Changes in particles of coffee powder and extensions to caking

Changes in the projected area (PA) of particles of an instant coffee powder were followed in real time by videomicroscopy and image analysis. Particles were placed inside a transparent acrylic chamber (7 × 7 × 3 cm) where the relative humidity RH (43%, 52%, 67% and 74%) and temperature T (20, 25 and 30 °C) were controlled and measured in the head space. A “caking index”, ϕ, was defined as the change in PA at any time t with respect to the initial PA. At any temperature, there was an increasing effect in ϕ as RH increased and the shape of ϕ versus time curves closely followed that of caking, measured by sieving techniques. The exponential kinetic model adequately fitted the data. The PA method could be used as a first approximation to predict the tendency to caking of amorphous particles.