Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment

Abstract

Espresso is a beverage brewed using hot, high-pressure water forced through a bed of roasted coffee. Despite being one of the most widely consumed coffee formats, it is also the most susceptible to variation. We report a novel model, complimented by experiment, that is able to isolate the contributions of several brewing variables, thereby disentangling some of the sources of variation in espresso extraction. Under the key assumption of homogeneous flow through the coffee bed, a monotonic decrease in extraction yield with increasingly coarse grind settings is predicted. However, experimental measurements show a peak in the extraction yield versus grind setting relationship, with lower extraction yields at both very coarse and fine settings. This result strongly suggests that inhomogeneous flow is operative at fine grind settings, resulting in poor reproducibility and wasted raw material. With instruction from our model, we outline a procedure to eliminate these shortcomings. The modern coffee market aims to provide products which are both consistent and have desirable flavour characteristics. Espresso, one of the most widely consumed coffee beverage formats, is also the most susceptible to variation in quality. Yet, the origin of this inconsistency has traditionally, and incorrectly, been attributed to human variations. This study's mathematical model, paired with experiment, has elucidated that the grinder and water pressure play pivotal roles in achieving beverage reproducibility. We suggest novel brewing protocols that not only reduce beverage variation but also decrease the mass of coffee used per espresso by up to 25%. If widely implemented, this protocol will have significant economic impact and create a more sustainable coffee-consuming future. Coffee extraction performed in an espresso machine is a process that depends on a delicate interplay between grind setting, coffee mass, water pressure and temperature, and beverage volume. Using a mathematical model based on the transport of solubles through a granular bed, paired with cafe-setting experiments, we elucidate the origin of inconsistencies in espresso production. Informed by the model, a protocol is proposed to systematically reduce coffee waste and beverage variation, resulting in highly reproducible shots.