First of all, the variation in % caffeine in a coffee bean based on roasting is negligible. Caffeine is stable up to 235°C. And a dark roast will not make it to this level, if it does the coffee will be completely charred, and if it reaches this temperature it will only be for a very short time. Which makes the % of caffeine approximately the same for a given coffee.
So for a fixed weight, for example 100g, you will need approximately 50 coffee beans and for dark coffee, 70 coffee beans. You will therefore have more caffeine per 100g with black coffee. Where there is a change, it is mainly in the water evaporation which makes the density of the bean different.
If you measure your coffee by volume, like with a spoon, you will need fewer coffee beans to fill your measure, because dark coffee takes up more space and is lighter. So the number of coffee beans will be smaller, so the % of caffeine too.
Generally, customers who contact us about this want to maximize the caffeine in their cup, so I suggest they use a full immersion brewing method ( piston , Clever ) with a long contact time, and use a blend which contains a certain percentage of Arabica and Robusta, the latter comprising approximately 4 times more caffeine.
The roast level does not change the caffeine %, but if the recipe is measured by weight, you will need more coffee beans for the same recipe than you would with brown coffee.
And if you want to discover our strongest coffees, these contain the most caffeine: Réveil Zombie and its colleague the Biologique Zombie
And they live up to their names!
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