Effect of nitrogen fertilizer on proso millet starch structure, pasting, and rheological properties

Nitrogen fertilizer is an important agronomic measure significantly affecting crop yield and grain quality. This two-year study aimed to explore the effects of four nitrogen levels on the morphology, lamellar and crystalline structure, pasting, and rheological properties of proso millet (PM) starch and to investigate potential food applications of PM. The results showed that the starch surface became uneven, and the structure of the starch granules shifted towards greater complexity with increasing nitrogen levels. Nitrogen increased the relative crystallinity, ordered structure, and average repeat distance, leading to a stable starch structure and a higher gelatinization enthalpy. Furthermore, nitrogen significantly increased peak, breakdown, setback, and final viscosities but decreased apparent amylose content, which caused the shear resistance and storage capacity of PM starch-based foods to deteriorate during processing and transport. Rheological analysis showed that PM starch exhibited the typical characteristics of a pseudoplastic fluid. Under nitrogen treatment, PM starch gels showed high potential for application in 3D printing products due to high resistance to shear thinning, gel strength, and yield stress, presumably owing to the significantly higher G´ than G´´, and the lower amylose content, which in turn promoted the formation of a stable network structure in water to immobilize more water. Our study provides a reference for applying PM starch in the food industry and for developing PM cropping-management programs to improve quality.