Critical examination of particle swelling during wetting of ground coffee

Swelling of roast and ground coffee particles during immersion in water was studied using laser diffraction and digital microscopy. In particular, swelling was investigated on a time scale typical of common brew methods (0.5–5 min). Time-dependent swelling behavior during brewing would change the packed coffee bed parameters and thus the dynamics of coffee extraction. The impact of roast level, decaffeination, water temperature, and water chemistry were examined. When accounting for the time dependency of particle entrainment into the recirculating water loop, laser diffraction did not detect statistically significant increase in particle size during wetting. In situ digital microscopy also observed no increase in particle diameter during wetting. These results suggest that individual coffee particles do not swell significantly during the time scale of typical brew methods. •Ground roast coffee particles release gas from their pores upon wetting.•Bubble formation impacts coffee size measurements in water.•Short-term apparent size increase of wet coffee is statistically insignificant.•None of the tested coffee/water combinations showed clear evidence of swelling.