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A new methodology to estimate the steady-state permeability of roast and ground coffee in packed beds

Corrochano, B.R., Melrose, J.R., Bentley, A.C., Fryer, P.J., Bakalis, S.
Journal of food engineering 2015 v.150 pp. 106-116
brewing, equations, mass transfer, methodology, particle size distribution, permeability, porosity
In an espresso-style extraction hot water (90±5°C) is driven through a coffee packed bed by a pressure gradient to extract soluble material from the coffee matrix. Permeability is a key parameter affecting extraction as it determines the flow rate through the bed and hence brewing and residence time. This may alter bed-to-cup mass transfer and therefore impact brew quality.In this work a methodology that will allow estimation of the permeability of coffee packed beds in steady-state was developed. Fitting measured flow rate – pressure drop data to Darcy’s law resulted in permeability values in the range of 10−13–10−14m2. Disagreement between the experimental and theoretical permeability, as estimated from dry measurements of particle size distribution and Kozeny–Carman equation, was found. Bed consolidation may have a larger effect on the packing structure than the mere decrease in bed bulk porosity. The Kozeny–Carman equation, corrected with a porosity-dependent tortuosity according to a power law, gave a good fit of the data.