All brewed coffee goes bad eventually, even if you refrigerate it. Depending on the method of brewing, your coffee could last as long as two weeks or less than one day. This is because of the different chemical reactions that take place when water extracts flavors from your coffee grounds, and in the end, it all comes down to a single deciding factor: heat.
Types of Brew
There are two ways to brew coffee: hot and cold. Hot brewing is what happens when you use an automatic drip coffee maker -- hot water quickly extracts flavors from your coffee grounds. Cold brewing does the same thing, but as the name implies, it does so using cold water. Consequently, the process takes significantly longer. Cold brewing prevents a specific chemical reaction from taking place during the brew, though, which has a significant effect on how long the brewed coffee lasts in the refrigerator.
Hot-brewed coffee is faster than cold brewing, but it also dissolves more of the coffee grounds' acidic oils. In fact, hot-brewed coffee is typically about 67 percent more acidic than cold-brewed coffee. Additionally, hot-brewing induces a chemical reaction in the liquid because of a compound called 2-furfurythiiol, which is potent when the coffee is hot, but wears off as the liquid cools. This is why coffee that was hot-brewed and then cools down has a stale taste. Even though it's safe to drink, the taste is not as appealing.
Because cold-brewed coffee doesn't undergo fluctuating temperatures like hot-brewed, the chemicals in the liquid don't degrade the same way, and the coffee consequently retains its quality much longer. Cold brewing is a process of steeping coffee grounds in cool water for an extended period of time -- usually about 12 hours, and usually in a higher concentration than in a hot-brew. This allows you to create a highly potent concentrate.
Storing the Coffee
Cold-brewing coffee leaves you with a concentrated liquid, so you can store it in the refrigerator and dilute it with water whenever you want to make a cup of iced coffee. The amount of water that you use depends on the strength of the concentrate, which is determined by both the beans and the portions of the specific recipe you followed. Keep the concentrate in a sealed airtight container, and it will last for about two weeks. After that, you should dump the remains and cold-brew a new batch.
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