Hang up your hats and tie up those high horses, the great caffeine debate can be put to rest.
All roasts are created equal.
That’s right. Under normal circumstances an 8oz cup of light roast has about the same amount of caffeine as the same serving of dark roast.
Ready to dive down the rabbit hole?
First a caffeine primer: Caffeine occurs naturally in more than 60 species of plants, primarily as a defense against insects. It is most commonly found in coffee, tea leaves, kola nuts, and cacao pods (though chocolate professionals are quick to point out the primary stimulant in cacao is theobromine, a remarkably similar but chemically distinct compound).
In coffee, the Canephora species (Robusta) produces roughly twice as much of it as Arabica. The Laurina variety (aka Bourbon Pointu) and the species Eugenioides (a parent of Arabica, with Robusta) are both naturally low in caffeine concentration; neither are widely commercially available due to their low productivity, both can taste sweet, have a naturally low acidity, and possess pointed, football shaped seeds.
Caffeine was first isolated and named by the German chemist Friedlieb Ferdinand Runge, after receiving a gift of coffee (presumably roasted beans from Yemen) from the prominent statesman-poet Johann Wolfgang von Goethe. Identified as an alkaloid – a class of naturally occurring and generally plant-derived, high pH compounds containing nitrogen – in its pure form caffeine is a bitter, white, water-soluble powder. Like most alkaloids (morphine, nicotine, cocaine, quinine, and strychnine, e.g.) it produces profound psychological effects in humans, but unlike most alkaloids its consumption is almost completely unregulated.
Caffeine has a melting point of 238℃/460.4℉, just a few degrees below combustion temperature for coffee. Even the very darkest of roasts don’t spend much time at all above 460℉. So it’s pretty unlikely that there’s a significant difference as a result of exposure to heat in the roaster.
When it comes to roasting and brewing coffee, let’s assume we’re talking about the same green beans – because caffeine content can vary dramatically based on genetics, regions, and processing/storage, well before the coffee is roasted & brewed.
It’s common knowledge that coffee loses more weight the longer and darker you roast. And anyone who’s cleaned the guts of a roaster with bare hands knows, you can get pretty caffeinated just by touching the crystallized dust that forms in the exhaust pipes.
There used to be a notion that there was a large difference between light and dark roast caffeine content – lighter roasts being denser thus retaining a higher percentage of caffeine. However, darker roasts lose more of their total weight during roasting, 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. You can see this easily by weighing 50 grams of a dark and light roast – the pile of dark roast will be bigger.
Brew strength (dose to volume ratio) can affect the caffeine content. Because the percentage of caffeine lost during roasting is pretty minimal, it is possible that brewing might account for some differences in dark/light caffeine content.
Like I said, it takes more dark roast (by volume) to achieve the same dose (by weight) for a brew. As such, volumetric measurements for brewing will skew coffee to water ratios when comparing dark and light roasts. Thus 5 tablespoons of dark roast will be lesser in weight than 5 tablespoons of a light roast, and the resulting brew of the light roast will be stronger both in total dissolved solid count and caffeine content because of the heavier dose.
Still with me? There’s another complication. The amount of caffeine that makes it into an average cup of coffee at a given retail outlet can vary wildly. A recent study showed that a 16 oz cup from a Dunkin Donuts shop had 100 mg less caffeine than the same size cup from a Starbucks café. And the same blend, ordered from the same Starbucks store one day to the next also showed dramatic changes: one cup measured 250 mg of caffeine one morning, another cup yielded over 500 mg the following day. The same coffee, presumably the same brew method. Yikes!
So, it’s a little complicated. But at the end of the day, it’s safe to say that (all other things being equal – green coffee specs, brew dose & method, etc.) a light roast and a dark roast will not significantly alter the amount of caffeine in a cup.