Effect of Acetylation, Hydroxypropylation and Dual Acetylation-Hydroxypropylation on Physicochemical and Digestive Properties of Rice Starches with Different Amylose Content

Four rice (Oryza sativa L.) starches widely differing in amylose content were subjected to acetylation, hydroxypropylation, and dual modification involving acetylation followed by hydroxypropylation. The starches showed a higher affinity to hydroxypropyl substitution. However, acetylation caused a significant alteration in the glycosidic matrices. The changes in physicochemical properties were most prominent in the dual-modified samples. Hydroxypropylated and dual-modified granules showed greater swelling power with structural retention. Degrees of acetylation were high in the amorphous regions of waxy and low amylose starches (0.04 and 0.05). Minor granular swelling was observed under SEM. Substitution partially dissociated side-chain superhelices, lowering crystallinity values by 3.12-4.58%, however retaining the native A-type XRD patterns. Glycosidic dissociation and enhanced hydrophilicity caused a significant lowering of gelatinization temperature (To, Tp, Tc), enthalpy (H), and cooking time as observed from DSC and RVA results. Low setback viscosity and low syneresis during freeze-thaw cycles indicated the decreased tendency of modified starch chains to realign. Dual modified starches could be cooked to thinner and more precise pastes, which are highly resilient to retrogradation. Significant increases in enzyme-resistant RS and SDS (up to 20% of each) were recorded. The dual modification method could suitably alter the properties of starches for food use.