Effect of Heat Moisture Treatment on the Rheological, in Vitro Digestive Properties and Structure of Quinoa Starch

Heat moisture treatment (HMT) is a widely used method for starch modification due to its safe, green, environmentally friendly, and efficient characteristics. In this study, quinoa starch was modified by HMT with varying moisture contents (20%, 25%, and 30%) at a temperature of 120 °C for 10 hours, and its rheological properties, in vitro digestive properties, and structure were further evaluated. The steady rheology results indicated that all the samples were shear-thinning non-Newtonian fluids. After undergoing HMT, the apparent viscosity of quinoa starch decreased significantly. Additionally, the samples' apparent viscosity decreased as the moisture content of HMT increased. The results of frequency sweep showed that the storage modulus G' of quinoa starch decreased with the increase of moisture content of HMT, while the loss modulus G'', and loss factor increased with the increase of moisture content of HMT. The thixotropic results showed that the thixotropic ring area of quinoa starch decreased with the increase of the moisture content of HMT. Additionally, the results of in vitro digestion showed that the content of rapidly digestible starch in quinoa starch decreased significantly after HMT (P<0.05), while the content of slowly digestible starch (SDS) in modified quinoa starch increased significantly with the increase of moisture content of HMT (P<0.05). Notably, at a moisture content of 30%, the highest content of SDS in the modified starch was 24.93%, which was 31.42% higher than that of the native starch. XRD results indicated that the crystal form of quinoa starch remained unchanged after HMT, with all crystals being A-type. However, the relative crystallinity of quinoa starch decreased significantly after HMT (P<0.05). The Fourier transform infrared spectroscopy results showed that the overall order of quinoa starch decreased, while the content of double helix structure in quinoa starch increased after HMT. SEM observations revealed that the modified quinoa starch tended to agglomerate, and some of the particle surfaces collapsed compared to native quinoa starch. The study results suggested that HMT could significantly alter the rheological properties of quinoa starch and increase its SDS content. The study found that the modification effect was primarily affected by the moisture content of the starch. These findings provided a theoretical basis for the application of quinoa starch.