Here's why coffee gets stale if it sits out for too long

Here's a mistake I'm embarrassed to admit; and one that I make far too often. Immediately after waking up, I get very excited for coffee so I brew a giant batch.

But then I realize halfway through that I can't finish it.

Not wanting to waste a jug of perfectly good brew, I leave it on the counter for safe keeping so I can heat it up and drink it later.

But: It's always a bad decision, but I keep doing it anyways even though it tastes disgusting every time.

Turns out that there's actually a scientific explanation for why this happens. And there are certain things you can do to avoid - or at least delay - your coffee from going stale.

Coffee grounds are chock full of various oils, chemical compounds, and acids. These compounds, referred to collectively as "solubles," give coffee its flavor. They're extracted from the grounds in the brewing process and give coffee its quintessential "coffee" taste and smell.

But once those beans touch air, the oxygen begins to zap their flavor and make them smell different almost immediately by causing coffee solubles to either degrade and oxidize - kind of like how iron becomes rusty when it's exposed to oxygen for too long - or because they drift away into the air.

This is why roasters package their beans in vacuum-sealed containers, so the beans are no longer in contact with oxygen from the air.

One of the most susceptible yet sweet-smelling solubles is a sulfur compound called methanethiol. It plays a key role in making coffee beans smell fresh, as well as masking less desirable aromas, such as the unpleasantly bitter smell that people associate with raw green peas. But then coffee continues to go stale when you mix coffee grounds with water. Coffee solubles dissolve best between 195 to 205 degrees Fahrenheit, so coffee brewed with hot water has a more full-bodied, flavorful taste profile than coffee brewed with room-temperature or cold water, which is also called cold brew.

But as the boiling water pulls out the solubles from the grounds, they continue to oxidize yet again, giving hot coffee more of a sour and bitter taste.

This process begins to happen the moment any water hits the beans, and it gets more intense the longer the coffee sits after you brew it. You can even notice the change in taste just an hour after you brew the coffee. This is why fancy coffee houses brew small batches of coffee to order, rather than reserving large jugs of it diner-style.

The easy way to prevent a stale-tasting brew is by locking in the freshness of your beans before you brew them by storing them in an air-tight container. And after you brew, transfer your coffee to an air-tight thermos if you want to maximize freshness for longer.

But as we all know, we can't stop the damaging effects of time. And eventually, we'll all be left with a cold pot of bitter, tasteless sludge - a good point to keep in your memory bank the next time you're itching to save a cup for later.