Influence of particle size distribution on espresso extraction via packed bed compression

Abstract

Espresso extraction is a diffusion-convection process involving both intra-granular pores, and the intergranular pores of the packed bed. Bed hydrodynamics and overall end-cup quality are governed by the bulk porosity, set by the dry coffee bed compression before extraction. In this work, the impact of the initial bed compression on the water permeability during espresso extraction is studied. Coarse fractions of the bimodal coffee particle size distributions were characterized by the particle mean size α and the size uniformity β of the distribution. It was found that under the same axial compression lower bed porosities were obtained with decreasing size uniformity β, even at larger mean size α. A model was developed to predict the dry packed bed porosity. Extraction experiments confirmed that low permeability desired during espresso brewing was found not to be exclusive to distributions with low mean particle sizes. Rather, it can also be achieved by tuning compression force, α, and β together. A modified Kozeny-Carman expression was derived to explain and predict the observed permeability results.