Thin-layer modelling of the convective, microwave, microwave-convective and microwave-vacuum drying of lactose powder

Lactose-water samples were dried under selected convective, microwave, microwave-convective and microwave-vacuum conditions in an experimental system (2.45 GHz, 90 W). Irrespective of the drying technique, a typical drying profile, with a constant drying rate stage followed by two falling rate periods, was exhibited. The magnitude of the drying rate, however, was dependent on the convective air temperature and velocity, and system pressure. The experimental moisture loss data were fitted to selected semi-theoretical and empirical thin-layer drying equations. The mathematical models were compared according to three statistical parameters, i.e. reduced chi-square, root mean square error and residual sum of squares. The drying characteristics were satisfactorily described by the Page, Logarithmic, Chavez-Mendez et al. and Midilli et al. models, with the latter providing the best representation of the experimental data.