It's annoying, or even disastrous when your food spoils, so one can imagine the reaction of the first person to discover their cheese went mouldy. The story was much the same when someone found their grape juice or barley had gone off, possibly with bubbles. Or their dough had been infected, making fluffy bread. If they were hungry or thirsty enough, they consumed it anyway and thus was born blue cheese, wine, beer and bread.
The process of making cheese begins when a starter culture such as Streptococcus lactis is added to pasteurised milk. This makes it acidic, causing it to coagulate from liquid into solid.
A few steps later, the cheese is inoculated with Penicillium roqueforti and Brevibacterium linens bacteria, which gives the cheese its distinctive flavour. It's then left in a cool, humid environment to ferment for 60-90 days.
Some cheeses develop a very strong, possibly unpleasant smell caused by ketones, which is a class of organic compounds including acetone. It's not surprising that these cheeses can small bad because the bacteria Brevibacterium linens is the same one responsible for foot and body odour.
High ketone levels can cause halitosis, largely due to acetone, which is also found in nail polish and urine. Bad breath occurs when you don't have enough insulin in your body to turn glucose into energy, so your body uses fat instead.
Of course flavour had many components, of which ketones are only a part. There are, for example, sulphur compounds which are predominant in varieties such as cheddars. Sulphur compounds are also responsible for the smell of rotting garbage, but thankfully most cheeses are better than that.
We might ask why some chemicals evoke responses in us because after all, they are just chemicals. An obvious part of that answer is we've learned to associate bad smells with things that are bad for us. Unlike chickens, we humans don't tolerate rotting food very well so we've evolved to associate certain smells with ill health.
Curiously, researchers have found that heavier, more spread-out molecules tend to smell worse than lighter, more compact molecules.
Response by: Rod Taylor
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