Advantages of ultrasonic-assisted extraction over hot water extraction for polysaccharides from waste stems of Rubia cordifolia L.: A comprehensive comparison of efficiency, structure, and bioactivity.

Rubia cordifolia L. (RCL) is a plant with both commercial and medicinal values, widely distributed worldwide. Its roots have been used to treat various diseases, including abnormal uterine bleeding, arthritis, and kidney stones. However, its stems are often overlooked and regarded as waste. This study employed response surface methodology to optimize the ultrasonic-assisted extraction (UAE) process for polysaccharides from RCL waste stems (RCLP), and compared energy consumption and carbon emissions with hot water extraction (HWE). The underlying mechanism of UAE for polysaccharide extraction was further explored. Additionally, the physicochemical properties and in vitro activities of RCLP extracted by UAE (UAE-P) and HWE (HWE-P) were evaluated and compared. The results showed that the optimal UAE conditions were as follows: temperature of 52.5 °C, extraction time of 43 min, power of 310 W, and liquid-to-solid ratio of 32 mL/g, yielding 5.97 ± 0.34 % of RCLP. Compared with HWE, UAE significantly improved extraction efficiency, with a 640.18 % increase in production rate and a 93.23 % reduction in specific energy. UAE promoted the exposure of cell wall functional groups (e.g., C=O, C-O-C) and disintegration of the crystalline structure, thereby enhancing extraction efficiency. UAE-P exhibited higher uronic acid content, smaller and more uniform particle size, and better stability than HWE-P. These structural characteristics were significantly correlated with superior in vitro antioxidant and hypoglycemic activities, as well as stronger bile salt binding capacity. In conclusion, this study highlights that UAE is a feasible method for efficiently extracting high-value polysaccharides from RCL waste stems.