Characterisation of commercial LM-pectin in aqueous solution

Fourteen commercial LM-pectin samples were investigated in this study. The degree of esterification (DE) varied from 9.6% to 42.0% and the degree of amidation (DA) from 0% to 25%. Chemical and structural characteristics were examined using atomic absorption (AA), dilute solution capillary viscometry, GPC-MALLS and dynamic light scattering. The intrinsic calcium was in the range of 47-1388 ppm, the intrinsic viscosity varied from 2.9 to 4.9 (dL/g) and the weight average molecular weight (M-w) from 113 to 290 (kDa). Most of the samples had Huggins constants of similar to 0.5. However, for samples having acidic pH, Huggins constant values greater than 1, which can be as an indication of aggregation, were obtained. The high Huggins constant value could be reduced to similar to 0.5 by the addition of 3 M urea, indicating that the aggregation was stabilised by hydrogen bonding. Shear flow viscosity revealed three types of rheological behaviour. Type A showed pronounced shear thinning behaviour, which was reduced by the addition of hydrogen bonding breaking agent urea. Type B with intrinsic Ca of 1 mM and pH similar to 4 showed two shear thinning regions, with significantly enhanced shear viscosity upon addition of calcium. Type C showed the least aggregation due to its pH similar to 4 and low intrinsic Ca, but could be converted to type B upon addition of Ca. The effect of Ca on the rheological behaviour of types B and C was further confirmed by CaCl2 and Ca-chelating agent (EDTA). Temperature affected the molecular conformation of all types and most significantly type A by eliminating the hydrogen bonding. (C) 2012 Elsevier Ltd. All rights reserved.