Coffee, as delicious as it is, has one major selling point: caffeine. For daily drinkers, a cup of coffee is as fundamental to our morning routine as, say, putting on pants or brushing our teeth. There might even be a few of us who would skip one of those other steps before forgoing our a.m. cup. And, after a night of Netflix bingeing or happy hours that turn into bar hopping, it can seem urgent to consume caffeine as quickly as possible. But is there a reliable way to figure out how much caffeine is actually in your various coffee drinks? We spoke with Christopher Hendon, an assistant professor of chemistry who has literally written the book (or, a book) on the science of coffee.
But First, Science
Caffeine, Hendon explains, is actually a poison. Plants produce it as a defense mechanism against bugs. In tea, it’s in the leaves to protect the plant’s energy source (cue up those photosynthesis memories from elementary school here). In coffee, the caffeine is in the seed (which we call the coffee bean), another vital part of the plant since it’s the reproductive organ. For insects, caffeine is toxic, but in the levels humans ingest it, it’s a mild stimulant.
The amount of caffeine in a bean does not change during the roasting process, though our ability to access the caffeine in the seeds might change through roasting and grinding. Just how much caffeine you are drinking, however, has much less to do with that and more to do with two other factors: water and time.
Slow And Steady
Besides being a toxin, in the right quantities, caffeine is also not particularly soluble in water. The biggest factor in determining how much caffeine is in any particular cup is how long the water is exposed to the grounds, says Hendon. Espresso, made from water forced quickly through finely-ground coffee, would typically have the least. Drip coffee, which extracts caffeine and other coffee flavors with gravity, has more, then followed by French press and finally by cold brew. The amount of water also has an effect — if there’s just a little bit of coffee and a lot of water, the drink will be weak and have less caffeine.
Temperature does not impact the amount of caffeine that is extracted, though steeping coffee too long with hot water will start to over-extract other flavors and make the coffee ultimately taste sour, as another coffee expert explained in our brewing guide. Cold brew allows coffee to extract caffeine over a long period of time (as long as 48 hours), and the room-temperature water allows the beans to give off a different flavor profile that’s not as sour as hot coffee brewed too long. Cold-brew coffee is often so strong it’s used as a concentrate, which, of course, does lower the overall caffeine level. While it’s impossible to generalize since brewing methods vary on time, ratio, and whether or not it’s used as a concentrate, generally speaking cold brew will still have more caffeine than hot-brewed coffee.
Light Vs. Dark
According to Hendon, there is one factor that doesn’t really seem to affect caffeine levels: The roast of the coffee. One theory says that light roast coffee, roasted for less time, doesn’t break down caffeine and thus has more. Another says that dark roast coffee actually results in more of the cellular structure getting broken down, allowing more coffee to be exposed. Hendon says that the studies on this are old and inconclusive, but that ultimately any effect would likely be negligible.
“Caffeine, I think, is a relatively subtle stimulant,” Hendon says. A lot of our reaction to coffee has to do with our perception of it: dark roast, with a stronger flavor, might elicit a strong reaction from the drinker who then feels like it has more caffeine than a lighter roast. Espresso, with a relatively low amount of caffeine, is a similarly intense experience. A cold cup of iced coffee, whether its cold brew or not, drunk quickly on a hot summer’s day could also have other factors in play besides caffeine that make the drinker feel more awake.
So if you’re looking for a more intense, caffeinated morning brew, consider upgrading to cold brew. But, Hendon says, the fact that caffeine levels are higher in slower brewed coffees shouldn’t change how you feel about your drinking method of choice. When it comes to that daily morning routine, your own perception might be just as important to which drink makes you feel the most alert.