Effect of ultrasound pretreatment and drying air temperature on the drying characteristics, physicochemical properties, and rehydration capacity of raisins

One of the most significant dried foods is raisin, used directly as ingredients in confectionary and in the form of raisin paste used in sauces, fillings, baked goods, microwaveable coatings, and other culinary products to color them naturally. The purpose of this study is to investigate the effects of ultrasound pretreatment (53 kHz, 300 W, 30 °C, 300 s, in a mixture of 5% K 2 CO 3 + 1% OO solution) and drying air temperatures (40–60 °C) on the drying behavior, color values, physicochemical properties (moisture content, VCC, TPC, RSC, and antioxidant capacity (DDPH)) and rehydration capacity of Sultani Çekirdeksiz grapes. Both the drying kinetic (drying time, drying rate, and effective moisture diffusivity) and the quality characteristics of grapes were significantly improved by ultrasound pretreatment ( p < 0.05). In addition, the vitamin C (0.75–3.86 mg/g) and total phenolic content (1.31–1.80 mg GA/kg) of raisins were highly protected with the high drying temperature and ultrasound pretreatment. The antioxidant capacity (1.15–5.50 mM Trolox) of samples increased with drying temperature whereas the antioxidant capacity decreased with ultrasound application. The rehydration time of all samples was found around 800 min. As a result of this study, it can be said that the ultrasound application may be used successfully for drying grapes and that a greater degree of the nutritional value of the grapes can be maintained. Graphical Abstract