Ethanol-induced opposite viscosity trends in psyllium seed husk polysaccharides: The critical role of rhamnogalacturonan-I in aggregation behavior

Ethanol induction typically results in the contraction of the polysaccharide molecular chain and a decrease in viscosity. However, this study reveals an unexpected dual behavior in psyllium seed husk polysaccharides (PSHPs). The viscosity of the cold ethanol-water extracted polysaccharide (CEP, containing 9.42 % rhamnose and 7.40 % galacturonic acid) increased by 154 % in the presence of 25 % ethanol compared to its viscosity in aqueous solution. In contrast, under identical conditions, the viscosity of the hot ethanol-water extracted polysaccharide (HEP, containing 1.21 % rhamnose and 1.53 % galacturonic acid) decreased by 99 %. Through multi-scale characterization, including intrinsic viscosity ([η]), radius of gyration (R_g), atomic force microscopy (AFM), and rheology, we observed that both CEP and HEP exhibited chain contraction and aggregation under ethanol-induced conditions. However, significant differences in rhamnogalacturonan-I (RG-I) content resulted in distinct degrees of contraction and aggregation behaviors. Upon increasing the ethanol concentration from 0 % to 20 %, the R_g of CEP decreased by 30 %, whereas the R_g of HEP exhibited a more substantial reduction, by 71 %. For CEP, ethanol substantially mitigated the steric hindrance effect of RG-I, thereby enhancing intermolecular interactions and increasing the entanglement network density. In contrast, the pronounced chain contraction of HEP disrupted the original entanglement network structure. This RG-I-dependent aggregation behavior represents a significant addition to the traditional solvent effect theory and also provides a robust theoretical foundation for the diverse applications of PSHPs.