Hitting the Pounds: The Right Way to Make Coffee, According to Science


The best part of waking up is, of course, the hot bean juice in your cup. But, as Dr. Kate explains “The Chemist” Biberdorf in her new book It’s elementary, if you still want to enjoy the best cuppa joe you can make – perfectly caffeinated and not too bitter – a little math is needed. And it’s not just coffee. Biberdorf takes readers on a journey through mundane moments in everyday life, illustrating how amazing they are – if you stop to examine the chemistry behind them.

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Extract of It’s elementary by Kate Biberdorf, Copyright © 2021 by Kate Biberdorf. Published by Park Row Books.

Coffee and tea are much more powerful sources of caffeine than soda. In a cup of coffee, you’re likely to ingest around 100 mg of caffeine, but that can go up to 175 mg with the right coffee beans and technique. The whole process of making coffee beans (and the coffee itself) is quite fascinating if you’ve never given it much thought. For example, espresso machines and percolators get the most caffeine from lighter roasted beans, but the drip method is the best way to get the most trimethylxanthine from darker beans. However, in general, light and dark roast coffees generally have the same relative number of caffeine molecules in each cup of coffee (excluding espressos).

Let’s take a look at the roasting processes to find out why. When the grains are initially heated, they absorb energy in what we call 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 returning it to the atmosphere for roasting. When this happens, the settings should be adjusted on the equipment, in order to avoid excessive roasting of the beans (which sometimes results in a burnt-tasting coffee). Some roasters will even switch the beans between the endothermic and exothermic reaction several times, to achieve different flavors.

Over time, the roasting of the coffee beans slowly changes from green to yellow and then to a number of shades of brown. We call the darkness of the bean its “roast,” 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. Lighter beans are heated to about 200 ° C (392 ° F) and darker roasted beans to about 225–245 ° C (437–473 ° F).

But just before the beans begin, for lack of better words, to be lightly roasted, the coffee beans experience their first “crack”. This is an audible process that occurs at 196 ° C (385 ° F). During this process, the grains absorb heat and double in size. But since water molecules evaporate from the grain when subjected to high temperatures, their mass actually decreases by about 15%.

After the first crack, the coffee beans are so dry that they easily stop absorbing heat. Instead, all of the heat energy is now used to caramelize the sugars on the outside of the coffee bean. This means that the heat is used to break the bonds of sucrose (sugar) into much smaller (and more fragrant) molecules. Lighter roasts, like Cinnamon Roast and New England Roast, are heated just after the first crack before being removed from the 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 this happens, you can usually hear it with a second “pop”. Dark roasts are generally classified according to grains that have been heated beyond the second crack, such as French and Italian roasts. In general, due to the higher temperatures, darker beans tend to have more of their caramelized sugars, while lighter beans have less. The variation in flavor due to these methods is wild, but it doesn’t really affect how they react in the body, only the taste.

Once you’ve bought your perfectly roasted coffee beans, you can do the rest of the chemistry at home. With a cheap coffee grinder, you can grind your coffee beans in different sizes, which will definitely affect the taste of your morning coffee. Small, fine grinds have a large surface area, which means caffeine (and other flavors) can be extracted from miniaturized coffee beans with ease. However, this can often lead to excessive caffeine extraction, which makes the coffee taste bitter.

On the other hand, coffee beans can be coarsely ground. In this case, the insides of the coffee beans are not exposed to about 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 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 brew coffee is to add extremely hot water to the coarse coffee grounds. After soaking in water for a few minutes, the liquid can be decanted from the container. This process, called a decoction, uses hot water to dissolve the molecules in the coffee beans. Most current methods of brewing coffee use a decoction version, which allows us to drink a hot cup of coffee instead of munching on roasted beans. However, because this method does not contain a filtration process, this version of coffee – affectionately known as cowboy coffee – is prone to coffee bean floats. For this reason, it is generally not the preferred method of brewing.

By the way, did you notice that I avoided the term boiling? If you are trying to brew a half decent cup of coffee, hot water should never be boiled. Instead, the ideal water temperature is around 96 ° C (205 ° F), which is just below boiling (100 ° C, 212 ° F). At 96 ° C, the molecules that provide the aroma of coffee begin to dissolve. Unfortunately, when the water is only four degrees hotter, the molecules that make coffee taste bitter also dissolve. This is why coffee nerds and baristas are so obsessed with the temperature of their water. In my house we even use an electric kettle which allows us to select the temperature we want our water to be.

Depending on how strong you like the taste of your coffee, you may have a soft spot for the French press or another steeping method. Like cowboy coffee, this technique also soaks the coffee grounds in hot water, but these grounds are a bit smaller (coarse versus extra large). After a few minutes, a plunger pushes all the grounds to the bottom of the appliance. The liquid remaining on top of the grounds is now perfectly clear and deliciously tasty. 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 drips onto coffee grounds, the water absorbs the aromatic molecules before dripping into the coffee cup. This process, aptly 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 the scent and aromatic molecules (the ones that give your coffee its distinctive smell) cannot dissolve in the water. The result is called Dutch iced coffee, a drink that is ironically favored in Japan, and takes around two hours to prepare.

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