Quality evaluation of different pre-treatments and combined convective-freeze drying of sour cherry (Prunus Cerasus L.)

In this study, the influences of various pre-treatments and combined drying (CD) on the drying kinetics and quality parameters of sour cherry were investigated. Prior to freeze drying (FD), sour cherry was pre-treated by blanching in boiling water (100 °C, 3 and 6 min), immersion in sugar solution (20% w / w , 10 and 20 min) and blanching (100 °C) in sugar solution (20% w / w , 3 and 6 min). Freeze-drying of raw samples was taken as a control. Combined drying (CD) can be accomplished in two distinct ways: freeze drying assisted with hot-air drying application (FD-HAD) and hot-air drying followed by freeze drying (HAD-FD). The quality of the dried sour cherry was evaluated in terms of water activity, color, texture and rehydration. Our research results have shown that all of the pre-treatments can effectively reduce the freeze drying time, the best treatment methods are FD-HAD, HAD-FD and blanching in boiling water (6 min). CD involves much less drying time and energy consumption than conventional freeze drying methods. The highest values of hardness and rehydration were found in blanching in sugar solution with water at 100 °C for 6 min. The better color retention was observed for samples pre-treated with blanching in boiling water (6 min).