The science behind a perfect cup of coffee

How often have we found ourselves with eyes half-open, scrambling to make a quick cup of coffee before logging on to that zoom lecture? And then dissatisfied and disgruntled, wondering if it’s worth ordering from your favourite café or to just pack up and head to Starbucks? Or why even the coffee from the same place can taste different throughout the week? A recent study by mathematician Jamie Foster and chemist Christopher H. Hendon aims to mathematically explain how to brew the perfect cup of espresso each time. By resolving the inherent inconsistency of common brewing methods, their findings can lead to baristas not only brewing a delicious cup of coffee but also reducing waste and increasing their profits!

Broadly, the quality of a cup of coffee is based on the coffee’s origin, the roast, the water chemistry, and the brewing method. This study focuses on improving quality through the coffee’s brewing method, specifically the espresso brewing method. Espresso is complicated method as it requires careful measurements, but it is quite popular as it forms the basis of cappuccinos and lattes. Multiple parameters control the balance of thousands of chemicals to brew an espresso. Baristas have to make decisions about the quantity of coffee and water to use, how finely to grind the coffee, the temperature and water pressure of the espresso machine. Finally, to make the espresso, hot water is forced through the finely-ground bed of coffee. However, despite following all the correct steps, there may be large variations in taste between different final cups of coffee. This is where Foster and Hendon formulated a mathematical model to create a delicious reproducible cup of coffee each time.

They first simulated brewing espressos in realistic café conditions to predict the amount of solid coffee that ultimately ends up dissolved in the cup. This percentage is called the extraction yield and is the key metric to assess coffee recipes in the industry. They found that their prediction model is almost always accurate, except for when the coffee is ground very finely. As the water flow through the espresso bed is unpredictable, sections of the bed become clogged resulting in parts of the coffee that are under-extracted (low extraction yield) or over-extracted (high extraction yield). Hence, consistency is determined by examining the extraction yields of different shots of espresso. They discovered that to make consistently tasty brews, baristas should use less coffee and instead grind the coffee to be marginally coarser. The coarse grind and reduced coffee mass would allow the hot water to flow marginally faster, making the extraction more uniform and predictable.

As not everyone may enjoy the same flavour profile each time, the researchers also suggest some methods for the baristas to navigate different flavours within their coffee. This includes combining two shots of different extractions, or offering a selection of different roasts to accommodate different flavours. By following the researchers’ findings, baristas can reduce their coffee waste by up-to 25% per espresso shot without compromising on quality. This reduces costs thereby leading to higher profits, and is also better for the environment as it encourages using less coffee each time.

Further Readings:

Brewing a better espresso, with a shot of math

Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment

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