Modeling the drying kinetics of microwave vacuum drying of concentrated skim milk: correlation of dielectric properties, drying stages, and specific energy demand at pilot scale

This study establishes a correlation between dielectric properties, drying parameters, drying rate and energy demand for microwave vacuum drying (MVD) of concentrated skim milk (CSM). MVD was conducted using a EnWave nutraReV™ unit, at various specific power inputs (0.86, 1.29, 1.72 W g^-1), drying pressures (30 to 147 mbar), and CSM layer thickness (1 to 4 mm). The drying rate based on moisture ratio was estimated using the Page model. The specific energy consumption was lowest as CSM dried from 50 to 15% moisture, then rapidly increased as the drying rate decreased due to a low ε” at <15% moisture. The loss factor ε” of CSM at 2.45 GHz increased up to 35% total solids, then steadily decreased up to 95% solids. The specific electrical energy demand of the MVD unit, which accounted for all utility flows at full capacity, was 2.85 kWh kg^-1 water evaporated, with the magnetrons accounting for 2.14 kWh kg^-1. This study provides valuable data on drying kinetics and energy demand during MVD of CSM, and an approach that can help processors assess the energy efficiency of MVD for various food applications.