.
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Recalibrate the chemical dosing pump and verify NaOH concentration by titration before every wash. Restore the temperature to 70 °C, check the heat exchanger, and confirm 10 minutes contact and 1.5 m/s flow. Inspect the spray ball for blockage or worn jets. Carry out an acid wash to remove milkstone, then re-clean and swab ATP at the valve, manhole and outlet. Release the silo only after two consecutive ATP passes below 150 RLU and a satisfactory microbiological swab. Update the CIP record to require the actual titration value, temperature chart, flow and a verified signature; retrain the operator.