Constrained viscous sintering of melting grains

Abstract

Viscous sintering of food grains can occur spontaneously and cause undesired caking or be induced to drive a change in food product structure. The dynamics of this phenomenon are still poorly understood, particularly when the sintering is coupled with heat or mass transfer. The aim of this work is to investigate experimentally the effects of heat transfer and grain size on the sintering dynamics of partially melting fat-based food products, i.e., dark chocolate. Two spherical particles were heated through forced convection using an air stream at different temperatures and velocities. Optical and IR thermal cameras were employed to capture the bridge growth and surface temperature evolution. Subsequently, the optical images were analysed to extract the evolution of the size of the bridge between the grains and a first order kinetics model was used to describe the sintering dynamics. The experimental observation showed that the viscous sintering is faster for smaller spheres, and at higher air flow temperature and velocity. When the spheres are heated from ambient conditions, a lag time is observed, which was found to be related to the time required by the material to reach a temperature at which its viscosity decreases to about 54 Pa·s allowing flow. Therefore, the lag time is temperature-dependent and decreases with decreasing particle size and increasing air temperature and velocity. Moreover, a semiempirical model involving a temperature dependent viscosity was proposed to describe the viscous coalescence. It is shown that considering some correction parameters accounting for the experimental conditions leads to an accurate prediction of the bridge growth. The model could be incorporated into the discrete element method to simulate the viscous sintering of a bulk of chocolate grains.