Caffeine

Caffeine is the bitter alkaloid that makes coffee a stimulant. The plant produces it partly as a natural pesticide, which is why beans grown under more stress tend to hold more of it. By dry bean weight, arabica sits around 1.2 to 1.5%, while robusta runs roughly 2.2 to 2.7%, often nearly double. That difference is one reason robusta tastes harsher and is common in cheap blends and some espresso.

How much ends up in your cup depends far more on dose, grind, and brew method than on the bean alone. Caffeine is highly water-soluble and extracts early, so a strong, long brew pulls more than a quick one. A common myth is that dark roast has much more or less caffeine; roasting changes it only slightly, and by mass versus volume the picture flips depending on how you scoop. For the practical per-cup numbers and the roast question, see caffeine-101 and is-dark-roast-stronger. If you want the stimulant gone, look at decaf.