Mass Transfer Effects of Particle Size on Brewing Espresso

The extraction process for coffee is complicated and serves as a great model to study mass transfer, fluid mechanics, and thermodynamics. In this paper, we studied the brewing of espresso both experimentally and through validation with a mathematical model. First, the particle size distribution of finely ground coffee grinds was determined using a particle size analyzer. The ground coffee was shown to have a bimodal distribution, with smaller particles (50 – 75 micron range, representing cells or cell fragments) mixed in with larger particles (200 – 400 microns). Researchers have posited that significantly different mass transfer mechanisms exist between the two particle size regimes, with fast dissolution occurring from the cells and a slower, diffusion limited mechanism dominating for the larger particles. To test this hypothesis, espresso was brewed using both normally ground coffee and coffee that was ground and then sieved to remove a significant portion of the larger particles. As the hydraulic permeability of this sieved coffee was too low, microspheres of inert glass (roughly 300 microns in diameter) were mixed into the fines to lower flow resistance. Different masses of espresso were then collected and converted to mass ratios by dividing by the original mass of coffee grinds. A refractometer was using to measure the total dissolved solids in the espresso and determine the degree of extraction. From these values, it was determined that the smaller coffee particles extract at a faster rate, and are capable of reaching higher levels of total dissolved solids, when compared to the normal grind sized espresso. High-Performance liquid chromatography and mass spectrometry were also performed to study whether chemical differences arose between the different brewing techniques. The results showed that the compounds extracted for both brews are mostly identical. Finally, data from these experiments was then used to test a recently developed mathematical model for espresso brewing.