Molecular size and aggregation behavior of Erwinia gum in aqueous solution

Abstract

Erwinia (E) gum, a stabilizer and thickening agent of food, is composed of glucose, fucose, galactose, and glucuronic acid (1: 0.1: 0.05: 0.3 by molar ratio). The apparent weight-average molecular weight M w and intrinsic viscosity [η] in 0.2 M NaCl aqueous solution were measured to be 7.83 × 10 5 and 268 mL g -1 , respectively, by light scattering and viscometry. The aggregation behavior of E gum in aqueous solution was investigated by gel permeation chromatography (GPC) and dynamic light scattering. The results showed that 7.5% E gum exists as an aggregate, whose diameter is 12 times greater than single-stranded chain, in aqueous solution at 25°C, and the aggregates' content decreased with increasing temperature or decreasing polymer concentration. The aggregates at higher temperature were more readily broken than in exceeding dilute solution. GPC analysis proved that a significant shoulder, corresponding to a fraction of higher molecular weight due to chain aggregation, appeared in the chromatogram of E gum in 0.05 M KH 2 PO 4 /5.7 × 10 -3 M NaOH aqueous solution (pH 6.0) at 35°C, and decreased with increasing temperature, finally disappeared at 90°C. The disaggregation process of E gum in aqueous solution can be described as follows: with increasing temperature, large aggregates first were changed into the middle, then disrupted step by step into single-stranded chains.