Physicochemical properties and in vitro antioxidant and anti-inflammatory activities of graded polysaccharides from finger citron obtained by cascade membrane technology

Finger citron ( Citrus medica L. var. sarcodactylis Swingle), a common edible fruit, demonstrates strong potential for development as a functional food. Polysaccharides represent one of its major bioactive constituents; however, the structure–activity relationships of these polysaccharides remain unclear. A sequential membrane separation strategy was applied in the present work to fractionate finger citron polysaccharides into fractions of differing molecular weight. By successively passing the crude extract through ultrafiltration membranes with cut-off values of 100, 50, 10, and 1 kDa, four distinct fractions—designated FSP-1 through FSP-4—were isolated. Comparative analysis was subsequently performed to evaluate the physicochemical characteristics of each fraction alongside their capacities for antioxidant and anti-inflammatory action. FT-IR spectra confirmed that all four fractions displayed the typical absorption bands associated with polysaccharide structures; nevertheless, marked differences emerged among the fractions with respect to monosaccharide profiles and other structural attributes. Among them, FSP-1 showed the highest yield (50.41%), the largest particle size (153.55 nm), the highest relative molecular weight (106.04 kDa), the most favorable zeta potential (− 32.95 mV), as well as the highest total sugar content (67.31%) and uronic acid content (41.21%). In addition, all four fractions exhibited notable anti-inflammatory and antioxidant activities. Across the 25–200 μg/mL concentration range tested, the extent to which each fraction suppressed nitric oxide (NO) generation in LPS-stimulated RAW264.7 macrophages ranked as follows: FSP-1 (49.59%) > FSP-4 (29.63%) > FSP-2 (23.05%) > FSP-3 (22.50%). FSP-1 demonstrated the strongest hydroxyl radical ( · OH) scavenging capacity (IC 50 = 1.24 ± 0.24 mg/mL), as well as pronounced DPPH · and ABTS · radical scavenging activities. Furthermore, this fraction demonstrated superior efficacy in mitigating H 2 O 2 -triggered oxidative injury within Caco-2 cells relative to the others. This protective capacity of FSP-1 is plausibly explained by its ability to substantially boost the cell's intrinsic antioxidant machinery—encompassing catalase (CAT), superoxide dismutase (SOD), and glutathione (GSH)—while concurrently lowering levels of oxidative stress indicators such as malondialdehyde (MDA) and reactive oxygen species (ROS). These bioactivities may be associated with the combined compositional and physicochemical characteristics of FSP-1, including its relatively high galacturonic acid content and large particle size. The above findings collectively indicate that finger citron polysaccharides with different molecular weight ranges exhibit considerable variations in physicochemical properties and in vitro anti-inflammatory and antioxidant activities. Among them, within the tested concentration ranges, FSP-1 showed the most favorable overall bioactivity, suggesting its potential as a health-promoting ingredient for functional food and nutraceutical applications.