I have been asked many times about caffeine in coffee. Does light roast have more caffeine because it is roasted less? Does dark roast coffee have more caffeine because it is stronger? When it comes down to it, it really isn’t so cut and dry. First you have to ask yourself…
What is caffeine?
According to Nehlig A, Daval JL, Debry G (1992), caffeine is a central nervous system (CNS) stimulant. A methylxanthine class purine base, or nitrogen-containing biological compound, caffeine is chemically related to the adenine and guanine bases of DNA and RNA. According to Mitchell, Diane C., et al. (January 2014), It is found in many seeds, nuts, and leaves of a number of plants native to South America and East Asia, helping to protect them against predator insects and to prevent germination of nearby seeds. Coffee is the most notable source of caffeine, being consumed widely because of caffeine’s cognitive and physical performance enhancing properties, the most notable of which is it’s improvement of our wakefulness. So, if we want the most caffeine, how do I know which coffee has the best caffeine content? You would have to ask yourself…
What factors determine caffeine levels in coffee?
The amount of caffeine in an 8oz cup of coffee can vary between about 80 to 150 milligrams, due to a variety of factors. The first and foremost factor that affects caffeine content is the altitude at and region in which the coffee beans in question were sourced. Ironic to the popular idea that good coffee has a lot of caffeine, higher quality, high altitude coffees contain less caffeine. These high quality beans take longer to grow in the colder, oxygen-depleted atmosphere, making them smaller and denser than beans grown in warmer, lower altitude regions. They are also less susceptible to fungus, as the colder, low-oxygen atmosphere does not promote fungal spread such as coffee rust. When taken care of, coffee plants grown at high altitudes produce significantly more character, having less caffeine and more aromatics, acidity, and depth of flavor. This image from DT Coffee Club shows variance of flavor profile with altitude:
Caffeine is a very stable molecule, meaning that it is not very reactive to the natural surrounding environment, and thus is not generally lost to the atmosphere during oxidation and other chemical processes, and stays in the coffee at close-to original concentration for years, even after roasting, assuming it stays in a whole-bean form (pre-ground coffee has a larger atmosphere-exposed surface area, which accelerates the removal of caffeine, but at a nominal rate). So what else makes the amount of caffeine vary? For one, the roasting process. Very small amounts of caffeine are lost during roasting to sublimation, where some of the coffee mass is turned straight from a solid into a gas. More caffeine is lost with the shedding of the coffee chaff (the outer ‘skin’ of a coffee bean), which causes the amount of coffee overall to lose mass (This means different roasting methods can result in varying caffeine – air roasting is a method that does well at completely removing coffee chaff from a batch of roasted beans). Coffee beans also expand while roasting, spreading out the caffeine and lowering the amount of caffeine present by volume, but not by weight. The expansion of the coffee bean has a big effect on cups of coffee brewed by volume, but a nominal effect on cups of coffee brewed by weight. A lot of baristas will tell you light-roasted coffee has more caffeine than dark roast because of this, which in some sense is true, but what they don’t explain is that when measuring coffee by weight, they overcome the difference in volume and therefor caffeine by using more beans when brewing dark roast coffee than light roast coffee. Apart from physical amounts of coffee, it should be noted that different concentrations of caffeine are just as important on the effect that caffeine has on your body; a 2oz shot of espresso might have close to the same caffeine content as an 8oz cup of coffee, but the 2oz shot is much more concentrated and can have a much greater effect.
So, what is the consensus?
The caffeine is generally predetermined by the altitude of the coffee. The roast can have a greater effect on caffeine, but it depends on the brewing method, grain size of the coffee, how the coffee is measured, and the coffee to water ratio. That said, the original caffeine content of the coffee will be of more importance: a low-altitude, warmer climate sourced coffee roasted dark could still have more caffeine than a very high altitude, colder climate coffee roasted light. You should consider both the concentration and amount of the caffeine when considering it’s enhancing properties, rather than just the amount, namely in espresso vs. drip brewed coffee where the coffee to water ratio varies drastically.
The best way to determine the amount of caffeine is to look at the sourcing of the bean. If your coffee was made from a blend of beans, you would have to find the caffeine content from each source and calculate a proportioned caffeine content. If you get your coffee from a standard thrid-wave store that weighs their coffee, you will likely not get much variance from roasting, whereas coffee from a second-wave store like Starbucks will likely be measured by volume, and as such the roast will play a large part in the caffeine content. All in all, it depends on the situation.