Learn Basic Coffee Chemistry

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Video: ACS Reactions 

What’s going on in that cup of coffee? It might seem boring, but the closer you look the more complex and surprising it becomes.

First off there are trillions upon trillions of molecules in the air around that coffee. Those hot vapors arising from the cup carry more than a thousand different types of molecules. A few dozen of those contribute to coffee’s one of a kind aroma. Pyrazine has an earthy smell. Methylpropanil is fruity and spicy. There’s Vanillin, you can probably guess what that smells like. Methional smells like a baked potato. Methanethiol is the odor of cabbage or garlic thanks to a sulfur group that also shows up in rotten eggs and skunk spray. Most of those molecules are made when the beans are roasted. The heat of roasting provides the energy to convert coffees bitter Chlorogenic Acid into all these other compounds making a richly flavorful, fragrant and just awesome beverage.

Many chemical reactions require some energy to get going. Even the ones that release energy like combustion. After all spending energy is how we make things happen. Cars drive, legs walk, and coffee brews. Which brings me to caffeine, which is probably the reason most of us drink coffee anyway. The story of caffeine is also a story of evolution. Plants didn’t evolve caffeine to help us get through 9am meetings. Like so many of the molecules plants make, caffeine, which is actually pretty bitter on its own, is a chemical weapon that can disable or even kill insects that threaten plants. Citrus plants use caffeine a different way. Thanks to caffeine’s memory boosting and ever so slightly addictive kick, bees and other pollinator’s remember the citrus flowers they visit. That gives those plants an advantage in spreading their genetic material. Evolutionarily speaking that’s what life is all about. So caffeine just doesn’t give you that pleasant morning buzz. You can also thank it for your orange juice. But, back to our own brains. Caffeine blocks the nerve receptors that signals to the brain that it is time to sleep. That’s what helps us power through our bedtime.

And speaking of energy, watch that milk swirl and blend with the coffee. You’re looking at diffusion the process of particles moving from an area where there is a lot of the same stuff to where there isn’t. Diffusion is how breathing keeps us alive. Oxygen diffuses from air into our bloodstream in the lungs and from capillaries into our cells throughout the body. The way the milk moves here is driven by a few things. It falls due to the force of gravity pulling it down from the carton and into the mug. But then its motion through the coffee is disrupted by the stickiness of milk and coffee molecules interacting. That and the walls of the mug help set up those swirling eddies. Convection is at play here too. Colder denser milk sinks to the bottom of the warmer less dense coffee. This causes the same kinds of loops of motion in liquefied rock far beneath the earth’s surface. This motion keeps our planet geologically active.

But even in the absence of any of these phenomenon, milk and coffee would eventually mix anyway. The particles of milk and the little un-dissolved coffee grounds dance thanks to Brownian Motion. That’s the random movement of particles as they crash and collide against each other. Studying this dance is one of the clues that Einstein used to prove the existence of atoms. This spontaneous process of mixing follows the second law of thermodynamics. Entropy, or disorder, increases over time. Your coffee and milk go from two separate substances to a mixture, which is less ordered because it is made from two things all mixed up together. This disorder increase is even more obvious with the sugar, which goes from a solid crystalline state where each molecule is locked into a pattern to a dissolved form with sugar molecules all over the place.

And so it goes with the rest of the universe. Rocks crumble to dust. Ice melts and your place keeps on getting messier. Left to itself long enough, the entire universe will one day reach a state of maximum disorder, a bit like this milk and coffee.

What are we getting at here? From the largest scale to the smallest, chemistry is every where and once you see it you will never look at the world the same way again.








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