Quantitative contactless photothermal monitoring of drying in foodstuff materials

Abstract

In this work a quantitative method for the study of drying in foodstuff systems is presented. This study relies on the fact that changes in moisture content influence the effective thermal transport properties of porous or fibrous materials. Here, we analyze frequency dependent thermal wave signals recorded on the basis of infrared detection at several drying states by considering that the foodstuff system is not perfectly opaque at the surface. The consideration of transparency allows an asymptotic linear analysis that gives direct information about combinations of optical and thermal parameters. This therefore simplifies a further theoretical numerical approach for the signals within the scope of a two-layer model. Under the condition of constant optical properties at the system surface and neglecting the moisture dependence of thermal diffusivity in comparison with changes induced upon the thermal effusivity, the number of effective parameters necessary to follow the drying progress reduces to two: the thermal diffusion time at the drying surface layer and the thermal effusivity of the bulk.