Let’s dispel the most common myth right off the bat: A dark-roasted bean contains more caffeine than a light-roasted bean due to its stronger flavor. Not true. Actually, the caffeine content in both is virtually the same. An opposing view held by many is that the darker the roast level, the lower a bean’s caffeine since much of it is lost or "burned off" during roasting. Yet caffeine changes very little during a roast. Any significant variation would require a roasting temperature above 600° F. Since temperatures rarely exceed 470° F, a bean’s caffeine content remains relatively static across all roast levels.
But wait. Even though a bean’s caffeine content changes little during roasting, a bean’s caffeine per volume and per weight is altered considerably—not because the caffeine changes but because the size and weight of the beans change. The longer a bean is held in the roaster, the darker in color, lighter in weight, and larger in size it becomes.
So when do differences in caffeine content come into play? This happens when roasted coffee is measured for brewing or packaging. Since a bean loses weight (mainly water) during roasting, its caffeine content by weight increases while its caffeine content by volume decreases. Confused yet? Let’s put this principle into practice by measuring some coffee.
Dark-roast coffees measured by volume with a scoop actually contain fewer coffee beans due to their larger size, resulting in a weaker brew and less caffeine per cup than a light-roast coffee measured in the same manner. Bottom line, you're not getting the most from a dark-roast coffee if you measure it by volume.
So, does a dark-roast or a light-roast coffee have more caffeine? A 12 oz. brewed cup of dark-roasted Arabica coffee will contain more caffeine if it has been weighed prior to brewing as opposed to a lighter roasted Arabica coffee taken to the same weight. Yet, it all depends on how you compare coffees—by bean, volume, weight, or coffee varietal.
On the other hand, dark-roasted coffees measured by weight require more coffee beans for brewing since each bean weighs less than a coffee bean that has been roasted lighter, resulting in a full-flavored brew and more caffeine per cup than a light roast. Measuring coffee by weight is the method adopted by many folks devoted to their joe and strictly adhered to by any reputable coffeehouse. Not convinced? Prove this to yourself by weighing 50 grams each of a dark roast and a light roast. You'll find that the pile of dark roast will be larger since it has lost more water during roasting than the light roast--but the dark roast has not lost its caffeine. So, it takes a higher bean count (volume) of dark roast to equal the same weight as a light roast when you’re dosing to brew coffee.
COMPARING CAFFEINE BY COFFEE VARIETAL
One more caffeine comparison ought to be mentioned: Robusta coffee versus Arabica coffee. Robusta—a harsh-tasting, inexpensive coffee variety (or varietal)—contains nearly twice the caffeine of Arabica. A less expensive, supermarket-type brand offering a blend of both represents more caffeine per cup than a 100% Arabica coffee when both are identically prepared and brewed. What's more, if that supermarket blend happens to be roasted dark and its coffee grounds are measured by weight prior to brewing, a brewed cup will deliver the biggest caffeine dose of all.
Note: This blog post, "Which Has More Caffeine: Light or Dark Roast?" was written and posted by Scribblers Coffee on April 6, 2011. We reposted this blog on our new website in 2017.