Why do some coffee beans have more caffeine?

Caffeine is a secondary plant metabolite—specifically an alkaloid—that functions as a natural insecticide. For the coffee producer, this means the plant’s genetic makeup is the primary determinant of caffeine density. Robusta varieties, which are often grown at lower altitudes where pest pressure is higher, have evolved to produce significantly higher concentrations of caffeine (roughly 2–4% by dry weight) compared to Arabica (1–2%). This is a survival mechanism; the bitter alkaloid discourages herbivory and can even inhibit the growth of competing plants in the soil surrounding the coffee tree.

Common misconceptions often center on the roasting process. Many consumers believe darker roasts contain less caffeine because the beans are 'burnt,' but the reality is more nuanced. While roasting does cause minor caffeine degradation, the primary change is physical: beans lose moisture and mass during the roast. Because caffeine is relatively stable under standard roasting temperatures, a scoop of dark roast beans may actually contain slightly more caffeine by volume than a scoop of light roast, simply because the beans are less dense. Ultimately, the farmer provides the genetic and environmental baseline, but the brewer controls the final concentration through grind size, water temperature, and extraction time. The idea that 'strong' flavor equates to high caffeine is a marketing myth; flavor intensity is a product of Maillard reactions and caramelization, while caffeine is a stable, bitter compound largely independent of those sensory profiles.