Effect of microwave vacuum puffing conditions on the dielectric properties of berries.

The objective of this study was to study the effect of microwave vacuum puffing (MVP) conditions on dielectric properties of berries. A central composite response surface experiment, with four factors and five levels, was carried out to assess the effect of puffing conditions on the dielectric properties. The microwave volumetric heat, dielectric constant and dielectric loss factor were, accordingly, evaluated based on the dynamic dielectric property index model of berries. As a result, the initial moisture content had significant effects on the dielectric constant and dielectric loss factor of the berry crisps. Specifically, in the early stage of puffing, the dielectric constant decreases with increasing initial moisture content. However, in the later stage of puffing, the dielectric constant first decreases and then increases as the initial moisture content rises. Meanwhile, the dielectric loss factor increases consistently with increasing initial moisture content throughout the puffing process. The MVP conditions exert a complex effect on dielectric properties of berries. Based on these findings, a theoretical for the dynamic processing mode and equipment design for MVP of berries can be provided.