Effects of ultrasound-assisted osmotic dehydration as a pretreatment and finish drying methods on the quality of pumpkin slices

Abstract

In this study, ultrasound-assisted osmotic dehydration (UAOD) pretreatment in combination with different drying methods (hot-air, vacuum, and freeze-drying) has been applied in order to achieve better quality of dried pumpkin slices. UAOD pretreatments were carried out in sucrose solutions of 40 and 60% for 40, 80, and 120 min. The solution concentration and pretreatment time had significant effects on weight reduction, water loss, and solid gain of pumpkin. The effects caused by UAOD pretreatment on L*, a*, b*, chroma, and hue values of dried samples varied depending on the drying method. UAOD resulted in lower rehydration values. Peleg model was used to describe the rehydration process. Freeze-drying method provided superior color and rehydration ability but required the longest drying time. UAOD shortened the hot-air and vacuum drying times; however, it had negative impact on freeze-drying time. Scanning electron microscopy technique was used to analyze the microstructure of samples.

Practical applications

Water loss during UAOD pretreatment was considerably affected by osmotic solution concentration and pretreatment time. UAOD pretreatment may be used to shorten the processing time during hot air drying and vacuum drying to some extent, and it may have a positive effect on minimizing energy cost. However, this does not apply to freeze drying method. The use of UAOD as a pretreatment in the drying of pumpkin provided darker and more red dried products and adversely affected the rehydration ability of dried samples. The UAOD pretreatment conditions applied in this study cannot be suggested as a complementary treatment that may be applied prior to drying of pumpkin. However, UAOD may be considered as an effective method for the production of partially dried product since it can remove up to 36.4% of water in the fresh pumpkin.