Effects of Ultrasound Pretreatment on Physicochemical and Biophysical characteristics of Dried Grapes using Hot Air and Freeze Dryer

Drying of grapes as one of the agricultural strategic products in Iran makes different challenges such as fruit color change, spoilage and contamination and time consumption of the drying process. In this study, effects of ultrasound pretreatment at two frequencies of 40 and 59 kHz on physicochemical and biophysical quality of dried grapes have been investigated using hot air and freeze dryer as the most appropriate dryer for drying fruits.

Experimental sample drying was carried out in hot air dryer with temperature of 70 °C and air speed of 1.5 m/s and freeze dryer with pressure of 0.1 mbar using ultrasonic pretreatment process at three levels (without ultrasonic pretreatment and pretreatments at 40 and 59 kHz). Data were statistically analyzed using Design Expert Statistical Software and graphs were drawn using Excel 2013.

Results showed that increasing ultrasonic frequency to 59 kHz and then drying with hot air until the product reached a moisture content of 5.5% based on the dry weight decreased the drying time (13 h), compared to the frequency of 40 kHz (drying time of 17 h) and the control sample (drying time of 18 h). In contrast, ultrasound included no effects on decreasing time in freeze drying. Moreover, ultrasonic pretreatment in the two types of dryers decreased the antioxidant activity (DPPH), compared to the control sample. Independent and interaction effects of the ultrasound pretreatment and dryer type on the EC50 of dried grape samples were significant at 99% probability level (p < 0.01). Dried samples in the freeze dryer included more desirable sensory characteristics than that the hot air dryer did. Hot air dryer included negatively significant effects on sensory characteristics at 99% probability level (p <0.01).

Regarding antioxidant activity, less shrinkage and overall acceptability of sensory characteristics, samples dried in the freeze dryer with 59 kHz pretreatment were more desirable than the samples dried in hot-air dryer.