Sour Milk

Over the weekend, I bought 2 x 2 liters of PnP milk from PnP Heidelberg (GP). We started using them on Monday and there was nothing wrong as the expiry date is the 10th of Feb. This morning when we started to eat our porrige, we tasted that the milk was off. So we threw away 4 bowls of porrige and tea that was made, opened the other bottle that smelled fine, but had the same outcome. This is now the 3rd time that we have an issue with PnP milk. The previous time it happened, we took the bottle back and lodged a complaint with management. Fortunatly we had some longlife milk and used that. Please follow up on this. PS.....Just the other day I was send a survey form regarding PnP and its no name products, in which I complemented PnP for an exemplery product as it improved a lot since my previous bad encounters.