ABSTRACT
The study aims to determine drying of sweet potatoes using multifrequency ultrasound (US) pretreatments (20, 40, and 60 kHz) at three different infrared (IR) drying temperatures (60, 70, and 80°C) and evaluate the phytochemical and textural quality of the dried product. Drying time was significantly decreased in moderate US frequency (40 kHz) at 70°C with the increasing drying temperature. Comparing to the fresh samples, the dried samples showed the highest amount of phytochemical contents. The antioxidant activity of the samples increased especially at 60 kHz and 80°C, while US-IR treatments shown a positive impact on total carotenoids contents and ß-carotene. For phenolic compounds, Ellagic acid and Rutin were quantified in higher amount while Quercetin-3-rhamnoside and Quercetin 3-ß-D-glucoside were two new compounds identified for the first time in sweet potatoes. FTIR spectra showed the successful synthesis of OH group and phenolics in samples treated with the US at 20 kHz. PRACTICAL APPLICATIONS: This study investigated the effects of multifrequency ultrasound with different infrared drying temperatures. The study provides evidence that infrared drying application in synergy with ultrasonic pretreatments can improve drying efficiency and food quality much better than using each method alone. Total phenolic contents and total flavonoid contents remained stable at US 40 kHz and 60°C conditions. The findings showed that moderate ultrasound frequency (40 kHz) at 60°C improved phytochemical properties while antioxidant activities showed better preservation response at 80°C with 60 kHz. In addition, the samples treated with the same US treatment at 40 kHz showed less cell breakage in SEM analysis.
