Figure 3
Distribution of coffee in a coffee grain. The coffee grains make up a volume fraction \(1-\phi_{h}\) of the bed. The grain volume consists of (intragranular) pores of volume fraction \(\phi _{v}^{*}\) and solids of volume fraction \(\phi_{s}^{*}=1-\phi_{v}^{*}\). The schematic shows how the solid volume fraction is divided (intragranular pores are not represented for clarity). In diagram (a), the solid is divided into a soluble volume fraction \(\phi_{c}^{*}\) and an insoluble volume fraction \(\phi_{s,i}\). In diagram (b), this soluble volume fraction is broken into a volume fraction near the surface of grains \(\phi_{s,s}^{*}\) and a volume fraction in the kernels (bulk) of grains \(\phi_{s,b}^{*}\).