Can you attempt a rescue operation by cutting off the mould or should the whole lot go in the bin?
The answer to some extent depends on how you balance your approach to a potential health risk versus your desire to avoid wasting food.
If the cheese is a hard cheese, it's probably safe just to cut the bad bit off, says Dr Ailsa Hocking, of CSIRO Agriculture and Food.
The bread though, is probably better off thrown away, she believes.
Assessing the risk
It's not just an awful taste you're risking if you eat mouldy food.
Actively growing mould can release toxins into food.
Since the spread of the tiny mould tips is not always visible, it might be hard to know where it (and hence the toxin) is.
So how do you decide what to do when you haven't got a food safety expert on hand?
Two factors that should guide you are the moisture content of the food and how densely it's structured, Dr Hocking says.
The low moisture content and dense structure of hard cheese means mould will usually survive only on the surface, rather than spreading invisibly into the cheese, Dr Hocking says.
So it should be safe to cut around the affected area and eat the rest of block. But she advises cutting with a margin of a couple of centimetres, just to be on the safe side.
But many other foods, including mouldy bread, are better off thrown away as the mould is more likely to be growing beyond the areas you can see.
Also, if a food like yoghurt is old enough to have mould, it might also contain harmful bacteria which could cause a bout of diarrhoea.
"If you can see a mould growing, there can be other microbes such as bacteria and yeast in the food as well. It's really not worth making yourself sick," Dr Hocking says.
Low-acid foods such as dairy products – think yoghurt, cottage cheese, dips – are the foods where this risk is greatest.
What's the concern?
There are many different types of moulds that can grow on our food; the most common include Aspergillus and Penicillium, and Botrytis which you might see as a fur on your strawberries.
So what do toxins from such moulds do to us?
While it's rare, they can cause severe illness with symptoms including excessive sweating, tremors, muscle weakness, twitching, headache, fever, and vomiting.
One elderly man who became ill after eating canned soup contaminated with mould was reported in the Medical Journal of Australia as saying he felt so sick, he thought he would die.
Most toxins from moulds are heat-resistant and so may not be destroyed in the cooking process.
Eating mouldy food has also been known to cause illness in livestock and to kill dogs.
Domestic dogs are at particular risk when they have access to household waste.
But most toxins from moulds are only a risk if we eat them over a long period of time.
A lot of these compounds are carcinogenic, and ongoing exposure has been linked to liver cancer, Dr Hocking said.
That would be very unlikely to happen in Australia, where we have very strict food production standards.
But it is seen in developing countries where communities sometimes inadvertently use grain which is contaminated with mould.
If it's mouldy soft cheese, casserole leftovers or soft fruit like say berries, nectarines or peaches, it pays to know the high moisture content of these items means there is a greater chance branches of the mould have grown deeper into the food, where you won't necessarily be able to see them, Dr Hocking says.
The same rule goes for porous foods such as bread and cakes which have become mouldy. All of these foods should be binned if you spot mould on the surface.
Of course, certain domesticated moulds are deliberately introduced into our food as part of the production process — think blue cheese, for example.
While these are safe to eat, if a soft cheese that has been made with domesticated mould starts growing other types of mould, it should be discarded. (It can be tricky to tell, but Dr Hocking suggests looking out for any patches that are a different colour from the rest of the cheese.)
The jam and jelly debate
One area of some debate is whether fruit jams and jellies can be rescued if they have developed mould on their surface.
You can try to scoop off the affected jam, but its wobbly consistency means you risk dislodging air-borne spores that all moulds produce, that can float around, settle and start a fresh crop of mould.
The US Food Safety and Inspection Service says moulds on jam could be producing toxins and the whole lot should be discarded.
However, Dr Hocking expects Australian jams would have a lower moisture content and are therefore less of a risk.
To eat or not to eat?
Moulds can grow in the fridge and will even survive freezing. They can also survive in salty, sugary and acidic environments.
As mould on our food is so hard to avoid, here are some general guidelines from the US Food Safety and Inspection Service on responding to the problem:
Discard all of these foods if mouldy:
- Luncheon meat, bacon, and hot dogs.
- Yoghurt, sour cream and soft cheese.
- Soft fruits and vegetables
- Bread and baked goods.
- Peanut butter, nuts and legumes.
- Jams and jellies - but note Dr Hocking has a slightly different view for Australian jams.
These foods can be saved from mould:
- Hard salami (the dry, aged type) – scrub mould from the surface.
- Hard cheese – cut off at least 2.5 centimetres around and below the mould. Don't let the knife touch the mould and recover the cheese with fresh wrap.
- Firm fruit and veg – small mould spots can be cut off.