How does roasting affect caffeine content?

The persistent myth that dark roasts contain less caffeine than light roasts stems from a misunderstanding of bean physics rather than chemistry. Caffeine is remarkably heat-stable; its sublimation point is approximately 352°F (178°C), but within the porous structure of a coffee bean, it remains largely intact throughout the roasting process. The real shift occurs in the bean's physical properties. As a producer roasts a bean, it loses moisture and organic mass, causing it to expand and become more porous. If you measure coffee by volume (e.g., a scoop), a dark roast will have fewer beans than a light roast because the dark beans are larger and less dense, leading to a lower caffeine concentration in the cup. Conversely, if you measure by weight, the dark roast may actually yield a higher caffeine concentration because there is more coffee mass per gram. Recent research indicates that caffeine levels often peak in medium roasts—where mass loss is roughly 14–16%—before slightly decreasing due to increased porosity and sublimation at higher temperatures. For the farmer and producer, this highlights that the 'strength' of a coffee is less about the roast degree and more about the extraction efficiency. The increased porosity of darker roasts makes them easier to extract, but the lower overall density often results in a net loss of caffeine per serving compared to a denser, light-roasted bean. Ultimately, the roast profile is a tool for flavor development, not a reliable dial for caffeine content.