Gums tuning the rheological properties of modified maize starch pastes: Differences between guar and xanthan

The experimental setup aimed at gaining a more fundamental knowledge on the effects of two frequently used food gums, guar and xanthan, on the rheological properties of modified waxy starch systems. With regard to starch/non-starch hydrocolloid combinations, chemically modified starches receive much less attention than their native counterparts, despite their commercial and scientific value. The use of cross-linked waxy starches allows to treat the combined systems as dispersions of undisrupted, swollen granules, surrounded by the gum solution. This hypothesis was verified by using confocal scanning laser microscopy with fluorescently labeled gums. The average swollen granule sizes were derived by laser light scattering, and the corresponding total volume fraction occupied by the swollen starch (Φ) was calculated. The specific effects of the gums appeared to strongly depend on the starch content. Both gums, which were effectively increased in concentration due to the starch swelling, dominated the dynamic rheological properties and flow behavior at low Φ. At higher Φ, granule interactions governed the rheological properties. Whereas both gums could weaken the granule network, they contributed to the elastic behavior by their own entanglements. Moreover, xanthan gum appeared capable of reducing the granule interactions within flow, which results in lower viscosities at high starch volume fractions and high shear rates.