Modelling swelling effects in real espresso extraction using a 1-dimensional coarse-grained model

Swelling of coffee particle is difficult to measure and control, but it could have significant effects on espresso extraction. In this article, we incorporate a particle-level swelling model to a one dimensional bed-level extraction model to investigate the effects of swelling on extraction. Realistic geometric parameters and brewing parameters are used. A small degree of swelling is assumed to be present (approximate to 3.6% in size). Our simulation results show that swelling only slightly affects the yield and the strength if the flow rate is fixed. However, when the pressure drop is fixed swelling will considerably enhance the strength at both fixed brewing time and fixed brewing mass. The finer the coffee particles the more pronounced the enhancement. By tuning the partition coefficient, the one-dimensional extraction model is shown to be able to capture very well the extraction kinetics of a fixing-flow-rate espresso machine in low beverage mass regime (M-c < 30 g). Our results suggest that better yield and strength control can be achieved by fixing the flow rate in an espresso machine. Moreover, it is indicated that when properly incorporated with relevant physical effects, numerical extraction simulations can be used to predict the extraction kinetics of espresso machine and guide their design and production.