Granule size distribution and rheological behavior of heated modified waxy and unmodified maize starch dispersions

The effect of granule size distribution, supported by microscopic analysis, on the steady shear and dynamic shear rheological behavior of 5% gelatinized modified waxy maize starch (MWMS) dispersions heated at 80C for time periods from 0.5 to 30 min was examined and data obtained with 5% unmodified maize starch (UMS) dispersions was used to provide additional insight. The MWMS dispersions showed a faster and more extensive swelling than the UMS dispersions at 80C. They also exhibited rheopectic (anti-thixotropic) time-dependent and shear thinning with yield stress behavior. UMS dispersions showed shear thickening characteristics after short heating times that gradually turned to shear thinning as the heating time was increased. Dynamic tests on MWMS dispersions showed G' was about ten times higher than G ''. G' showed a small dependence on frequency over 0.6-63 rad/s while G '', after an increase at low frequencies, became less sensitive. The gelatinized MWMS dispersions can therefore be classified as 'weak gels. ' The flow behavior index of UMS dispersions was strongly dependent on the granule size distribution characteristics (average granule diameter and standard deviation), which are, in turn, dependent on the heating time. For MWMS dispersions no such relationship was observed, due to the different extent of swelling of starch granules and their rigidity.