Effect of Extrusion on Physicochemical Properties, Digestibility, and Phenolic Profiles of Grit Fractions Obtained from Dry Milling of Normal and Waxy Corn

Extrusion behavior of grits obtained from 3 successive reductions of dry milling of 2 normal corn types and 1 waxy corn was studied at different extrusion temperatures (ET). The grit from each reduction stage (RS) showed the presence of base-hydrolyzed bound protocatechuic acid, p-coumaric acid, sinapic acid, ferulic acid, and quercetin, and the concentrations of these decreased after acid hydrolysis and that of gallic acid increased. African tall grit from all RSs showed the highest average specific mechanical energy (SME) and torque, while waxy corn grit had the lowest. Corn extrudates showed an increase in water solubility index (WSI) and a decrease in water absorption index (WAI), as well as in expansion ratio (ER) with an increase in ET. Extrudates from each corn type showed an increase in rapidly digestible starch (RDS) and a decrease in slowly digestible starch (SDS) and resistant starch with an increase in ET. Most of the phenolics were present in bound form in extrudates, and quercetin, and catechin observed in grit were not present in extrudates. Frying of extrudates showed lower L-* and higher a(*) and b(*) when compared to color parameters of raw extrudates. Lower expansion of fried extrudates from waxy corn than those from other corn types may be related to its higher protein and fat contents and to lower amylose content.