Rheological Behavior of Pectin Gels Obtained from Araçá ( Sabine) Fruits: Influence of DM, Pectin and Calcium Concentrations.

In this work, purified pectins from Araçá fruits ( Sabine) were obtained and characterized after partial demethylation. On each prepared sample, the carboxylic yield was obtained by titration, the degree of methylation (DM) by H-NMR, and the molecular weight distribution by steric exclusion chromatography (SEC). Then, the gelation ability in the presence of calcium counterions was investigated and related to DM (59-0%); the pectin concentration (2-10 g L); and the CaCl concentration (0.1-1 mol L) used for dialysis. The critical pectin concentration for homogeneous gelation was above 2 g L when formed against 1 mol L CaCl. The elastic modulus (G') increased with pectin concentration following the relationship G'~C in agreement with rigid physical gel network predictions. The purified samples APP and APP-A with DM ≥ 40% in the same conditions released heterogeneous systems formed of large aggregates. Gels formed against lower concentrations of CaCl down to 0.1 mol L had a higher degree of swelling, indicating electrostatic repulsions between charged chains, thus, counterbalancing the Ca cross-linkage. Compression/traction experiments demonstrated that an irreversible change in the gel structure occurred during small compression with an enhancement of the G' modulus.