You're in a building. Any kind of building, really -- an office space, the campus cafeteria, a waiting room, etc..
You see an old metal coffee dispenser with a pump, next to some hastily stacked packets of 'sweeteners' & dehydrated 'cream' that has been there since the 18th Century (well, we might be exaggerating here).
A sign that says "Free coffee" -- No one's looking, so you go on ahead and have yourself an ungodly slurp.
Internally, you think to yourself "Holy *$%!, what on Earth did I just put in my body?!"
(Don't lie. This has happened to you at least once!)
You think "Yeah, never doing that again!" -- But you might. You don't know when or where, but this is an all too-common consequence of hot brewed coffee.
The point being, is that often times people are drawn to coffee in public settings because it's complimentary, not because it's good!
But the DRINKING it part? That is not complimentary. That's torture. No one should have to endure... "The Sludge".
"The Sludge" happens when hot brewed coffee has sat. . . For a while. Even at home. After a batch of hot brewed coffee has sat in the carafe for a few hours (maybe even a day or two), you'll notice that what sits at the bottom is hardly coffee.
Business Insider also shares some insights as to how the degradation of coffee works.
"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 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."
What is it that makes hot brewed coffee eventually turn stale?
Acid + Oxygen.
Coffee that's brewed hot releases a lot of various acids into the drink. The acids are the cause for both the bitter flavors, as well as its poor ability to 'keep'. Want to avoid that? Want to brew coffee that you can actually save? Cold brewing coffee's final results are remarkably less acidic, and last way longer in the fridge (2 weeks).
Coffee Brew Guides actually makes a special note of how oxygen plays a roll here -- "Oxidation (decay via oxygen) also affects your coffee once it’s brewed. "
The Cure To The Sludge
You've probably guessed it by now -- Cold brewing coffee is the strongest combatant to "The Sludge". Due to its inherently low levels of acid, it slows down the deterioration oxygen can set up -- so it's a no-brainer!
Cold brewing lasts longer AND tastes better.