This question comes up so often, that I feel it needs a more lengthy response than my typical “If you judge by volume, light roast, if you judge by weight, dark roast”. Even this response requires an explanation, so let’s dive in!
Caffeine is one of many compounds found in green coffee, as well as many other species of plant around the world, including tea and cocoa. There is certainly more to coffee than just it’s effective caffeine conveyance, but this is what people tend to focus on because we’re all taught that “coffee wakes you up in the morning”. The amount of caffeine in green coffee can vary depending on the variety of coffee plant and where it is grown. Regarding variety, it’s more than Arabica versus Robusta, though we can say in general terms that Robusta has about twice the caffeine of Arabica at around 2.2% compared to Arabica’s average 1.2%. But even within the Arabica classification, there are a number of distinctive varieties, some that naturally evolved, and some hybrids created through a conscious effort to improve disease resistance and better yields. In terms of “where it’s grown”, caffeine is a natural insecticide, disturbing the behavior and growth of numerous insects and their larvae. Coffee trees growing in more hospitable environments will have a greater need for protection against insects, so it follows reason that the same coffee plants will tend to have higher caffeine levels. The higher in altitude a coffee is grown, the less caffeine it will tend to have. Okay, with these basics covered, let’s move on to the roasting!
When you roast coffee, there is a cascade of chemical reactions that take place, by some reports, as many as 1,500 or so different reactions. This happens because more complex molecules are broken down into simpler molecules, which can then combine to create new compounds. One reason all of these changes happen is because each coffee bean is like a tiny pressure cooker. The moisture, lipids, carbohydrates, proteins and enzymes inside the bean are trapped in a confined space. This space, by the way, is like a cellulose sponge with many small compartments where these chemical reactions take place. Eventually, the moisture inside the coffee expands so much that it is explosively released by fracturing the cellulose structure of the coffee bean and escaping into the open air. This stage of roasting is known as “first crack” and it’s where the coffee really starts to develop its deliciousness. A coffee roasted up to first crack but no further will taste grassy and vegetal, and will be thin tasting because the flavor won’t extract very well. This coffee will be more like an herbal tea, and there are people who actually enjoy this, but I digress.
From first crack and beyond, chemical reactions continue to take place, and the caffeine is just hanging out, watching all of this activity around it. The sublimation point of caffeine is around 347° F, and first crack occurs at a point somewhat above that, so you might think that the caffeine gets boiled off along with the moisture, but this isn’t the case because we’re dealing with caffeine inside a pressurized system, not at atmospheric conditions. Just like your gasoline-fueled automotive engine, the boiling point of coolant is much higher because it is pressurized. In reality, very little caffeine escapes during the roasting process. A little does, yes. But it happens for both light roasted and dark roasted coffee, with the difference between the two being inconsequential. In fact, other factors, such as your brewing method, fineness of grind, temperature of water, and length of contact time with water has more influence over the amount of caffeine in your cup than the degree of roast.
So why do we say that light roasted coffee has more caffeine if you measure by scoop, and dark roasted coffee has more caffeine if you measure by weight? This has to do with another thing that happens during the coffee roasting process. As moisture is driven off coffee beans expand, growing more as coffee is roasted darker. So while a small, light roasted coffee bean has 1.2% caffeine, a big dark roasted coffee bean also has 1.2% caffeine. What’s different is the size of the coffee bean. Light roasted beans take up less space, so if you are measuring out your coffee by the scoop, each scoop will have more coffee beans in it, thus more caffeine. Conversely, if you measure by weight, it takes more dark roast beans to reach a weight of 20 grams than light roast beans, so you will have more coffee beans, thus more caffeine, when you measure by weight.
The amount of caffeine in a coffee continues to be debated, and perhaps we will never all agree on a single answer, but my hope is that this explanation will at least give you some perspective. When people ask me which has more caffeine, I tend to say “It really doesn’t make that much difference. If you want more caffeine, just enjoy a few more sips of delicious coffee!”.