Coffee Decoded: The Chemistry of Decaffeination
Welcome to Coffee, Decoded, the Specialty Coffee Association’s weekly column on science, research, and all things coffee knowledge. I’m PETER GIULIANO, and each week I answer coffee questions, interpret research, and dive deep into coffee science.
Consumers love coffee for its stimulating properties, and for its flavor. But many coffee drinkers, for a variety of reasons, want to avoid the stimulation of caffeine while still enjoying the flavor of our favorite beverage. We’ve all heard about the different decaffeination methods, often referred to by the name of a brand or processing technique. Sometimes coffee experts describe these various processes, explaining in detail how decaffeination is done. But often these descriptions don't include the chemistry and mechanics of how decaffeination actually works. Today, we’ll dig into details of decaf.
The Makeup of a Coffee Bean
First, the basics: a coffee bean has hundreds of chemical compounds locked inside the seed's cells. Roasting, grinding, and brewing unlock these chemicals so we can extract them into our daily cup. Caffeine is just one of those compounds. So how on earth do we pull a single compound out of such a complex mix? Most decaf methods rely on two principles: dissolving caffeine and adsorbing it. Let's look at both.
Dissolving Caffeine
Some solids dissolve completely into a liquid, forming a uniform mixture called a solution. We make solutions all the time: sugar in water makes a sweet one, and salt in water makes a salty one. The solid that dissolves is called the solute, and the liquid doing the dissolving is the solvent. In both above examples, water is the solvent. Water is so good at dissolving that it's nicknamed the "universal solvent." There are other solvents too, such as alcohol (which we use in liquor and cosmetics) and acetone (which we use in paints and varnishes). We often associate the word “solvents” only with these, but, in fact, water is the most common and useful solvent of all.
Compounds dissolve in solvents differently. You can observe this at home: Take a teaspoon of table salt and a teaspoon of sugar (your solutes) and dissolve them each in a separate glass of water (your solvent). Stir, and you’ll notice something interesting: the sugar dissolves much more easily than salt. If you tried a teaspoon of sand, that would not dissolve at all. This demonstrates that some solutes are much more soluble in water than others, and still others are not water soluble at all.
It turns out that caffeine is very, very soluble in water. For this reason, water is often used to decaffeinate coffee. The problem is, many other coffee compounds—including the ones that make coffee taste good—are also soluble in water. One way to get around this is to use other solvents, which do just as good a job at dissolving caffeine, but don’t dissolve flavor compounds very well. The two most common solvents used for this are methylene chloride and ethyl acetate, both of which can be highly effective at dissolving caffeine but leave coffee flavor compounds relatively untouched. Another method uses highly concentrated carbon dioxide—so concentrated it behaves like a liquid—to dissolve caffeine. In any case, dissolving caffeine uses a solvent—whether water or another one—to remove caffeine from coffee.
Adsorbing Caffeine
But there’s another trick that can be used in caffeine removal. Imagine this: what if you had a kind of magnet for caffeine? Or, better yet, a filter that the caffeine molecules would stick to, letting flavor molecules flow through untouched? It turns out this is possible, via a process called adsorption, our term for when a chemical “sticks” to another at a microscopic level. Some materials just attract certain chemicals, and caffeine is no exception.
One material that is especially good at adsorbing caffeine is a kind of carbon often called “activated carbon” or “activated charcoal”: its porous surface effectively traps caffeine molecules. And, if the material is prepared properly, it can be very selective, trapping mostly caffeine and letting other molecules pass. When water-based decaffeination processes mention “carbon filtration” this is what they mean; they can use water as a solvent for coffee, remove the caffeine from the water, and then use the flavor-rich decaffeinated water to decaffeinate future batches. A caffeine adsorbent can even be used as a brewed coffee filter, removing caffeine after the coffee is brewed.
Almost all commercially available decaffeination methods use some combination of dissolving and adsorbing caffeine, so understanding these two processes is critical to understand all the decaf methods. See if you can pick them out in your favorite decaf process!
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