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How Do You Brew The Perfect Espresso? Ask A Bunch Of Scientists

It's a pressing question for the caffeine connoisseurs of the world, and an unlikely team of scientists thinks they've brewed the shot of your dreams. 

Despite being one of the most widely consumed cups of coffee, the path to perfecting an espresso isn't as straightforward as it may seem.

There is a lot to think about -- from the type of bean to the grind, to the water pressure and amount of water used.

But a group of mathematicians and physicists from Australia and across the world think they might just have it.

Teaming up with some baristas (for good measure), they developed a mathematical formula that challenges the commonly-held belief the perfect brew comes down to using lots of finely-ground coffee beans.

Instead, they think we should be doing the opposite.

What's the secret to a perfect brew? Use less coffee and grind it more coarsely. Image: Getty

"Most people in the coffee industry are using fine-grind settings and lots of coffee beans to get a mix of bitterness and sour acidity that is unpredictable and irreproducible," lead author and computational chemist Christopher Hendon said.

"It sounds counter-intuitive, but experiments and modelling suggest that efficient, reproducible shots can be accessed by simply using less coffee and grinding it more coarsely."

According to the scientists, the norm for brewing an espresso shot is to grind about 20 grams of coffee beans as finely as possible. The logic goes, the finer the grind, the greater the surface area of beans that are exposed to water when it's pushed through.

This means more flavour is extracted through the yielding process -- or the amount of grounds that dissolve and end up in the final cup of coffee.

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But the researchers found this method can clog up the coffee bed, meaning some grounds aren't reaching the water and the results are inconsistent.

Testing factors such as the amount of water and dry coffee, the fineness or coarseness of the grounds and the water pressure, they reached a recipe they claim maximizes extraction and ensures a consistent brew, from one cup to the next.

"One way to optimise extraction and achieve reproducibility is to grind coarser and use a little less water, while another is to simply reduce the mass of coffee," Hendon said.

The researchers also drew on electrochemistry to work out their extraction model, likening how caffeine and other molecules dissolve out of coffee grounds to how lithium ions move through the electrodes of a battery.

The findings, published in the Matter Journal on Thursday, suggest their approach has both sustainable and economic benefits.

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They claim dropping the single mass of dry coffee used from 20 grams to 15 grams could save a small cafe up to $1.1 billion per year.

At a time when coffee supply is under threat from climate change in historic production areas, the researchers said the approach could also help to reduce waste.

But they maintain their results are not a one-size-fits-all approach.

"Though there are clear strategies to reduce waste and improve reproducibility, there is no obvious optimal espresso point," Hendon said.

"There is a tremendous dependency on the preferences of the person producing the coffee."