Ultrasound-assisted hot water extraction of yellow leaves of Pu’er tea polysaccharides: Extraction process, characterization, antioxidant and hypoglycemic activity

Yellow leaves of Pu'er tea are classified as lower-grade Pu'er tea, and polysaccharides being its principal active components. Pu'er tea polysaccharides (PTPS) were optimally extracted from yellow leaves of Pu'er tea using ultrasound-assisted hot water extraction with the following parameters: solid-to-liquid ratio of 1:25 (w/v), ultrasonic temperature of 70 °C, ultrasonic power of 500 W, and ultrasonic time of 40 min. Two fractions, PTPS-4A and PTPS-5A, were clearly isolated and purified. Their antioxidant and hypoglycemic effects were compared in a lab after examining their structural features. PTPS-4A and PTPS-5A were mostly composed of fucose, rhamnose, arabinose, galactose, glucose, mannose, and glucuronic acid. They manifested as flexible chains and assumed a linear spline curve suggestive of an earthworm-like conformation. The nuclear magnetic resonance investigation found α- and β-glycosidic linkages (δ4.30-5.95 ppm and δ4.29-4.97 ppm, respectively) in PTPS-4A and PTPS-5A. PTPS-4A had enhanced DPPH and hydroxyl radical removal, with IC₅₀ values of 1.79 ± 0.07 mg/mL and 6.42 ± 0.16 mg/mL, respectively. The intermediate concentration value of 5.22 ± 0.08 mg/mL of PTPS-4A indicated a stronger inhibitory impact on α-glucosidase. PTPS-4A and PTPS-5A did not affect α-amylase in any way. The PTPS-4A fraction from this study may be further processed to create polysaccharide-based hypoglycemic and antioxidant products.