Effect of xanthan on the small-deformation rheology of crosslinked and uncrosslinked waxy maize starch

Small-deformation oscillatory measurements have been used to characterise the effect of ordered xanthan (in 0.1 M KCl) on the rheological properties of gelatinised waxy maize starch (crosslinked and uncrosslinked). The uncrosslinked material shows a single DSC endotherm on heating, centred at similar to 70 degrees C. The overall enthalpy change on gelatinisation of the crosslinked starch is the same (Delta H = 13.2J/g), but occurs in two endothermic processes (centred at similar to 60 and similar to 74 degrees C), tentatively attributed to thermal dissociation of, respectively, chemically modified and native granule structure. The thermal transitions for both samples are complete by 80 degrees C, which was therefore chosen as the maximum temperature in the rheological investigations, to minimise loss of granule integrity and release of starch polysaccharides into the xanthan matrix. Both samples, after gelatinisation in water, gave gel-like mechanical spectra at volume fractions below 20%, suggesting association between the swollen granules, in addition to the steric interactions which become dominant at higher concentrations. The magnitude of the increase in moduli observed on progressive addition of starch (1-5 wt %) to solutions of xanthan (0.25 or 0.50 wt %) is far too great to be explained by increased concentration of the xanthan phase on swelling of the starch granules. Comparison with the concentration-dependence of moduli for starch alone, however, suggests that xanthan acts by promoting association between the gelatinised granules, possibly by a depletion flocculation mechanism. Copyright (C) 1996 Elsevier Science Ltd