Influence of pH and salt on the rheological behavior of dispersions prepared from the galactomannans extracted from Gleditsia triacanthos

BARRERA G; SCIARINI LS; MOLDENAERS P; ITURRIAGA L; RIBOTTA PD

Cordoba

Congreso; VII Congreso Internacional de Ciencia y Tecnología de los Alimentos; 2018

Ministerio de Ciencia y Tecnología de la Provincia de Córdoba

The objective of this work was to study the effect of pH and salt on the rheological properties of dispersions prepared from the galactomannans extracted from Gleditsia triacanthos seeds. The yellowish powder obtained from the extraction of Gleditsia triacanthos milled-seeds with hot water and ethanolic precipitation was used for the experiments without any further pre-treatment. The endosperm extract from Gleditsia triacanthos (Gledi) was dispersed (liquidwas added to the solid; stirring: 30 min at 25°C + 1h at 90°C) in water (Gledi-H2O) and in aqueous solution of NaOH (5x10 -6 M; pH=8) (Gledi-NaOH), HCl (1x10 -4 M; pH=4) (Gledi-HCl) and NaCl (0.4M) (Gledi-NaCl) to obtain samples at 2% w/w. The pHs and salt concentration were chosen considering the values typically found in food matrices. The systems structure was evaluated by steady shear and oscillatory shear rheology. Steady shear-viscosity: 0.01-100 s-1; 50s at 100s-1; 100-0.01 s-1. Strain-sweep: 0.1 to 1000% strain at 1Hz. Frequency-sweeps: 0.01-100 Hz at 1% strain (LVR). Temperature sweep: 25-90°C 3°C/min; 90-25°C 3°C/min at 1Hz and 1% strain. Geometry: plate/plate PP25; Gap=1 mm. The samples showed a shear thinning behavior. The apparent viscosity of Gledi-NaCl, Gledi-HCl and Gledi-NaOH dispersion was higher than Gledi-H2O. In all samples, G'' was higher than G' during the strain sweep. The samples showed a linear viscoelastic behavior over a wide strain region. There were not practically differences between the samples in both moduli profile. Strain thinning behavior was observed in both moduli in all samples. G'' and G' moduli did not show crossover in any sample. The samples showed a concentrated macromolecular-solution behavior with viscoelastic-fluid behavior at low frequencies and viscoelastic-solid behavior at high frequencies. The cross-point frequency decreased as a consequence of lower pH and salt presence. Gledi-NaOH, Gledi-HCl and Gledi-NaCl dispersions showed higherG'' and G' moduli than Gledi-H2O. During the thermal process the samples showed a viscoelastic-fluid behavior. Both moduli decreased during heating and increased during cooling. The G' slope was higher than G'' during heating and cooling. The difference between G'' and G' increased during heating and decreases during cooling. The loss of polymeric interaction during heating was not recovered after cooling in any sample in the test conditions. Gledi-HCl, Gledi-NaOH and Gledi-NaCl dispersions had higher G'' and G' moduli than Gledi-H2O during all thermal processes. Gledi-NaCl and Gledi-HCl samples showed the lowest Ea, indicating that Gledi-NaCl dispersion was the less fluid material and the less sensitive to the T° changes during heating process. The viscosity at infinite T° of Gledi-NaCl and Gledi-HCl dispersion was higher than the other samples. The results suggested the formation of a more entangled polymer network as a consequence of the chemical-environment changes of the galactomannans aqueous-dispersions.