Hank Green: You know when you got, like, three quarters of a loaf left and then you see that little blue green fuzzy spot and you're like "Ahhh, should I, should I not eat", but, you know, you probably shouldn't eat it, but why? And why can I eat this (blue cheese) but not this (moldy bread)? It's a moldy world out there, especially in the kitchen, and we have answers for you.

(Intro)

Food mold, like all molds, are microscopic fungi. They grow from tiny spores that float around in the air all the time and they're all round you at this very moment. Seriously. Mold spores are between 3 and 40 microns long. Your hair is about 125 microns wide -- so tiny. When conditions are right (there's enough moisture, warmth, and nutrients), the spores will set up shop. Unlucky for us, mold prefers the same kinds of temperatures that we prefer, and even the coolness of a refrigerator won't prevent mold from forming eventually.

Let's say our mold has found a delicious peach on which to grow. In its early stages, the spore releases root threads of the mold fungus deep into the fruit. By the time you see the first signs of mold, those threads, called mycelia, have already penetrated the inner depths of that peach. These roots are difficult, if not impossible, to see.

The signs of mold, whether it be weird fur, green dots, or white dust, are a result of the stalk of the fungi rising above or sitting on the surface of our now not-so-delicious peach. The spores that form at the end of the stalk are what give mold its color.

Mold is an efficient organism, growing quickly as enzymes released by the mycelium break down whatever organic matter it has invaded. Unlike other fungi, mold digests its food first and then eats it, allowing it to grow at a faster pace.

Now, you may have heard that mold isn't dangerous if you just cut away the ugly parts and eat the rest of the food. This is generally true with harder foods like apples, potatoes, onions, and hard cheeses like cheddar and Swiss, where the mycelia can't quickly penetrate their host, but I would suggest not trying to cut or scrape away the mold off of soft cheeses, berries, meats, and other produce. You may very well become ill if you eat that kind of thing.

The reason is mycotoxins, poisonous chemical compounds produced by several kinds of mold. Mycotoxins are produced around the mycelium, and not only can they survive a really long time but most aren't even killed when the food it has invaded is processed or cooked. The molds that produce mycotoxins are mostly found in grains and nuts, but have been known to invade celery and other produce as well.

One of the most dangerous types of mycotoxin is called aflatoxin, which is produced by two kinds of mold. This naturally-occurring poison, which has been known to cause cancer, is typically found in field corn, wheat, oil seeds, and peanuts. In fact, many scientists believe that those dangerous peanut allergies we're always hearing about are the result of a reaction to aflatoxin, not the peanut itself.

Other food-born mold may cause less severe allergic reactions -- rashes or nasty infections. Of course, some molds don't produce mycotoxins and are totally safe to eat, notably the ones you see in some smelly cheese like blue cheeses and Gorgonzola and Stilton, eurgh. These are actually created by the introduction of specific mold spores. One of these, Penicillium roqueforti, comes from the same genus as some guy used to make the group of antibiotics known as penicillin. The mold in these cheeses breaks down complex organic molecules into simpler ones, which smooths out the fibrous structure of the cheese and also results in their unique flavor and smell.

But moldy cheeses are not immune to other molds, so be careful next time you're about to dig into that six month old block of Roquefort.

So, let that be a lesson to you, and thank you for watching this episode of SciShow. If you have any questions, comments, or suggestions, you can find us on Facebook and Twitter and down in the comments below, and if you wanna keep getting smarter with us here at SciShow you can go to youtube.com/scishow and subscribe.