Efficient drying of laser-treated raspberry in a pulse-spouted microwave freeze dryer

Abstract

Pulse-spouted microwave freeze drying (PSMFD) has been shown to be more energy efficient than the conventional vacuum freeze dryer. PSMFD yields higher drying rate and better energy efficiency due to volumetric heating. Because of the mass transfer resistance due to the berry’s epidermal barrier during drying, the raspberry skin was subjected to laser pretreatment so as to perforate the surface with pre-selected number of fine holes generated by a CO2 laser. The perforations allow vapor generated within the bulk of the berry to escape with no resistance of the waxy skin. In addition to studying the effect of perforation parameters on drying time; therefore, quality indicators of the dried product (shrinkage ratio, hardness, rehydration capacity, color, flavor, and anthocyanin content) were also explored. The micropore parameters in experiments were set as follows: P-0 (no perforations), P-3 (12 perforations), P-6 (24 perforations), and P-9 (36 perforations). As expected, greater the number of perforations per berry shorter is the drying time, by up to 23.08%. Moreover, the shortened drying time has a positive effect on product quality as well. The shrinkage rate could be reduced by about 3.95% and the retention rate of anthocyanin could be increased by 20.02%.