As foods age, bacteria start to multiply rapidly – especially in warm environments. Some of these bacteria release ‘off’ smells – gases and compounds which the human nose has evolved to associate with unsafe foods. You know that mouldy cheese that your friend loves? The one that smells like a footballer’s sock? The reason that some cheeses smell so off is that they contain high levels of these ‘off’ smelling bacteria.

However, these bacteria are often innocuous to humans (That cheese might smell like death, but it probably won’t kill you). By the same token, sometimes a product can be absolutely teeming with pathogenic nasties, and lack any distinct odour. Bacteria like Salmonella, Listeria and E. Coli don’t produce a distinctive smell – despite being highly dangerous to humans.