What are signs of coffee oil oxidation?

Coffee is rich in volatile oils, which carry the vast majority of the fragrance and flavor experience we associate with a great cup. Oxidation is the chemical reaction that occurs when these oils are exposed to oxygen. Over time, this transforms the compounds that give coffee its bright, fruity, or chocolatey characteristics into molecules that taste papery, cardboard-like, or even slightly rancid.

One of the most obvious signs that oxidation has taken hold is in the smell. Fresh coffee, even before you grind it, should be aromatic and pungent. Oxidized coffee often has a muted, 'dusty' scent. When you grind it, that fresh, vibrant smell is noticeably absent or replaced by an unpleasant, stale aroma. If you taste the coffee, it will feel one-dimensional. The acidity that once added structure to the cup will taste like flat vinegar, and the inherent sweetness will have vanished.

Darker roasts, which have more oils on the surface, are more susceptible to rapid oxidation than light roasts. If you look at a dark roast bean that has been sitting out, you might see it transition from a glistening oil-covered surface to a matte, dull finish. That is a visual indicator that the surface oils have evaporated or reacted with the air.

In practice, you can fight oxidation by protecting your beans from their primary enemies: air, light, heat, and moisture. Store your coffee in an opaque, airtight container, and only buy as much as you can consume in a few weeks. If your coffee has reached the point where it smells like old books or paper, it is a sign that the oils have degraded. While it is technically safe to drink, the experience will never match what the roaster intended. Don't be afraid to discard coffee that has clearly gone past its peak.