A multi-stage combined heat pump and microwave vacuum drying of green peas

The effect of multi-stage heat pump fluidized bed atmospheric freeze drying (HP FB AFD) and microwave vacuum drying (MVD) on the drying kinetics, moisture diffusivities, microstructure and physical parameters of green peas was evaluated. The results were compared with those obtained for microwave vacuum drying (MVD) and hot air convective drying (HACD). In case of combined method, the initial drying rate was about 0.04 l/min. The application of MVD increased the drying rate to the values 0.08 l/min. The drying rates of green peas dried by MVD and HACD were 0.59 and 0.20 l/min, respectively. MVD samples were characterized by a structure with minimal changes in respect to fresh samples. However, HP FB AFD and MVD satisfied important requirements, such as high product quality (due to low material temperature during AFD and low pressure during MVD), and increased drying rates in the final stage due to application of microwave heating.

► The multi-stage drying of green peas has been developed.
► The effect of multi-stage drying on the drying kinetics has been studied.
► The effect of drying methods on the moisture diffusivities of samples has been analyzed.
► Structural changes occurring during drying at micro and macrolevel have been discussed.
► Heat pump/microwave vacuum drying represents an alternative method for food processing.