This is an episode from the predecessor of the regular column ‘5 questions’: QED. Each instalment focused on a single question.
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Cut away a little mould and the cheese will last a bit longer. But is it always safe to eat around the mould?
A mouldy bag of bread that emerges from a school bag after a week leaves little room for doubt: throw it away. Yet discarding food with just a little mould on it often seems wasteful. Remove the mould and the problem is solved. Or is it?
‘To be on the safe side, you are better off always throwing away mouldy food’, says Marcel Zwietering, Professor of Food Microbiology at Wageningen University. ‘That certainly applies to liquid, softer or airy products. Only for hard products, such as hard cheese, can cutting generously around the mould be a solution.’ To spread, moulds form threads, also known as hyphae. ‘Those hyphae become so fine at their tips that you cannot see them with the naked eye’, Zwietering explains. ‘Through something like jam or butter, the threads can penetrate easily. In a hard cheese, that penetration is much more difficult; if you cut away a few centimetres all around the visible mould, you should be fine.’
In general, ingesting a small amount of mould is not particularly harmful. ‘Most moulds are harmless’, says Zwietering. ‘Some moulds are even delicious, such as those used in brie and Roquefort. Others merely impart a musty flavour, and a few produce toxins: mycotoxins. The good, the bad and the ugly, as I sometimes call it. Unfortunately, there is no simple test to determine which mould you are dealing with. Even once you have identified the species, you still need to analyse the DNA to find out whether that particular specimen also carries the genes to produce mycotoxins. By the time you have worked that out, your mouldy food will truly be past saving.’
‘The good, the bad and the ugly, as I sometimes call it’
Those toxins are the principal reason why eating mouldy food is unwise. ‘If you ingest a large amount, it can cause stomach ache, and with repeated exposure the risk of cancer may increase slightly. That does not happen very readily here, but in warmer countries where people suffer more from hunger, you do see that people are reluctant to throw away mouldy food, and cancer caused by mycotoxins is more prevalent locally’, Zwietering explains. Since mycotoxins can diffuse relatively easily through semi-liquid products on their own, that is a further reason why scooping away the mould does not solve the problem. Heating does not help either, because although moulds themselves are poorly tolerant of high temperatures, most mycotoxins are heat-stable.
Heat treatment
Unopened food products are usually free of mould. They have often undergone a heat treatment, but moulds also require oxygen to grow. ‘Bread is ideal: a large surface area and pleasantly airy. Moulds grow more slowly than the competition – bacteria and yeasts. But they are better able to tolerate acidic and low-moisture environments, which is why you find them more readily on, for example, bread, fruit or jam. By producing extracellular enzymes, they also have greater capacity to penetrate surfaces such as apple skin. At low temperatures, moulds do grow more slowly’, says Zwietering. Vacuum packaging is therefore the recommended approach, followed by storage in the refrigerator.
Common moulds found on food include Aspergillus, Penicillium and Fusarium, according to Zwietering. Aspergillus can produce the dangerous aflatoxin, but the other two can also secrete toxins. At the same time, Penicillium moulds are popular in blue cheeses, and some can be used industrially to produce, for example, antibiotics. ‘Mouldy products often taste unpleasant and there is a small chance they will make you ill. But we should not be afraid of moulds alone, because they also do a great deal of good’, Zwietering concludes.
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