Effects of Pre-Germination Assisted with Ultrasound on the Total Flavonoids and Water Absorption Process and Structural Characterization of Starch in Tartary Buckwheat Seeds

Tartary buckwheat is a characteristic minor grain crop rich in flavonoids, while pre-germination is an effective strategy to improve its nutritional quality. Ultrasound, as a green physical pretreatment technology, can modulate seed germination and starch structural properties. However, the multi-scale structural evolution of Tartary buckwheat starch and nutrient accumulation during ultrasound-coupled pre-germination remains insufficiently understood. This study explored the influences of ultrasound-assisted pre-germination on the nutritional components and total flavonoid content (TFC) of Tartary buckwheat seeds. Meanwhile, the variations in water absorption behavior, internal water distribution, and multi-scale starch structural characteristics were systematically analyzed under different ultrasonic treatment durations. The results indicated that soaking prior to ultrasound treatment better promotes water absorption and TFC of pre-germination Tartary buckwheat. Under the optimal processing conditions of 3 h soaking time, 60 min ultrasound treatment and 20 h pre-germination time, the contents of protein, fat, total starch, amylose and TFC in the Tartary buckwheat seeds peaked. However, there were no significant differences in protein and fat contents between the 50 min and 60 min ultrasound treatments, nor were significant differences observed in amylose and resistant starch contents between the 60 min and 70 min ultrasound treatments. Furthermore, ultrasonic treatment for 40–50 min increased the fraction of small starch granules. Upon extending sonication to 50–60 min, intermolecular hydrogen bonding, relative crystallinity (RC) and short-range ordered structure were all enhanced. Compared with 60 min ultrasound treatment, prolonged sonication (70–80 min) promoted the growth of amorphous regions and the formation of large granule agglomerates. Meanwhile, the short-range ordered structure decreased, and the crystalline framework was partially disrupted; nevertheless, X-ray diffraction (XRD) and Fourier transform infrared (FTIR) results demonstrated that ultrasound-assisted pre-germination did not alter the crystalline polymorph of Tartary buckwheat starch. This finding provides a theoretical basis for the green processing and quality improvement of Tartary buckwheat.