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All coffee beans of a specific type (genus) have more or less the same amount of caffeine, purely by weighing the caffeine only in each roasted bean....
All coffee beans of a specific type (genus) have more or less the same amount of caffeine, purely by weighing the caffeine only in each roasted bean, no matter how light or dark roasted. But when you measure ground coffee in grams (instead of looking at a single bean), the darker the roast, the more caffeine there is in each gram of ground up coffee. How can that be? It has to do with the processes green coffee goes through when it is roasted.
When you roast coffee, literally thousands of tiny changes are happening inside a coffee bean. Stuff is burning off (carbonizing); other stuff is phase changing (solid, to liquid, to gas). Stuff (mainly gasses) dissipates. And various chemicals inside the bean change into different chemicals. One chemical inside green coffee doesn't change much during the roasting process, and that is the caffeine component. In fact, caffeine is one of the more resistant components inside coffee, changing little during the roasting process. A roasted coffee bean contains nearly the same amount of caffeine as it did when it was a green bean.
However, other things in the coffee bean can change dramatically during the roasting process. It's the reason why a coffee bean will lose between 10 and 25% of its weight during the roasting process, depending on how dark your roast: the darker the roast, the more weight loss per bean. And therein lies the reason why the darker you roast coffee, the more caffeine is in the resulting grounds. Let me explain.
Take two beans that were identical in size and weight when “green” (before roasting). Roast one to City (medium), and roast the other to French (dark). At this point, if you were able to extract 100% of the caffeine inside each individual bean, you'd end up with an nearly equal amount between the City roasted bean and the French roasted bean (the French roast might have .1mg less!). But the French roasted bean can weigh about 10% less than the City bean, because of the increased weight reduction it went through with the darker roast.
So if you want 100 grams of ground coffee for your big pot of brewed goodness, you're going to be using (roughly) 10% more actual beans from the French roast than you would be from the City roast to get the same weight volume. And that means... ta da! (roughly) 10% more caffeine in the French roast 100g sample compared to the 100g City roast sample! It's also important to note that the darker a coffee is roasted, the higher each individual bean's percentage of caffeine is (compared to all the other components in that bean). This is responsible for what I call a side myth: some people believe the darker a coffee is roasted, the more caffeine is created by the roasting process! Roasting doesn't create caffeine. But caffeine sticks around much longer than other things inside the roasting bean, so it's overall percentage in the bean climbs the darker you roast.
Short answer? Dark Roast, gram for gram, has more caffeine than a City (or Medium) roast, because caffeine likes to stick around during the roasting process, when many other things in the bean want to run away.
*source: Illy's Espresso: The Chemistry of Qual