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You know that horrible, sour smell that comes from expired milk? Yeah.

You only need to smell it once to make sure it never happens again. Luckily, many food manufacturers put some kind of Sell By, Best By, or Use By date on their products to tell you... well, actually, they don’t tell you much. These labels are mainly for quality and flavor purposes, not safety.

And a lot of common foods are edible past what it says on the label. This can make it difficult to figure out what you can eat and what needs to go in the compost bin. But understanding what’s happening chemically when foods go bad, both before and after those dates pass, can help you out.

A product’s real expiration date is tricky to predict, because it can be affected by everything from the sanitation of the factory to the temperature of your fridge. But a lot of it depends on what kind of food you’re buying. Produce, for instance, can become unsafe anywhere from hours to weeks after you buy it; a huge variation most labels don’t account for.

Most of this has to do with when it gets moldy, and not about things like brown spots. Although those don’t look pretty, they’re harmless. Mold, meanwhile, can actually hurt you.

Some strains produce substances called mycotoxins, which can cause everything from skin infections to internal bleeding and death. And because produce is such a water- and nutrient-rich home for mold, it can start growing as soon as 24 hours after you bring your produce home. And it grows even faster in warm temperatures, like on your sunny kitchen counter-top.

Extending the shelf life of your produce is pretty easy, though: Just throw it in the fridge. Chemical reactions happen more slowly at cold temperatures, so it takes longer for mold to grow and reproduce in there. But it will grow eventually.

And when it does, whether it’s after a day or a few weeks, it’s time for the compost bin. Dropping the temperature might be a great trick for a lot of food preservation, but if you want to really push back the expiration date, you have to kill bacteria before letting them reproduce. That’s not easy with produce, but it’s exactly how we treat milk.

According to the label, milk is usually at its best for around two weeks after you buy it. But depending on the kind you buy and how you treat it, it could really expire a few days later, or earlier. Traditional milk goes through the process of pasteurization, a heat treatment process that denatures enzymes in living bacteria and renders them harmless.

And it does a good job, but it’s not perfect. Some of those bacteria survive pasteurization, and over time, they convert the sugars in milk into lactic acid. That’s what gives spoiled milk that sour smell.

Normally, this process takes a couple of weeks and, typically, it’s still drinkable for around five days after the Sell-By date. But if the milk gets exposed to slightly warmer temperatures somewhere along the shipping route, or if you run your refrigerator a few degrees warmer, it can spoil faster. And while you’re probably not going to catch anything life threatening from drinking spoiled milk, you can catch some brutal food poisoning.

The good news is, it’s easy to tell if milk’s gone bad. Just give it a whiff, and take in that lovely lactic acid smell. Finally, while heat treatments are awesome for getting rid of bacteria in milk, you’ll need a different strategy for meat.

Typically, the Best or Use By dates on these products is for a week or so past purchase. But while it’s often safe past then, it doesn’t last for that much longer. That’s because meat is a prime breeding ground for bacteria.

It’s a wet, nutrient-dense home with all kinds of fuel sources. For a few days, these bacteria won’t cause you any trouble because there aren’t that many of them. But over time, they’ll multiply.

And that’s when you get problems. Bacteria, like Psuedomonas or Lactobacilli, feast on the meat’s surface, where there are free floating sugar molecules and light weight compounds to nom on. Some of these bacteria can make you sick, but they do at least announce their presence.

As they break those compounds down, they leave behind some nasty smelling byproducts. These have all kinds of shapes and compositions, but they all fall into a category called volatile organic compounds, or VOCs. Normally, these VOCS smell like rotten meat.

And because you find them when bacteria have been chowing down and multiplying, there’s a pretty strong sign your meat has gone bad. Besides the smell, you can also probably tell your meat has expired based on how it looks. For example, VOCs like hydrogen sulfide can convert the natural muscle pigment in meat into something that looks more green.

Typically, all this happens a few days after the Use By date on that sticker. But if you notice any of those funky smells or a little slime on your beef… it’s time for it to go, no matter when it happens. Food science can be tricky, and those ambiguous labels don’t always help.

Knowing a little bit about how and why food expires can help you keep your fridge full for longer. But if you’re really uncertain… it’s okay to play it safe. Thanks for watching this episode of SciShow!

Scishow is a production of Complexly, which also produces channels like The Financial. Diet where Chelsea, Erin, and Lauren dive into personal finance topics with no fear. If you want to learn other ways to help keep your fridge fuller longer, and on a budget, check out their video on Rules for Mastering The Grocery Store.

From the emotional side of money to technical financial terms, you'll learn tricks to improve your life by getting a better handle on money. Check out new videos every Tuesday and Thursday on The Financial Diet. [♪ OUTRO].