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Coffee grinding

23 June 2020
Dr Karl's curious mind: science in everyday life

Dr Karl Kruszelnicki takes a close look at what it takes to make a cup of coffee great.

We’ve been drinking coffee (the world’s most popular legal drug) since at least the middle of the 15th century.

But after six centuries, we still didn’t know how to make a consistent and high quality cup of coffee. At the highest levels of international professional barista competitions, the competitors have to make four identical espressos - but each one can taste quite different!

Why?

Luckily, curiosity and two years’ research gave us the answer.

Close up coffee machine dripping coffee

Five Senses Coffee (Australia)

Look at a single grain of finely ground coffee, nestled in between many other coffee grains.

These grains take up about 82% of the available space, leaving about 18% free for the hot pressurised water to force its way through. The grains vary enormously – between 10-1,000 µm in diameter. (A hair shaft is about 50-70 µm across.) About 99% of the grains are less than 100 µm across. These smaller grains account for about 80% of the surface area. Each grain of ground coffee contains (at last count) some 2,000+ recognised chemicals.

Those chemicals have to leach out from each grain into the hot pressurised water being forced past it.

I had always imagined that when you made coffee, the very hot water would flow from top to bottom of the coffee basket. It would diffuse through nice and evenly, kissing up against ALL of the grains of coffee. The result would be a democratic and fair removal of coffee goodness from all the grains.

Surely, a finer grind gives more surface area, for the hot pressurised water to give more coffee goodness and a wonderful taste?

No.

“Random clumping” of coffee grains is the enemy of consistency. The combination of fine grind and high water pressure “chokes” up the espresso machine.

Espresso machine dispensing two cups of coffee

When the coffee grains are too fine, they “wedge” in between the bigger particles. The hot pressurised water randomly coagulates some of the coffee grains into several almost-solid lumps. The water then takes the path of least resistance, opening up channels around that solid lump. The hot water gushes through the channels that it forced open – unfortunately bypassing many coffee grains.

You get very little coffee goodness from those coffee grains that had coagulated into a lump. Even worse, you extract too much of those 2,000+ chemicals from the ground coffee lining the channels.

For a more perfect and consistent espresso, use ¾ as much coffee (15 grams, not 20), and grind the coffee beans less finely. In addition, run the water at a lower pressure (6 atmospheres, not 9) and run the espresso for a shorter time (15 seconds).

Of course, the proof is in the drinking. Already, some baristas are saying that this new regime produces espresso that is better tasting and more consistent, from one cup to the next.

It's not exactly an independent double-blind trial, but after 600 years of taking a shot in the dark we might be on track for the perfect espresso every time.

Written by Dr Karl Kruszelnicki

Julius Sumner Miller Fellow, School of Physics

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