Non-thermal effect of radio frequency treatments verified by the multi-scale structure and in-vitro digestibility of sweet potato starch

To verify whether there is a non-thermal effect in the radio frequency (RF) heating process, a heat block (HB) system was used to simulate the RF process for excluding the influence of thermal effect. Sweet potato starch (SNS) slurry was treated to 60 °C by RF and HB systems with different heating rate, while the heat uniformity was determined to ensure the feasibility and reliability of the method and results. The long- and/or short - range structural, molecular, and digestive characteristics of samples were investigated to elucidate the difference of the RF and HB treatments. The X-ray diffraction patterns (XRD) analysis demonstrated that crystal pattern remained A-type, but the crystallinity of treated samples was significantly reduced. Compared with HB treatment, the RF- treated starch had a more compact, or ordered structure, higher z-average radius of gyration (Rz), peak viscosity (PV) and setback (SB) based on small angle X-ray scatter (SAXS), Fourier-transform infrared spectra (FTIR), size-exclusion chromatography (SEC) and pasting properties observations, respectively. Especially, RF- treated starches with higher heating rate had lower weight-average molar mass (Mw), rapidly and slowly digestive starch content, resulting in the highest resistant starch content and polydispersity index (PDI) in comparison with other samples. The structural characteristics and digestibility of starch were significantly different between RF and HB treatments under the same conditions. This study may provide valuable insights into the thermal and non-thermal effects of RF treatment on starch structure to broaden the application of RF heating in the food processing filed.