Stale coffee is what you get when roasted beans (or grounds) have been oxidized from exposure to oxygen; the more intense the exposure, the more quickly oxidized and stale your coffee will become. Other factors such as heat and moisture can also make oxidation occur. Roasters suggest that you brew your coffee beans as soon as possible after roasting so that you can get the freshest, and best out of your coffee.

Why Does Coffee Go Stale?

Coffee grounds are 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 unique"coffee" taste and smell. But once those beans touch air, the oxygen begins to exstract their flavor and make them smell different almost immediately by causing coffee solubles to either degrade and oxidize — similar too how iron becomes rusty when it's exposed to oxygen for too long This is why roasters package their beans in vacuum-sealed containers, so the beans are no longer in contact with oxygen from the air.

Tell if your coffee beans are freshly roasted

Look for a glossy appearance

When coffee beans are roasted, the intense heat evaporates moisture out of the heart of the bean and simultaneously draws out the oil-like substances, which then coat the outside of the bean.  

Check for residue

Pick up a handful of coffee beans if they leave a residue on your hands that means they are oily, and have been freshly roasted. Lighter roasts aren't as oily, while a darker roast will have more residue.  

Check for a valve in the sealed bag

When beans are roasted under high heat and then cooled, they release a ton of carbon dioxide (CO₂). This release of gas can last anywhere from a few days to several weeks after roasting, known as the degassing period. If your sealed bag does not have one of these valves, that likely means that your coffee beans aren't actively giving off CO₂ — and are not likely to be fresh.   If buying in bulk, pop them in a resealable bag and see what happens. Put a half-cup of beans into a resealable plastic bag, press out the air, and let them sit on the kitchen counter over night. If they've been freshly roasted within the past seven to 10 days, the bag will puff up from an outgassing of carbon dioxide. If they're not so fresh, the bag will remain flat.

Keep those beans fresh

Whole coffee beans typically start losing freshness after three weeks, and ground coffee can begin to go stale within an hour of grinding. You can decelerate the rate at which they go stale by keeping them away from excessive air, moisture, heat and light. Transfer the beans or grounds to an airtight container; avoid plastic or metal containers, which can alter the flavor of the coffee. Store the container in a dark, dry spot, such as the bottom shelf of a pantry or a rarely used cupboard. For large quantities of beans or grounds that you won't immediately use, separate them into small portions, wrap them in airtight packages and place them in the freezer for up to a month.

Grinding fresh with a quality coffee grinder is really important for making better coffee.