The role of fines in espresso extraction dynamics

Abstract The impact of particle size distribution of coffee grounds on espresso extraction was explored. Finely ground coffee for espresso has a characteristically bimodal particle size distribution. For a given median grind size, different grinding technologies can yield a different share of fines (particles < 100 µm). We performed espresso extractions for a range of median particle sizes and by systematically varying the share of fines in the ground coffee by adding sieved fines to the coffee grounds. Dynamic beverage weights, extraction percentage, extraction time and dynamic headspace PTR-MS (proton-transfer mass spectrometer) analysis and sensory evaluation of the resulting brews were measured. We show that the share of fines plays an important role in the espresso extraction flow rate. An increase of share of fines decreases coffee bed permeability and leads to reduced flow rates and longer extraction times. Statistical model using partial least squares regression of the particle size distributions of coffee grounds confirms that previously an arbitrary choice of fines at below 100 µm is an appropriate one for characterizing coffee grounds. The PTR-MS analysis shows a non-linear increase of aroma compounds content in the final cup with increasing extraction yield. Our hypothesis is that both extraction efficiency and post-extraction evaporative losses of aroma compounds influence the final aroma compound concentrations in the cup.