What is oxidation in coffee?

Oxidation is the chemical degradation of aromatic compounds and flavour-active molecules when they're exposed to oxygen. It's one of the primary mechanisms by which both green and roasted coffee lose freshness and quality over time.

In green coffee, oxidation is a slow process that occurs alongside moisture exchange as beans age in storage. The cellular structure gradually breaks down, losing volatile aromatic compounds and undergoing chemical changes that produce a past-crop character - flat, papery, or woody - in the cup.

In roasted coffee, oxidation proceeds much faster. As CO₂ degassing slows down after roasting, oxygen penetrates and reacts with lipids and aromatic compounds, causing the coffee to go stale. Proper packaging - valve bags, sealed containers - and cool, dark storage significantly slow the process, which is why both packaging choice and storage conditions matter for preserving quality.