Good news for fellow Parmesan lovers ― if you’re worried about the green fuzz on hard cheese, it’s usually safe to cut off the part affected by mould and use the rest of the block. 

Issues only arise with softer cheese, like brie and cream cheese, as their higher water content means the spores spread further. 

But what about hard-boiled eggs which have formed that copper-green ring around their yolks? Surely green eggs are a never-ever-eat situation?

What causes the green ring around the yolks of hard-boiled eggs?

According to the University of Minnesota, the colour change happens because the egg white contains a lot of hydrogen, while the yolks have sulphur (which some may associate with that unfortunate “eggy” smell). 

These combine together when boiled, and if the egg is boiled at too high a heat for too long, the hydrogen-sulphur interaction (which creates ferrous sulfide) becomes visible in the form of a greenish ring. 

Sometimes, a higher level of iron in the water which the egg boils in can lead to discolouration too. 

Sharing her expertise on the University of Illinois Urbana-Champaign’s site, food scientist Dr Stephanie Smith said that not only are they safe to eat, but they’re sort of fortified as well. 

“The ferrous sulfide that’s formed when the egg is overcooked is actually the same form of iron you find in iron supplements,” she told the college. 

“From a food safety perspective, it’s actually safer to eat overcooked eggs rather than undercooked eggs, which can harbour bacteria.”

How can I prevent the green tint and chalky yolk?

Though the egg might technically be a little safer, most boiled eggs are good to eat.

So if you hate the disconcerting hue and the chalky yolk and rubbery whites which usually come alongside it, you may be wondering how to make sure you don’t have to face it of a weekday morning. 

The University of Illinois Urbana-Champaign says that putting eggs in cold water, bringing them to a boil, and then covering them and letting them stand for at least 12 minutes can banish the risk of green eggs.

You should also let your hard-boiled eggs sit in an ice bath after cooking to stop the heating process (I’ve tried and loved that trick before).