Rheological behaviour of an amide pectin

Forced-oscillation measurements of aqueous dispersions of an amide pectin were carried out at pH 3.83 in the presence of 50% sucrose. The results show a predominant viscous behaviour corresponding to the so-called terminal region of the general mechanical spectrum of biopolymer solutions. The polymer chains have been described by the bead-spring model with dominant hydrodynamic interactions among beads. At pH 3.31, samples are more elastic than viscous and the mechanical spectra reveal an entanglement network (weak gel), although not dense enough to store the stress during a long period of time after having been deformed. At low pectin concentration, the plot of loss moduli versus frequency shows a minimum that suggest monodisperse junction zones and the existence of two regions with different relaxation times. The short relaxation process corresponds to movements of galacturonic units inside the junction zones (local configuration adjustment), and the longer one to movements of the junction zone as a whole (rearrangements of couplings through entanglement and cross-linking). The comparison of the results obtained here at different sucrose concentrations and temperatures with those for methoxy pectins, suggests that the hydrogen bonds formed by amide groups have an important role in the formation of the gel. The role of the hydrophobic interactions in the stability of the junction zones has also been discussed.