Why does my coffee taste stale?

Stale coffee is the bane of the specialty coffee experience, and it is usually a result of how the coffee is treated after the roasting process. Coffee is highly porous and essentially acts as a sponge for its environment. Once the beans are roasted, they begin to lose their carbon dioxide, which—while initially protective—eventually gives way to the relentless effects of oxygen. Oxidation is the chemical reaction that ruins coffee. It essentially strips away the delicate oils that carry the flavor and aroma. When you smell a bag of fresh coffee, that "bloom" of scent is exactly what you lose as the coffee stales. When these aromatic compounds evaporate or oxidize, you are left with a cup that lacks depth and character—it tastes "flat" or "dull," often with a lingering sensation of dry paper or cardboard. Storage is the most significant factor in staving off staleness. The "trinity of enemies" for coffee freshness is oxygen, heat, and light. If you keep your coffee in a clear container on a sunny countertop, or in a cabinet above the stove, you are accelerating the oxidation process. The best way to store coffee is in an opaque, airtight container in a cool, dark place. While many people turn to the freezer to keep coffee fresh, this can be a double-edged sword. If the coffee isn't stored in a truly airtight container, it will pick up the odors of other foods in the freezer. Furthermore, the constant temperature fluctuations when you take the bag out and put it back in can cause moisture condensation, which effectively "wets" the beans and destroys their quality. Ultimately, freshness is relative. Ground coffee stales much faster than whole beans because the increased surface area allows oxygen to reach every particle simultaneously. If you want the freshest cup possible, buy in smaller quantities that you can consume within two to four weeks of the roast date, and grind your beans only when you are ready to brew. If your coffee tastes consistently flat, it is almost certainly a storage or age issue, rather than a problem with the brewing method itself.