Why Old Coffee Can Smell Like Fish

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Coffee can develop a surprisingly fishy smell as it gets older, and the culprit can be its own oils. Roasting brings oils to the surface of coffee beans, with darker roasts generally showing more visible oil. Over time, exposure to oxygen can change those oils and other coffee compounds, creating stale or rancid odors. This is one reason older dark roasted coffee can develop unpleasant smells that were not present when the beans were fresh. Research on stored coffee also links oxidation with changes in aroma and a decline in sensory quality. The grinder can make the problem more noticeable because coffee oils and residue can build up on burrs and other surfaces, leaving old material behind even when newer beans are being ground. The smell is not inevitable, and many people never notice a fishy note in coffee at all. Freshness, roast level, storage, and equipment cleanliness can all affect how coffee smells and tastes.