Physical modification of waxy maize starch by dry heating with ionic gums

As an alternative process to the chemical modifications commonly practiced for food starches, dry heating of a starch with a presence of ionic gums such as sodium alginate, sodium carboxymethylcellulose (CMC), and xanthan was investigated. Waxy maize starch was dispersed in a dilute gum solutions (1% gum based on starch solids, 36% starch in the solution). The dispersion was dried (45 deg C overnight) to moisture contents less than 10%, and then heated (130 deg C for 2 h) in a convection oven. Paste viscosity and crystal melting of the heat-treated starch powders were measured using a Rapid Viscoanalyzer (RVA) and a differential scanning calorimeter (DSC), respectively. Under RVA viscograms, xanthan behaved like a cross-linking agent reducing the peak viscosity but increasing the shear stability, whereas CMC and alginate raised the peak and final viscosities as the chemicals for starch substitution did. The overall changes in the paste viscosity were most significant when xanthan was added. The pH of the starch-gum dispersion affected the paste viscosity of the heat-treated starches. A mild acidity (pH 6) was favored for the viscosity changes when sodium alginate was added, whereas a mild alkalinity (pH 8) enhanced the viscosity increase when xanthan was used. By using gum mixtures such as xanthan-CMC and xanthan-alginate, the increases both in final viscosity and in shear stability could be achieved as the effect similar to the dual chemical modifications (substitution and crosslinking). But the dry heating with ionic gums did not provide substantial changes in the melting of the starches (DSC), indicating that the crystalline regions in starch remain relatively inert to the dry heating process. The interactions between starch and gum molecules induced by the heating responsible for the viscosity changes might occur only in the amorphous regions.