The science behind the perfect espresso

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Espresso is one the most popular ways to drink coffee, but it is also one of the most complicated to make well. 

Skilled baristas know that creating the perfect complex flavour profile for a tasty shot of espresso is as much of an art as science. The process depends on a delicate combination of factors including: grind setting, coffee amount, water pressure, temperature and beverage volume.  

Despite the art of coffee making coming down to barista known-how, scientists are challenging current espresso production methods. New research suggests that processes are not cost effective and use unnecessary amounts of coffee beans, resulting in less than the best brew. 

Most people in the coffee industry use a ‘fine grind’ setting and a relatively large amount of coffee beans (about 20 grams) to produce espresso. The logic is, the finer the bean, the more surface area exposed to the brewing liquid, which should in theory enhance the ‘extraction yield’ - the percentage of ground coffee that actually dissolves and ends up in the final drink. 

This process creates a mix of bitterness and sour acidity that is unpredictable and variable in taste, according to scientists.

Researchers say that using fewer coffee beans and grinding them more coarsely will produce the best and most consistent espresso.

A new study published in the journal Matter found grinding coffee beans as finely as the industry standard suggested clogged the coffee portafilter, which as result reduced extraction yield, wasted raw material and caused a wide variation in taste in the final product.

"One way to optimise extraction and achieve reproducibility is to grind coarser and use a little less water, while another is simply to reduce the coffee mass. This way, all the cups of espresso will have the same flavour and intensity”, explained Christopher Hendon, a computer chemist at the University of Oregon and co-author of the study.

Aside from improved and more consistent flavour, there money to be saved by opting for a coarser grind. According to the study, reducing the amount of dry coffee per espresso from 20 to 15 grams per cup, taking into account the current price of roasted coffee beans, would save a small coffee shop a few thousand dollars a year and $1.1 billion (around one billion euros) a year if this reduction was rolled-out across the whole of the US coffee industry.

Reference: Michael I. Cameron, Dechen Morisco, Daniel Hofstetter, Erol Uman, Justin Wilkinson, Zachary C. Kennedy, Sean A. Fontenot, William T. Lee, Christopher H. Hendon, Jamie M. Foster. Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment. Matter, 2020; https://www.sciencedirect.com/science/article/pii/S2590238519304102

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