Effects of extrusion on structural properties, physicochemical properties and in vitro starch digestibility of Tartary buckwheat flour

Tartary buckwheat is a promising pseudocereal with health benefits. However, native Tartary buckwheat flour (NTBF) cannot satisfy the specific physicochemical properties required for food processing due to its gluten-free character. This study investigated structural and physicochemical properties, the starch digestibility of Tartary buckwheat flour pregelatinized with traditional extrusion processing technology (TEPT), and improved extrusion processing technology (IEPT). TEPT had a higher destructive effect on the starch structure than IEPT. Amorphous flakes and incomplete starch granules were observed in pregelatinized Tartary buckwheat flour (PTBF). After extrusion, the relative crystallinity, endothermic enthalpy, the slowly digested starch (SDS) and resistant starch content decreased by 23.76–70.44%, 48.91–100.00%, 28.43–88.65% and 53.23–78.79%, respectively. However, the amylose molecular weight of branched starches, the degree of gelatinization (DG), the water absorption index (WAI), the starch digestibility, the rapidly digested starch content, and the glycemic index increased by 29.00–119.04%, 26.05–1279.88%, 4.2–188.70%, 9.2–19.96%, 113.04–236.25%, and 25.55–37.00%, respectively. For IEPT, crystallinity decreased with the increase in extrusion temperature and the decrease in feed moisture, whereas the DG and WAI showed a contrary trend. Compared to TEPT (5.05%), PTBF obtained with IEPT retained a higher SDS content (11.28–31.85%). Starch digestion of all samples possessed a combination of sequential and parallel digestion patterns. Correlation analysis suggested that an increase in the destructive effect on the starch structure from IEPT to TEPT was responsible for the changes in physicochemical properties and improvement of in vitro starch digestibility. This study provides a scientific basis for the quality improvement in Tartary buckwheat starchy foods following extrusion.