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

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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 10−13–10−14 m2. 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.


Original languageEnglish
Pages (from-to)106-116
Number of pages11
JournalJournal of Food Engineering
Early online date22 Nov 2014
Publication statusPublished - 1 Apr 2015


  • Coffee extraction, Espresso coffee, Under/over extraction, Packed bed, Permeability