You know coffee is serious business when mathematicians, physicists and materials scientists start working together on its protocols – and an international team at that.
Researchers from the US, the UK, Ireland, Australia and Switzerland are challenging common espresso wisdom, declaring that fewer coffee beans, ground more coarsely, is the key to a brew that is cheaper to make, more consistent from shot to shot, and just as strong.
Their findings, and supporting evidence, are presented in a paper in the journal Matter.
Conventional wisdom has it that a fine grind exposes more surface area to the brewing liquid, boosting the extraction yield – the amount of ground coffee that actually dissolves and ends up in the final drink.
However, when the researchers developed a mathematical model to explain the yield based on the factors under a barista’s control – such as the masses of water and dry coffee, the fineness or coarseness of the grounds, and the water pressure – it became clear that the relationship is more complex.
Grinding as finely as the industry standard suggests clogged the coffee bed, reducing extraction yield, wasting raw material, and introducing variation in taste by sampling some grounds and missing others entirely.
At a basic level, extraction yield depends on how water flows under pressure through a mass of coffee grounds, but that mass is made up of millions of individual grains of different shapes and sizes, jammed irregularly into a single filter bed.
“You would need more computing power than Google has to accurately solve the physics and transport equations of brewing on a geometry as intricate as a coffee bed,” says mathematician Jamie M Foster, from the University of Portsmouth, UK.
So the researchers turned to electrochemistry, likening how caffeine and other molecules dissolve out of coffee grounds to the way lithium ions move through the electrodes of a battery.
Modelling methodologies from battery work led to a rigorous coffee extraction mode capable of making powerful and testable predictions, the researchers say.
They stress, however, that they don’t want to reduce all of espresso making down to a single set of brewing conditions or a lone flavour profile.
“Though there are clear strategies to reduce waste and improve reproducibility, there is no obvious optimal espresso point,” says computational chemist Christopher Hendon from the University of Oregon, US.
“There is a tremendous dependency on the preferences of the person producing the coffee; we are elucidating the variables that they need to consider if they want to better navigate the parameter space of brewing espresso.”
Nick Carne is the editor of Cosmos Online and editorial manager for The Royal Institution of Australia.