Modeling of the relationship between freezing kinetics and sublimation kinetics in coffee brews

Abstract

The aim of this work was to develop a model that relates freezing kinetics of coffee brews to their sublimation kinetics. This has been achieved by assuming that the mean size of the ice crystal formed during freezing is equal to the mean pore size left by the subliming ice. The sublimation kinetics and the heat and mass transport properties of the coffee brews were determined by carrying out experiments using brews with three different solids content (0.1, 0.2 y 0.4 kg solids / kg solution) and three different freezing temperatures (-18, -30 and -70 ºC). The sublimation model developed in this work is based on the well known uniformly-retreating-ice front (URIF) model and includes transient variations in physical properties and permeability. For the latter, a semi-theoretical equation has been developed in this work, which also includes the mean pore size. It is concluded that the mean hydraulic radius of ice crystals formed during freezing can be used to link freezing kinetics with sublimation kinetics and temperature patterns within the drying material, which can give considerable insight into the quality changes occurring in the process.