The best part about waking up, of course, is hot bean juice in the cup. But like Dr. Kate “The Chemist” Biberdorf explains in her new book It’s Elemental, a bit of math is required if you are to consistently enjoy the best cup of coffee you can make – perfectly caffeinated and not too bitter. And it’s not just coffee. Biberdorf takes the reader on a journey through banal moments of everyday life and shows how incredible they actually are – when you pause to examine the chemistry behind them.
Excerpt from It’s Elemental by Kate Biberdorf, Copyright © 2021 by Kate Biberdorf. Published by Park Row Books.
Coffee and tea are much stronger sources of caffeine than soda. You are probably getting around 100 mg of caffeine in a cup of coffee, but with the right coffee beans and technique it can be as high as 175 mg. The whole process of making coffee beans (and making the coffee itself) is pretty fascinating if you’ve never thought about it before. Espresso machines and percolators, for example, get most of the caffeine from lighter-colored roasted beans, but the drip method is the best way to get the most trimethylxanthine from darker beans. In general, however, light and dark roast coffees typically have the same relative number of caffeine molecules in each cup of coffee (other than espresso).
Let’s look at the roasting processes to find out why this is so. When the beans are initially heated, they absorb energy in what is known as an endothermic process. However, at around 175 ° C (347 ° F) the process suddenly becomes exothermic. This means that the beans have absorbed so much heat that they are now radiating the heat back into the atmosphere of the roasting machine. In this case, the settings on the device must be adjusted to avoid overroasting the beans (which sometimes leads to burnt coffee). Some roasters even toggle the beans between the endothermic and exothermic reactions a few times to create different flavors.
Over time, coffee beans roast slowly change from green to yellow and then to a range of different shades of brown. We refer to the darkness of the bean as “roast”, with the darker roasted coffee beans being a much darker color than the lighter roasted beans (surprise, surprise). Their color comes from the temperature at which they are roasted. Lighter beans are heated to about 200 ° C (392 ° F) and darker roasted beans to about 225–245 ° C (437–473 ° F).
But shortly before the beans are lightly roasted for lack of better words, the coffee beans pass through their first “crack”. This is an audible process that occurs at 196 ° C (385 ° F). The beans absorb heat and double in size. However, because the water molecules evaporate from the bean at high temperatures, they actually decrease in mass by about 15%.
After the first crack, the coffee beans are so dry that they can no longer absorb heat. Instead, all of the thermal energy is now used to caramelize the sugar on the outside of the coffee bean. This means that the heat is used to break the bonds in the sucrose (sugar) into much smaller (and more fragrant) molecules. The lightest roasts – like cinnamon roast and New England roast – are heated shortly after the first crack before they are taken out of the coffee roaster.
There is a second crack that occurs during frying, but at a much higher temperature. At 224 ° C (435 ° F), the coffee beans lose their structural integrity and the bean itself begins to collapse. When this happens you can usually hear it with a second “pop”. Dark roasts are usually categorized according to beans that have been heated past the second crack – like French and Italian roasts. In general, because of the higher temperatures, darker beans tend to have more sugar caramelized, while lighter beans have less. The taste variations due to these methods are wild, but they don’t really affect how they react in the body – just the taste.
Once you’ve purchased your perfectly roasted coffee beans, the rest of the chemistry can be done at home. An inexpensive coffee grinder allows you to grind your coffee beans into different sizes, which definitely affects the taste of your morning coffee. Small, fine grinds have a large surface area, which means that the caffeine (and other flavors) can be easily extracted from the miniaturized coffee beans. However, this can often result in too much caffeine being extracted, which gives the coffee a bitter taste.
On the other hand, coffee beans can be coarsely ground. In this case, the inside of the coffee beans is not exposed nearly as much as finely ground coffee beans. The resulting coffee can often taste sour – and sometimes even a little salty. But if you combine the right size of coffee grounds with the right brewing method, you can make yourself the best cup of coffee in the world.
The easiest (and easiest) way to make coffee is to add extremely hot water to coarse coffee grounds. After they have been soaked in the water for a few minutes, the liquid can be dispensed from the container. This process, known as decoction, uses hot water to dissolve the molecules in the coffee beans. Most current methods of making coffee use some type of decoction that allows us to have a cup of warm coffee instead of chewing on some roasted beans. However, since this method does not involve a filtration process, this type of coffee – affectionately called cowboy coffee – tends to have coffee bean floaters. Because of this, it’s usually not the preferred brewing method.
By the way, did you notice that I avoided the term cooking? When trying to make a reasonably decent cup of coffee, the hot water should never really be boiled. Instead, the ideal temperature of the water is around 96 ° C (205 ° F), which is just below the boiling point (100 ° C, 212 ° F). At 96 ° C, the molecules that give the coffee aroma begin to dissolve. Unfortunately, when the water is only four degrees hotter, the molecules that give coffee a bitter taste also dissolve. This is why coffee nerds and baristas are so obsessed with their water temperature. In my house we even use a kettle that allows us to choose the temperature we want for our water.
Depending on how strong you like your coffee, you might like the French press or some other soaking method. As with cowboy coffee, with this technique the coffee grounds are soaked in hot water, but these are slightly smaller (coarse versus extra coarse). After a few minutes, the entire grist is pressed to the bottom of the device with a plunger. The remaining liquid above the bottom is now perfectly clear and deliciously tasty. Since the coarse coffee grounds are used in this method, more molecules can dissolve in the coffee solution, which gives us a more intense flavor (compared to cowboy coffee).
Another technique: when hot water is dripped over the coffee grounds, the water absorbs the aroma molecules before it drips into the coffee cup. This process, aptly referred to as the drip method, can be done manually or with a high-tech machine such as a coffee maker. But sometimes this technique is used with cold water, which means that the fragrant, aromatic molecules (which give your coffee its distinctive smell) cannot dissolve in the water. The result is called Dutch Iced Coffee, an ironically popular drink in Japan that takes about two hours to prepare.
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