Valorization of Whey as a Natural Functional Ingredient in Gluten-Free Rice Biscuits: Formulation, Optimization, and Chemical Profiling

The present study investigates the effect of whey powder incorporation on the nutritional composition, structural characteristics, and functional properties of rice flour-based gluten-free systems. Composite flours and biscuits were formulated by substituting rice flour with 5%, 10%, and 15% whey powder. Proximate composition, mineral profile, and structural modifications were evaluated using standard analytical methods, complemented by Fourier Transform Infrared Spectroscopy (FTIR) and Small-Angle X-ray Scattering (SAXS). The results showed that whey addition significantly improved the protein content of both flours and biscuits, increasing from 8.45% in the control to 15.06% at the highest enrichment level. Whey powder showed elevated phosphorus (912 mg/kg), sodium (434.65 mg/kg), and calcium (526.49 mg/kg) contents compared to rice flour. Consequently, mineral levels increased progressively in the composite flours, with phosphorus rising from 528 mg/kg to 647 mg/kg, sodium from 105.66 mg/kg to 132.81 mg/kg, and calcium from 102.15 mg/kg to 137.33 mg/kg as the whey incorporation level increased. Iron content showed minor variations among the gluten-free biscuit formulations (76.01–95.16 mg/kg). Whey incorporation led to a progressive increase in copper content, from 8.91 mg/kg in the control biscuits to 15.50 mg/kg, while zinc levels decreased from 27.47 mg/kg to 18.47 mg/kg with increasing whey addition. FTIR analysis revealed clear structural changes associated with whey addition, including the progressive intensification of amide I and II bands and a reduction in starch-specific signals, confirming the incorporation of whey proteins into the starch matrix and the formation of protein–starch interactions. These findings were supported by SAXS analysis, which indicated modifications in the internal structural organization of the systems. Sensory evaluation indicated good overall acceptability of the fortified biscuits at moderate whey incorporation levels, while higher whey addition slightly reduced taste scores due to the characteristic salty flavor associated with acid whey. Overall, the study demonstrates that whey powder is an effective functional ingredient for enhancing the nutritional and structural properties of gluten-free products. However, achieving an optimal balance between improved nutritional quality, technological performance, and mineral composition remains essential for the development of high-quality gluten-free formulations.