Microwave and spouted bed drying of frozen blueberries: The effect of drying and pretreatment methods on physical properties and retention of flavor volatiles

Frozen blueberries (Vaccinium corymbosum L cv. 'Elliott') were dried in a microwave and spouted bed combined dryer (MWSB) at 70C air temperature and 3.7 W/g microwave power (wet material). The effect of pretreatment using a 2.5 % Ethyl Oleate & 0.2 NaOH dipping solution followed by sucrose osmotic treatment was investigated. The drying kinetics of MWSB drying was compared with spouted bed (SB) drying with dipping treatment, and with tray drying. The rehydration ratio, the color, and the bulk density of MWSB dried blueberries were compared with those of freeze, tray, and SE drying. The drying time needed to reduce blueberry moisture content from 82.5 % to 15 % (wet basis) using MWSB drying was 1/19 and 1/24 (with and without pretreatment) of the time for tray drying. The MWSB drying resulted in a low bulk density and more reddish and less blue color compared with other methods. MWSB dried frozen blueberries exhibited a higher rehydration ratio in short soaking times. Analysis of flavor volatiles by GC/MS identified ten heat-generated compounds. Microwave heating generated three unique flavor compounds (2-Butanone, 2-methyl butanal, and 3-methyl butanal). Freeze-dried frozen blueberries lost several flavor compounds including the typical blueberry aroma, the 1, 8-Cineole.