Coffee is so deeply integrated in our societies it has become what’s often called the world’s most popular psychoactive drug. Despite this, I didn’t really understand how coffee is made or what it does to our bodies. I knew how to dump grounds in a filter and water in a boiler, but that’s the very last step in a long process and where most people’s knowledge stops. What does “dark roast” even mean? What about caffeine keeps me awake for morning meetings? I’ve found out that the secrets of coffee chemistry lie in three things: the bean, the roast, and the brew.
To get to the bean, a coffee plant cherry must be pitted. A red bauble holds a green coffee bean pit, carefully cultivated and then removed. The two most commonly used coffee plants are coffea arabica, “Arabica” beans, first made in East Africa and used mostly for its rich flavors, and coffea canephora, “Robusta” beans, which have two times the caffeine and acid levels as Arabica. Components of the bean that make it useful to coffee drinkers are the caffeine and chlorogenic acids, which both serve their natural purpose as protectors of the plant from bugs and other consumers. Unfortunately, or not, for the plant those chemicals are exactly why we like coffee so much. Specifically, the bean contains phenylindanes and lactone for bitterness, and citric and malic acid for sour and fruit-type tastes. These compounds are like the sour acids in a not-yet ripe lemon.
Once the beans are harvested, the next step is to roast them, a process that propels the first chemical reactions into action. Largely driven by the same browning reaction in your cooked meats and French fries, called the Maillard reaction, roasting converts sugars and proteins in the bean into the flavors and aromas we’re familiar with. For a light roast, the beans are heated at 205 degrees Celsius until you hear the sharp sound of what coffee makers call the “first crack.” If the roast stops here, the origin of the bean better be good since in a light roasting the origin flavors from climate and soil are more pronounced.
For the medium roast, heat makes it to 225 degrees to produce an even sharper “second crack” that form as the bean’s cell walls are breaking apart. The bean caramelizes, the burnt sugar compounds diacetyl and furanone emerging, and a better balance is made between acidity and body, for an earthy, well-rounded taste. The “body” describes the presence of the flavors in your mouth, which deepens with the increasing heat. Roasting at even higher temperatures results in the dark roast, where the beans become oily and completely stripped of origin flavors—all the taste coming from the roast. Longer roasting means there are less useful chlorogenic acids, but it creates the strong and classically aromatic pot that is such a recognizable staple around the world.
In each of the three roasts, new compounds such as quinic and caffeic acid develop from acids previously trapped in the fresh bean. The new acids enhance bitterness and have been shown to be effective antioxidants and anti-inflammatories, prevent cardiovascular disease, and make up the nasty burnt bitter taste when you leave coffee on heat for too long. Acetic acid, the active molecule in vinegar, and trigonelline is also released in the high temperatures. The acid contributes to raising stomach acids in our body, helping us digest faster. Trigonelline, one of the few non-acids created during roasting, is linked to blocking cancer cell growth.
The brew, the last step and the one most of us have done, activates another set of chemical reactions. This time it depends on the water and the method of extracting. The main two extraction methods used are filtering (common coffee pots) and pressurizing (espresso machines). The finer the grounds of the bean are, the slower the rate of water passing through the coffee, thereby changing the rate of the water’s reaction to it. There’s still some debate whether pure water or ionized water is better—since free ions can grab onto lingering flavor compounds in the grounds. When reacting with water, ground beans can produce 3-methylbutanal, methylpropanal, and acetaldehyde, compounds that evoke subtle fruitiness, spiciness, and floridness.
It’s at this point my favorite part of coffee is extracted from the grounds into the drink: caffeine. We consume about 260 million pounds of caffeine every year in coffee, tea, and energy products, making one of the most traded substance on Earth. This obsession is mostly because of the small neurotransmitter adenosine. Adenosine increases in the brain throughout the day while you’re awake, slowly making you feel tired until you go to sleep, when it slowly decreases until you wake up, refreshed. When you drink coffee, the caffeine immediately begins blocking adenosine receptors in the brain, enhancing brain activity by regulating dopamine and stopping adenosine from slowing you down. But perhaps unsurprisingly, the brain can retaliate by creating more adenosine receptors, meaning the more you use caffeine the more caffeine you’d need to feel the same non-drowsy effects as before. Additionally, consuming caffeine during peak cortisol levels diminishes the caffeine’s effect and increases the rate you become tolerant. So better to drink caffeine at least an hour after waking up, when cortisol has gone back down.
The brilliant collection of psychophysical influences that lies in coffee carries a lot of people through class, work, and regular daily life. Grabbing another cup is natural for me; I drink it for the all the health and energy benefits most people do, but also for the lovely chemical blend of rich flavors and aromas that take their time coming from the plant to my cup.