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What makes a good cup of coffee?

There’s no need to invest in fancy paraphernalia. Unpacking the science of roasts, grind, and why a shot of caffeine keeps you awake

Illustrated by Krish Ashok.

By Krish Ashok

LAST UPDATED 22.05.2021  |  07:45 AM IST

In the 1990s, a coffee percolator at the University of Cambridge’s computer lab became the very first object on the planet to be watched via a webcam by people on the internet. The folks at the lab originally set it up to keep an eye on the coffee level so that they didn’t waste a trip to the machine.

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About a billion people on the planet drink this beverage made by the high temperature water extraction of the roasted, sun-dried and fermented pits of a red-coloured cherry that originally used to grow in the Ethiopian highlands. Legend has it that a goatherd named Kaldi observed the high-energy twitchiness of the goats which ate these berries and gave them to a local monastery whose abbot concocted a drink that helped his monks stay awake during prayer time.

When you sip on a cup of coffee in the middle of a tiring workday, the following things happen. When you are tired, your bloodstream is awash with a chemical called adenosine that turns on a receptor in the brain responsible for shutting down the production of adrenaline, the Gurgaon SUV driver of hormones. It keeps you awake and aggressive. So, when production of adrenaline is shut down, we feel drowsy.

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Caffeine has a three-dimensional shape similar to adenosine and thus ends up blocking those same receptors, except it doesn’t shut down adrenaline production. So, the end effect is that you don’t feel drowsy. Coffee’s fundamental illusion is built on preventing sleep, not adding energy, although it sure does feel like the latter! It also prevents the re-absorption of dopamine by the brain, so it keeps you feeling high for a longer period—again, a trick of prevention rather than enablement.

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So what makes a good cup of coffee? Depending on how obsessed you are, the answer can range from something as simple as opening a sachet of freeze-dried instant coffee and adding it to water or milk, to buying the beans fresh, roasting and grinding them and brewing your own artisanal, single-source cup of fine coffee. Most people will fall somewhere in the middle, so let’s understand the science of every factor that goes into making a good cup of coffee.

We start with the beans themselves. Your coffee will either be full-flavoured and low on caffeine if it’s Arabica and medium-flavoured and high-caffeine if it’s Robusta. After that, it’s a function of how the beans are processed. If the pulp is just washed and the beans are removed and sun-dried, it has a less complex flavour than if it is allowed to ferment, a process that adds a layer of extra flavour to the coffee. Once the beans are dried, they are roasted. A light roast retains most of the original, fruity flavours of the coffee, while a darker roast introduces caramelly and chocolatey flavours thanks to the Maillard reaction. A medium roast gets you the best of both worlds, especially if you are using expensive beans. A dark roast is suitable for cheap coffee.

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The next factor is how fine you grind the beans. A coarse grind will result in a sour and grassy flavour profile because only the most rapidly water-soluble compounds are extracted, while a fine grind will result in a thicker, stronger and more bitter brew. Then there is water temperature. If the water is too cold, it will not extract enough flavours; if it’s too hot, it will destroy some of the aroma, so a temperature of 91-96 degrees Celsius, just short of a full boil, works best for coffee.

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So now that we have the hot water and the coffee powder ground to our preferred level, you can make coffee in one of several ways. The easiest is a percolator. It’s as simple as a paper filter on top of which coffee powder is added and hot water poured over it. The water spends very little time with the coffee and thus extracts a lighter brew. How you pour the water (a rotating motion and a slow pour rate is ideal) and how much water you pour will determine the final product.

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If you like your coffee stronger, then the idea is to let the water spend more time with the coffee powder before you filter it. A French press involves coffee powder sitting in hot water for as long as you want (typically a few minutes) before using a plunger that has a metal filter. A south Indian filter is a modified variant of the French press. It uses a small filter pore size that slows down the water and thus results in a stronger brew. South Indian filter coffee also uses a mixture of 80% coffee and 15-20% chicory to add thickness to the brew and reduce bitterness.

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For even more flavour extraction in the shortest amount of time, you need to increase the pressure at which the water goes through the coffee. An espresso (literally “express” in Italian) machine does exactly that. It pumps hot water at very high pressure through the coffee, resulting in a thick, creamy brew. But espresso machines are designed for commercial production where speed is critical.

An affordable option for the home is the Moka pot. This is a device that uses a hot water container at the bottom that pushes steam up the coffee chamber and condenses into decoction in the upper chamber. The resulting coffee is not as thick as espresso but still significantly stronger than percolators, or French presses.

So now you know what goes into making a cup of coffee. Just vary the bean source, roast level, grind size, water temperature, percolation time and pressure to get the flavour you like. Or just open a sachet of instant coffee. And oh, drink it while it’s still hot. Our tongues detect bitter tastes more strongly at room temperature.

Illustrated by Krish Ashok
Illustrated by Krish Ashok
Illustrated by Krish Ashok
Illustrated by Krish Ashok
Illustrated by Krish Ashok

Krish Ashok is the author of Masala Lab: The Science Of Indian Cooking.