Effect of combined freezing with heat-moisture treatment (HMT) on the modification of in vitro digestibility, morphostructural, physicochemical, and thermal properties of Adzuki bean starch

The objective of the study was to evaluate the effect of freezing methods at -20 °C (S20) and − 80 °C (S80) and liquid nitrogen at -196 °C (S196) applied to aduzki bean starch previously gelatinized by heat-moisture (HMT). It is characterized in terms of particle size distribution, functional properties, in vitro digestibility, specific surface area and thermal and morphostructural properties. Higher freezing rates (S196: 1.21 °C/min) resulted in a medium particle size (21–27 μm) and increased starch solubility (4.13–4.71%) and specific surface area due to rapid ice crystal formation. Freezing conditions had a significant impact (p < 0.05) on starch digestibility, with the lowest glycemic index obtained for S20 (68.36). X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) revealed that the freezing temperature did not modify the starch’s crystalline form (type A) and did not identify new functional groups. However, relative crystallinity (19.03–21.88%) and IR 1022/945 (0.82–0.91) were reduced compared to the control (23.06%, 0.95, respectively). Different surface damage caused by freezing, such as pores and roughness on the surface, induced by HMT and intensified by the freezing method, was observed. Finally, gelatinized adzuki bean starch frozen at -196 °C (S196) demonstrated high functionality for use in the development of frozen starch-based foods. By combining freezing with HMT, we introduce a novel approach to starch modification that offers potential benefits compared to traditional methods. Utilizing combined thermal pre-treatments allows for rapid modification of native starch without chemical reagents, thereby facilitating large-scale application due to process simplicity.