Development of a Microwave–Vacuum-Based Dehydration Technique for Fresh and Microwave–Osmotic (MWODS) Pretreated Whole Cranberries (Vaccinium macrocarpon)

Abstract

A bench scale microwave–vacuum (MWV) dryer was developed using a modified consumer-grade microwave oven. MWV dehydration was first tested as a standalone method on whole frozen–thawed berries. Subsequently, a new combination drying technique was developed employing microwave osmotic dehydration under continuous-flow medium-spray (MWODS) conditions together with MWV as a secondary drying operation. Fresh (frozen–thawed) and MWODS pretreated berries were dried under a range of MWV treatments employing continuous and decreasing microwave power settings (duty cycles). Initial microwave power density for all treatments was approximately 10.2 W/g and magnetron power-on and power-off times varied from 3 to 15 s and 27 to 15 s, respectively. Drying times to 20% (db) were recorded and energy consumption was calculated according to the total magnetron power-on time where overall it was found that drying times and energy consumption decreased with increasing MWV process intensity, where drying times for all MWV treatments were significantly shorter than those of conventional air drying. Drying kinetics were fit using two models (exponential and Page's empirical model), where Page's model better fit the experimental data. The quality of the berries was monitored visually through evidence of scorching in order to screen treatments and establish upper limits of treatment intensity for further studies.