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Hitting Books: The Correct Way to Make Coffee, According to Science

The best option is, of course, hot bean juice in your cup. But, as Dr. Kate “The Chemist” Biberdorf explains in her new book It’s Elemental, if you want to consistently enjoy the best joe cup you can craft – perfectly caffeinated and not too bitter – a little math is necessary. And it’s not just coffee. Biberdorf takes readers on a journey through the mundane moments of everyday life, illustrating how incredible they really are – if you stop to look at the chemistry behind them.

Harper Collins

Extracted from It’s Elemental by Kate Biberdorf, Copyright © 2021 by Kate Biberdorf. Published by Park Row Books.


Coffee and tea are much more potent sources of caffeine than soda. In a cup of coffee, you can ingest around 100 mg of caffeine, but it can be up to 175 mg with the right coffee beans and technique. The whole process of making coffee beans (and coffee itself) is quite fascinating if you’ve never thought about it too much. For example, espresso makers and percolators take the most caffeine from the lighter roasted beans, but the drip method is the best way to get the most trimethylxanthin from the darker beans. However, in general, light and dark roasted coffees typically have the same relative number of caffeine molecules in each cup of coffee (except espresso).

Let’s look at the roasting processes to determine why. When beans are initially heated, they absorb energy in what we call an endothermic process. However, around 175 ° C (347 ° F), the process suddenly becomes exothermic. This means that the beans have absorbed so much heat that they now radiate heat into the atmosphere of the roasting machine. When this happens, the parameters must be adapted to the equipment, to avoid over-roasting the beans (which sometimes gives a coffee a roasted taste). Some roasters also switch the beans between the endothermic and exothermic reaction a couple of times, to get different flavors.

Over time, the roasting of the coffee beans changes slowly from green to yellow, and then a series of different shades of brown. We refer to bean dark as its “roasted,” where darker roasted coffee beans are much darker in color than lighter roasted beans (surprise, surprise). Their color comes from the temperature at which they are roasted. The lighter beans are heated to about 200 ° C (392 ° F) and the darker beans roasted to about 225-245 ° C (437-473 ° F).

But just before the beans start to, for lack of better, slightly roasted words, the coffee beans go through their first “crack”. This is an audible process that takes place at 196 ° C (385 ° F). During this process, the beans absorb heat and double in size. But since water molecules evaporate out of the bean when they are at high temperatures, they decrease in mass by about 15%.

After the first crack, the coffee beans are so dry that they stop absorbing heat quickly. Instead, all the thermal energy is now used to caramelize the sugars on the outside of the coffee bean. This means that heat is used to break the bonds in the sucrose (sugar) into much smaller (and more fragrant) molecules. The lighter roasts — such as roasted cinnamon and roasted New England — are heated just past the first crack before being removed from the coffee roaster.

There is a second crack that occurs during roasting, 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 it happens, you can usually hear it from a second “pop”. Dark roasts are typically graded from any beans that have been heated past the second crack — such as French and Italian roasts. In general, due to warmer temperatures, darker beans tend to have more of their caramelized sugars, while lighter beans have less of them. The variation in flavor due to these methods is wild, but it doesn’t really have an effect on their reaction in the body – just the taste.

Once you’ve got your perfectly roasted coffee beans, you can do the rest of the chemistry at home. With a good-priced coffee grinder, you can grind your coffee beans to a number of different sizes, which will definitely influence the taste of your morning coffee. The small, fine grinds have a large surface area, which means that the caffeine (and other flavors) can be extracted from the miniaturized coffee beans with ease. 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 ground coarsely. In this case, the inside of the coffee beans are not exposed to almost the same degree as the finely ground coffee beans. The resulting coffee can often taste sour — and sometimes even a little salty. But if you associate the correct size of coffee grounds with the right method of preparation, you can make yourself the best cup of coffee in the world.

The simplest (and easiest) way to make coffee is to add extremely hot water to large coffee grounds. After soaking in water for a few minutes, the liquid can be decanted from the container. This process, called decoction, uses hot water to dissolve the molecules in the coffee beans. Most current coffee infusion methods use some version of decoction, which is what allows us to drink a cup of hot coffee instead of pouring it over some roasted beans. However, since this method does not involve a filtration process, this version of coffee – affectionately referred to as cowboy coffee – is prone to having coffee bean floaters. For this reason, it is usually not the preferred method of beer.

By the way, you noticed that I avoided the term boiling? If you’re looking to make a decent half cup of coffee, hot water should never really be boiled. However, the ideal water temperature is around 96 ° C (205 ° F), which is just below boiling point (100 ° C, 212 ° F). At 96 ° C, the molecules that provide the aroma of coffee begin to dissolve. Unfortunately, when water is only four degrees warmer, the molecules that give coffee a bitter taste also dissolve. That’s why coffee nerds and bartenders are so obsessed with their water temperature. In my house, we also use an electric boiler that allows us to select any temperature we want our water to be.

Depending on how strong you like the taste of your coffee, you may be partial to the French press or some other steeping method. Like cowboy coffee, this technique also impregnates coffee bottoms in hot water, but these bottoms are slightly smaller (thick versus extra thick). After a few minutes, a stand is used to push all the ground to the bottom of the device. The liquid remaining on the ground is now perfectly clear and tastes good. Since coarse coffee grounds are used in this method, more molecules can dissolve in the coffee solution, giving us a more intense flavor (compared to cowboy coffee).

Another technique: when hot water is dripped over coffee grounds, the water absorbs the aromatic molecules before dripping into the coffee cup. This process, appropriately called the drip method, can be done manually or with a high-tech machine, such as a coffee percolator. But sometimes this technique is used with cold water, which means that fragrant aromatic molecules (those that give your coffee its distinctive smell) cannot dissolve in water. The result is called Dutch iced coffee, a drink that is ironically favored in Japan, and takes about two hours to prepare.

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