Nutritional and Antioxidant Profile of Brown Eragrostis tef (Zucc.) Trotter Flour in Blends with Glycine max (L.) Merr. Flour

The still-growing demand for nutritious gluten-free products necessitates the development of a composite flour that addresses the nutritional deficiencies common in conventional gluten-free formulations. This study aimed to comprehensively characterize brown teff (Eragrostis tef (Zucc.) Trotter) and soybean (Glycine max (L.) Merr.) composite flours at 0%, 10%, 20%, 30%, and 40% soybean inclusion levels (w/w) to establish evidence-based formulation guidelines for future products. Proximate composition, antioxidant properties (total polyphenol content—TPC, antioxidant capacity vs. 2,2-diphenyl-1-picrylhydrazyl radical—DPPH and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid radical—ABTS, ferric reducing antioxidant power—FRAP), particle size distribution, pasting properties, color characteristics, and molecular fingerprints (Fourier transform infrared spectroscopy—FTIR) were evaluated. A principal component analysis (PCA) was employed to identify compositional–functional relationships. Soybean inclusion significantly enhanced protein content from 9.93% (pure teff) to 23.07% (60:40 blend, dry matter), fat from 2.14% to 10.47%, and fiber from 3.43% to 6.72%. The antioxidant capacity increased proportionally with soybean content, with a 40% inclusion yielding FRAP values of 5.19 mg FeSO4/g DM and TPC of 3.44 mg GAE/g DM. However, pasting viscosity decreased notably from 12,198.00 mPa·s (pure teff) to 129.00 mPa·s (60:40 blend), indicating a reduced gel-forming capacity caused by soybean addition. PCA revealed that nutritional composition (PC1: 70.6% variance) and pasting properties (PC2: 21.0% variance) vary independently, suggesting non-additive functional behavior in blends. Brown teff–soybean blends at a 20–30% soybean inclusion optimize the balance between protein enhancement, antioxidant preservation, and the maintenance of functional properties suitable for traditional applications, providing a nutritionally superior alternative for gluten-free product development.