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

• Published in: Journal of food engineering 2015-04, Vol.150, p.106-116
• Main Authors: Corrochano, B.R., Melrose, J.R., Bentley, A.C., Fryer, P.J., Bakalis, S.
• Format: Article
• Language: English
• Subjects: Coffee; Coffee extraction; Espresso coffee; Extraction; Flow rate; Mass transfer; Mathematical analysis; Methodology; Packed bed; Packed beds; Permeability; Pressure drop; Under/over extraction
• ISSN: 0260-8774; 1873-5770
• DOI: 10.1016/j.jfoodeng.2014.11.006

•A methodology to estimate permeability of coffee packed beds was developed.•The relation between permeability and particle size was dependent on bed bulk density.•Bed bulk porosity was assessed with a combination of porosimetry and pycnometry.•Conventional Kozeny–Carman equation over-predicted experimental results.•Bed consolidation had a larger effect than a mere decrease in bed 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.