All other things being equal, a light roast is going to have more caffeine in it than a dark roast.
That’s because the process of roasting beans causes some of the caffeine to escape.
Darker roasts take longer and use higher temperatures than light roasts. Both factors can cause caffeine loss.
The difference isn’t super large, though. One study comparing unroasted and severely roasted beans showed only a 5.4% difference in caffeine content.
The grind, the method of brewing, and the environment where the beans were grown all affect caffeine levels too.
In terms of things we have control over, the real issue is taste.
Unroasted coffee beans taste really nasty.
Coffee “beans” are really just the seeds of the Coffea arabica plant, found in its fruit. The problem is, those seeds will make a terrible, terrible-tasting cup of joe. That’s the reason for roasting our beans in the first place.
Roasting the beans allows them to undergo a complex set of reactions that produce brown colors and a multitude of flavor-bearing chemicals.
The browning color does not come, as it does in many other instances, from oxidation (like the browning of an apple, for instance). Instead, it comes from something called a Maillard reaction. Maillard reactions are a broad group of reactions that form from the combination of sugars and amino acids with heat.
The reactions produce hundreds of chemicals that then break down into many other complex chemicals, many of which contain distinctive flavors.
Bottom line: Don’t worry too much about the caffeine content of the roast you choose. The difference in caffeine will be negligible compared to the difference you experience in taste.
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