From the archives
Question:
I can never tell when feta cheese is growing mould because it is so white. Is it safe to keep it fully submerged in a jar of olive oil in the refrigerator?
Answer:
Mould growth on cheese and other fermented dairy products is a common and particularly unwelcome problem, given the price of many of them. Moreover, certain moulds and mould toxins cause significant health problems, so it’s highly advisable to avoid their formation and to avoid eating any foods showing mould.
Moulds are microscopic fungi that can grow on our food or any plant or animal matter. Most moulds are made of threadlike filaments, often visible to the human eye. Fungi also produce spores, which can be transported by air, water or insects, allowing them to spread.
They are a health issue because some moulds can cause allergic reactions and respiratory problems. In addition, some can produce mycotoxins, poisonous substances that make us sick.
And there is more to mould than meets the eye, because the threadlike structures of fungi behave like branches and roots, potentially growing deep into your food. So, although you may see only a tiny circle of mould on the surface, the threads may have carried poisonous substances much further inside. Therefore, cutting off the visible mould does not necessarily remove the infestation and inherent health risks.
The human body has a very competent system for destroying unwanted micro-organisms and removing toxins. First, our stomach’s acidic environment destroys many unwanted invaders, and then bile in our small intestine, which has a detergent-like effect, is squirted all over those that get that far. Toxins produced by fungi are detoxified by cells lining our gut – and if any sneak through these cells intact, the liver usually deals to them.
Still, if sufficient doses of toxins are consumed, they may overwhelm this highly efficient system and cause serious problems. Some mycotoxins produced by fungi are particularly nasty and can cause nausea, vomiting and, in exceptional cases, cancer and death.
However, not all fungi or moulds make us sick – some are used to produce antibiotics, and others in producing foods such as camembert and blue cheeses. These have been grown in controlled environments to ensure they do not produce harmful toxins, or, in the case of antibiotics, so their beneficial effects outweigh their toxic ones. So, a tasty bit of blue cheese is quite a different proposition from a piece of mouldy bread containing harmful mycotoxins.
Aflatoxin is one example of a toxic substance produced by a common mould called Aspergillus flavus and a related species, Aspergillus parasiticus. The International Agency for Research on Cancer classifies aflatoxins as a group 1 human carcinogen, placing them in the same cancer-causing category as alcohol and tobacco. There is no safe aflatoxin intake level, so every effort is made to minimise the contamination of human and animal food supplies.
Researchers have found a small proportion of moulds that grew on cheeses stored in home refrigerators were of the genus Aspergillus, and a subset of these was capable of producing aflatoxins. It is difficult to estimate, but previous studies have found anywhere between 0.3-8% of moulds on cheeses were capable of producing aflatoxins.
So soft cheeses such as feta that have visible mould are best discarded, as they can be significantly contaminated below the surface. In contrast, harder cheeses may be salvageable by cutting off the mould and at least another 2cm of cheese below and around the mould.
In the case of feta, yes, you can store it in oil in the refrigerator. But a more cost-effective way to reduce mould formation is to store it in a homemade brine solution. Feta is a pickled cheese, so storage in brine is typically part of the production process. You can make your own 5% brine solution by mixing 50g of salt with 1 litre of water, then safely submerge your feta in the solution (minus the packaging) to gain a longer mould-free storage time.
This article first appeared in the New Zealand Listener’s November 19, 2022 edition.
