Stabilization of emulsions by gum tragacanth (Astragalus spp.) correlates to the galacturonic acid content and methoxylation degree of the gum

Gum tragacanth samples from six species of Iranian Astragalus bush plants (“goat's-horn”) were evaluated for their emulsion stabilizing effects and their detailed chemical composition in order to examine any possible correlation between the make-up and the emulsion stabilizing properties of gum tragacanth. The six gum tragacanth samples were exudates from the species Astragalus parrowianus, Astragalus fluccosus, Astragalus rahensis, Astragalus gossypinus, Astragalus microcephalus, and Astragalus compactus. The six gum samples varied with respect to their levels and ratios of water-soluble (tragacanthin) and water-swellable (bassorin) fractions, their monosaccharide composition, methoxylation, and acetylation degrees. The gums from A. parrowianus and A. fluccosus had relatively high tragacanthin:bassorin ratios of ∼66:34 and ∼75:25, respectively, whereas in the other gums this ratio approached 50:50 (A. rahensis, A. microcephalus, A. compactus) or tipped toward higher bassorin than tragacanthin (A. gossypinus). The monosaccharide make-up of the six gums also varied, but all the gums contained relatively high levels of galacturonic acid (∼100–330 mg/g), arabinose (50–360 mg/g), xylose (∼150–270 mg/g), and galactose (∼40–140 mg/g), and also contained fucose, rhamnose, and glucose. The ability of the gums to act as stabilizers in whey protein isolate based emulsions varied. The best emulsion stabilization effect, measured as lowest creaming index ratio after 20 days, was obtained with the A. fluccosus gum. The emulsion stabilization effect correlated linearly and positively to the methoxylation degree, and galacturonic acid content of the gums, but not to acetyl or fucose content. A particularly high correlation was found between methoxyl level in the soluble gum part and emulsion stabilization. The work provides some important clues to the emulsion stabilization mechanisms in relation to the monosaccharide composition of tragacanth gums.