RHEOLOGY OF OKRA (HIBISCUS-ESCULENTUS L) AND DIKA NUT (IRVINGIA-GABONENSIS) POLYSACCHARIDES

Polysaccharide extracts were prepared from two traditional food thicke ners with extensive domestic use in central and western parts of Afric a: okra (Hibiscus esculentis L.) and the seed kernel from 'dika nut' ( Irvingia gabonensis). Both demonstrated typical polyelectrolyte behavi our in solution, and were therefore studied under fixed ionic conditio ns (0.1 M NaCl), yielding intrinsic Viscosities of [eta] = 7.6 dl g(-1 ) for okra and [eta] = 44 dl g(-1) for dika. Concentrated solutions ga ve mechanical spectra typical of entangled networks, with close Cox-Me rz superposition of eta(omega) and eta(gamma). The variation of 'zero -shear' specific viscosity with degree of space-occupancy (c[eta]) was also broadly similar to the general form observed for most disordered polysaccharides, but with greater separation of c and c** and steepe r slope of log eta(sp) vs log c above c (similar to 4.0 for okra and similar to 4.6 for dika, in comparison with the usual value of similar to 3.3). As found for normal disordered polysaccharides, the shear-th inning behaviour of dika gum could be reduced to a single 'master-curv e' for all concentrations above c*, but the absolute value of the ter minal slope of log (eta - eta(s)) vs. log gamma was unusually low (sim ilar to 0.58, in comparison with the normal value of similar to 0.76). Terminal slopes for okra gum were also unusually low, and varied syst ematically with polymer concentration. These departures from normal so lution properties are tentatively ascribed to compact macromolecular s tructures, coupled, in the case of okra gum, with a strong tendency to self-association. Copyright (C) 1996 Elsevier Science Limited.