Structure and rheological properties of acacia gum dispersions

The structure of acacia gum molecules was determined using size exclusion chromatography coupled to multi-angle laser light scattering, refractometry and viscosimetry. Results revealed the presence of many molecular species, including large aggregates, the arabino-galactan protein fraction (AraGP), the arabino-galactan fraction (AraG) and the glyco-protein fraction (GP). The M,, of the two major fractions AraGP and AraG were 2.3 10(6) and 2.7 10(5) g mol(-1), respectively. The [eta], R-g and R-h of the acacia gum sample were 0.23 dl g(-1), 14.2 and 11.9 nm, respectively. From the determination of R-g/R-h ratio (1.2) or the exponent nu (0.55) of the power-law relationship between R-g and M-w, it was tentatively suggested that acacia gum molecules displayed a random coil shape. Flow behaviour and viscoelastic properties of acacia gum dispersions at pH 4.2 were determined using shear-controlled rheometers with cone-plate or coaxial cylinders measuring systems. Acacia gum dispersions at concentrations between 3 and 32 wt% showed shear-thinning flow behaviour at shear rates < 10 s(-1). Time-dependent thickening flow behaviour was also demonstrated on 3 wt% acacia gum dispersions at shear rates < 1 s(-1). The effect almost vanished at gum concentrations greater than or equal to 20 wt% but increased adding dodecan onto samples to prevent dehydration. The eta(a) of acacia gum dispersions increased after ageing in the measuring devices and a stress overshoot was noticed during the measurement of eta(a). Acacia gum dispersions displayed structured liquid viscoelastic properties with a slope in the log-log plot of G' and G" = f (frequency) traces equal to 1.4 and 0.8, respectively. After ageing, dispersions displayed solid-like mechanical properties with G' above G" over the entire range of frequencies selected. These unusual results were thought to be due to the known surface properties of acacia gum molecules, especially to the ability of the gum to form elastic films at air/water or oil/water interfaces. These features were confirmed using a dynamic drop tensiometer by measuring during 1 h the interfacial dilational modulus \is an element of\ at air/water or dodecan/water interface. Our results indicated that an interplay could exist between bulk and surface theological properties of acacia gum dispersions. Whether such a trend is only a characteristic of acacia gum or could be generalized to other surface active molecules remains to be studied. (C) 2002 Elsevier Science Ltd. All rights reserved.