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The present study investigates the effect of guar gum on the digestibility of a waxy maize starch in vitro under simulated gastric and intestinal conditions. A detailed rheology and confocal scanning laser microscopy of the digesta were performed in order to study the possible mechanisms of interactions involved during in vitro hydrolysis of starch. No starch hydrolysis was observed under simulated gastric conditions, whereas more than 90% hydrolysis was observed at the end of digestion under simulated intestinal conditions. In the presence of guar gum, the starch hydrolysis was reduced by nearly 25% during the first 10 min and by 15% at the end of in vitro intestinal digestion. The rheological behavior of the digesta was significantly affected in the presence of the gum. The viscosity of digesta decreased during intestinal digestion; however, the extent of decrease was quite low in the presence of guar gum. The consistency index increased and flow behavior index of digesta decreased in the presence of gum after 1 min of intestinal digestion. All the samples (digested or undigested) displayed a pseudoplastic behavior as their apparent viscosity (η a ) decreased with an increase in shear rate. A negative correlation between the starch hydrolysis (%) and storage modulus for the starch sample without gum and a positive correlation for the starch sample with gum were found. Large granule remnants observed through confocal micrographs have shown that the solubilization of starch granule remnants during in vitro digestion is significantly reduced in the presence of gum.

Corn starch and starches separated from different potato cultivars were acetylated to evaluate the effect of plant source on the physicochemical, morphological, thermal, rheological, textural and retrogradation properties of the starches. Corn starch showed a lower degree of acetylation than potato starches under similar experimental conditions. The degree of acetylation for different potato starches also differed significantly. Morphological examination revealed that the granules of acetylated Kufri Chandermukhi and Kufri Sindhuri starches tended to appear as fused and less smooth than native starch granules. Acetylation of corn and potato starches decreased the transition temperatures and enthalpy of gelatinization and increased swelling power and light transmittance. However, the change in these was greater in the potato starches with higher percentage of small sized granules. Acetylated starches showed higher peak G', G'' and lower tan d than their counterpart native starches during heating. Among the starches from different cultivars, the change in the rheological parameters after acetylation differed to a significant extent. The retrogradation was observed to be negligible in the acetylated cooked starch pastes. Results implied that the change in functional properties of starches with acetylation depends on source and granule morphology of native starch.