He says once you’ve cut off the mouldy part (being careful that the knife doesn’t get contaminated by mould), cheddar and Parmesan are fine to eat.
As dry cheeses, they don’t provide the moisture that mould needs to thrive and it doesn’t normally penetrate below the surface.
Some cheeses, of course, are deliberately infected with fungi. Penicillium roqueforti gives blue cheeses such as stilton and Roquefort their flavour.
Soft cheeses are the really important ones to watch. 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.
Fruit normally lasts better than vegetables because the acid in fruit keeps harmful bacteria at bay but do be wary of apples.
They’ll keep for ages but if there’s a puncture in the skin, fungi can get in, says Patrick Hickey.
“There have been serious outbreaks in the past, incidents of people being poisoned by drinking bad apple juice, which contains a toxin called patulin.”
How about a soup made with those long-forgotten, slightly slimy courgettes and carrots lurking at the back of the fridge?
Best not. That slime is caused by bacterial colonies growing on the surface. “You’d probably develop terrible stomach ache in a couple of hours, followed by stomach cramps and diarrhoea,” says Patrick Hickey.
Mouldy nuts are particularly dangerous because they harbour a fungus called Aspergillus flavus.
“This fungus produces one of the most deadly toxins known to humankind,” says Patrick Hickey.
“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, then we should be fine.”
And the important bit: You can be flexible with “best before” or “sell by” dates but don’t risk serious food poisoning by eating foods past their “use by” date, warns Patrick Hickey.
The FSA advice
Some species of mould can produce toxins that are known to have adverse effects in humans as well as in animals.
While it is possible that removing the mould and a significant amount of the surrounding product could remove any unseen toxins that are present, there is no guarantee that doing so would remove them all.