Unraveling the changes of physical properties and nanostructures of rice starch incorporated with pregelatinized rice starch paste during gelatinization

Abstract

In this study, we aimed to determine the influence of pregelatinized rice starch paste (PRSP) on the gelatinization of native rice starch, including its rheology and nanostructure. Gelatinization of PRSP-incorporated rice starch in excess water while heating at 5 °C/min was analyzed via rheometry, differential scanning calorimetry (DSC) and synchrotron small angle X-ray scattering (SAXS). Rheometry revealed that the addition of PRSP resulted in a more stable rice starch–water system at sub-gelatinization temperatures. A decrease from around 3000 Pa to lower than 1000 Pa in G′ was observed in the samples with PRSP after heating over 80 °C compared to that of the native starch. The sol-gel transition point of the starch–water system was altered by PRSP. DSC results showed no significant differences between the PRSP-incorporated and native starch. SAXS analysis revealed that the scattering invariant up to the gelatinization onset temperature was increased by the addition of PRSP due to the large volume fraction of the lamellar stacks. Correlation function-based structural parameters of the samples with PRSP changed earlier than those of native starch during heating due to the annealing effect. Moreover, the melting temperature of lamella of native starch was around 70 °C while those of the sample with PRSP were around 65 °C. Analysis of the radius of gyration (Rg) of the micellar structure after complete gelatinization indicated that Rg of native starch sample increased from 3.9 to 6.2 nm while that of the sample with 20% PRSP remained unchanged (≈ 4.3 nm). Overall, our results suggest that PRSP inhibits the swelling of rice starch during gelatinization and changes its rheological properties.