Fractionation and physicochemical characterization of psyllium gum

Abstract

Psyllium gum has been used in the landscape industry as an environmental friendly natural binder. This research is focused on the basic chemical and some physical properties of psyllium gum. Psyllium husk was extracted with hot water (80 °C), and with 0.5 M NaOH, 1.2 M NaOH and 2.0 M NaOH solutions (at room temperature), respectively, to produce a series of psyllium gum fractions labeled as water extractable (WE), 0.5 M alkali extractable (AES 0.5), 1.2 M alkali extractable (AES 1.2), and 2.0 M alkali extractable (AES 2.0). The alkaline extracted solutions were neutralized, during which, a gel-like precipitate was observed in the 0.5 M NaOH extracted solution, which was then separated by centrifugation to give a soluble fraction (AES 0.5) and a gel fraction (AEG 0.5). The yields of fractions WE, AES 0.5 and AEG 0.5 were 18.6%, 8.7%, and 61.4%, respectively, based on the dry weight of husk. The combined yields from the 1.2 and 2.0 M NaOH extracted fractions were less than 1.5%. Monosaccharide analysis revealed that WE, AES and AEG contained mainly xylose and arabinose. Substantial amounts (∼15%) of uronic acids were found in the WE and AES 0.5 fractions compared to AEG 0.5, which only contains neutral sugars. Methylation analysis proved that WE and AEG 0.5 basically contain 1 → 4) and 1 → 3) linked β- d-xylopyranosyl residues in the main chain. The side chains are primarily composed of terminal arabinose and xylose connected to the main chain by O-3 and/ or O-2 linkage. In contrast, the backbone chain of AES 0.5 fraction was mainly composed of 1 → 4 linked β- d-xylopyranosyl residues and the side chains only contained arabinosyl residues where were linked only through the O-3 position of the 1 → 4 linked β- d-xylopyranosyl residues. Further study was carried out to evaluate the particle size of the molecules in solutions by dynamic light scattering (DLS). It was found that these fractions were very soluble in 1 M NaOH solutions, and the fraction AES 0.5 and AEG 0.5 were stable in such alkaline conditions, but slight degradation occurred for WE with time. Fraction WE also dissolved well in water, however, aggregation formed over storage. The apparent hydrodynamic diameters ( d h) determined by DSL were over 100 nm for all three fractions. Theses gums are of high molecular weight and can be expected to be good viscosity enhancers. The current study provided new structural information on psyllium gum, which will be helpful for explaining its unique functional properties.