Physicochemical, structural and functional properties of polysaccharides extracted from dried longan with different pretreatments: A comparison study

Polysaccharides are key bioactive compounds found in dried longan; however, the effects of pretreatments before drying on their properties remain unclear. The present study compared the physicochemical, structural and functional properties of polysaccharides derived from fresh longan (Fresh-P), unpretreated and dried longan (CK-P), and dried longan pretreated with blanching (BL-P), ultrasound (US-P), freeze-thaw (FT-P), and radio frequency (RF-P). Results indicate that US-P exhibited the highest total sugar (70.25 %), uronic acid (19.48 %) and particle size (180.80 nm), while the lowest levels of polyphenols (0.42 %), protein (0.45 %), molecular weight (1.95 x104 Da), and apparent viscosity among dried longan polysaccharides. BL-P and US-P showed distinct surface morphologies when compared to CK-P. The pretreatments did not significantly alter the main functional groups or the triple helical structure of the longan polysaccharides according to Fourier transform-infrared spectroscopy (FT-IR) and the Congo red test. However, both ultrasound and blanching enhanced the crystalline state of the dried longan polysaccharides, as evidenced by X-ray diffraction (XRD) analysis. Furthermore, FT-P demonstrated the highest antioxidant activity, while US-P exhibited superior probiotic activity. Additionally, US-P presented relatively strong alpha-glucosidase and alpha-amylase inhibitory activities among the dried longan polysaccharides. Collectively, these findings enhance our understanding of how pretreatments affect the properties of longan polysaccharides and provided scientific guidelines for the development of superior dried longan and its derivatives from the perspective of polysaccharides.