Rheology of okra (Hibiscus esculentus L.) and dika nut (Irvingia gabonensis) polysaccharides

Abstract

Polysaccharide extracts were prepared from two traditional food thickeners with extensive domestic use in central and western parts of Africa: okra (Hibiscus esculentis L.) and the seed kernel from 'dika nut' (lrvingia gabonensis). Both demonstrated typical polyelectrolyte behaviour in solution, and were therefore studied under fixed ionic conditions (0.1 M NaCl), yielding intrinsic viscosities of [77) = 7.6 di g1 for okra and [77) = 4.4 di g1 for dika. Concentrated solutions gave mechanical SJ?ectra typical of entangled networks, with close Cox-Merz superposition of 17 (w) and 77(y). The variation of 'zero-shear' specific viscosity with degree of space-occupancy (c[77)) was also broadly similar to the general form observed for most disordered polysaccharides, but with greater separation of c' and c'* and steeper slope of log 17sp vs. log c above c' (~4.0 for okra and ~4.6 for dika, in comparison with the usual value of ~3.3). As found for normal disordered polysaccharides, the shear-thinning behaviour of dika gum could be reduced to a single 'master-curve' for all concentrations above c'*, but the absolute value ofthe terminal slope oflog (77 - 17s) vs. logy was unusually low (~0.58, in comparison with the normal value of ~0.76). Terminal slopes for okra gum were also unusually low, and varied systematically with polymer concentration. These departures from normal solution properties are tentatively ascribed to compact macromolecular structures, coupled, in the case of okra gum, with a strong tendency to self-association.