Influence of microwave assisted foam mat drying conditions on drying characteristics of raspberry puree

To study the heat and mass transfer characteristics of microwave assisted foam mat drying ( MFD) berry puree, raspberries were used as raw material and the two orthogonal rotation center combination design was used in this study. The effects of drying conditions on the temperature, moisture content and dielectric properties were analyzed based on the law of heat and mass transfer in MFD. In the initial stage of MFD, the dissipation of microwave energy related to the dielectric constant, dielectric loss factor increased inside berry puree.Till the drying temperature of 70 ℃, the moisture content of berry puree had no obvious change.In the middle stage of MFD, the dielectric constant and dielectric loss factor of berry puree tended increasing trend followed by a decreased.There was no significant change of temperature and the moisture content changed from90% to 40%.In the later stage of MFD, the dielectric constant, the dielectric loss factor and the microwave energy absorbed by material decreased, and the moisture content of berry puree slowly decreased till 15%, but the temperature of berry puree tended to increasing trend.High efficiency of drying treatment could be achieved for the berry puree with high viscosity in the conditions of MFD.