we will not be eating any foods that have passed their “use by” date. To do so would risk serious food poisoning. With “best before” or “sell by” things are more flexible.
cheeses:
cheddar and parmesan and other dry cheeses: cut away the mouldy part, being careful that the knife doesn't get contaminated by mould. Mould needs moisture to thrive, it doesn't normally penetrate far below the surface.
blue cheeses: are deliberately infected with fungi. Penicillium roqueforti gives blue cheeses such as stilton and roquefort their flavour.
With most soft cheeses, however, unless the fungus has been deliberately introduced, the presence of mould suggests infection not just by unwelcome fungi but also harmful bacteria, such as listeria or salmonella. If that happens, throw it away.
Bread covered in small bits of white-and-blue mould: trim and toast? I think not.
vegetables (courgettes and carrots) covered in a horrible slime, which is caused by bacterial colonies growing on the surface. Discard, toxins present would persist after cooking and “you'd probably develop terrible stomach ache in a couple of hours, followed by stomach cramps and diarrhoea.”
fruit normally lasts better than vegetables because the acid in fruit keeps harmful bacteria at bay. The moulds you find on jam, are fine - just scrape them off. There is, however, one fruit to be wary of.
“Apples,” he tells me, “will keep for ages, but if there's a puncture in the skin, fungi can get in. There have been serious outbreaks in the past, incidents of people being poisoned by drinking bad apple juice, which contains a toxin called patulin.”
Mouldy nuts are particularly dangerous because they harbour a fungus called Aspergillus flavus. “This fungus,” says Hickey, “produces one of the most deadly toxins known to humankind. The toxin accumulates in the liver and can cause liver cancer. If there's no mould on the shell and the internal part is sealed and protected, which these are, then we should be fine.”
Rice. You should never reheat cooked rice that has been left hanging around at room temperature for any length of time, because it can harbour toxins that will give you food poisoning. But if leftover rice goes straight into the fridge after a meal it will be fine for a couple of days.