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

In this work, purified pectins from Araçá fruits (Psidium cattleianum 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 (1)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(−1)); and the CaCl(2) concentration (0.1–1 mol L(−1)) used for dialysis. The critical pectin concentration for homogeneous gelation was above 2 g L(−1) when formed against 1 mol L(−1) CaCl(2). The elastic modulus (G′) increased with pectin concentration following the relationship G′~C(2.8) 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(2) down to 0.1 mol L(−1) had a higher degree of swelling, indicating electrostatic repulsions between charged chains, thus, counterbalancing the Ca(2+) 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.