On 3 July last year I opened a 2-litre plastic bottle of semi-skimmed, homogenised milk, with a use-by date of 5 July. I poured out a small cup of what turned out to be curds: a sniff confirmed the milk was off. I guessed that it had been inadequately refrigerated but because I was making pancakes I continued to pour it out, suspecting the sour taste would not be noticed. To my surprise, the remainder was fine and still was a day later: the curdling was restricted to the top 4 centimetres of the milk, where the bottle was fairly narrow. How can this have happened?Bob Ladd, Edinburgh, UK