Structural Optimization of Whey–Brown Rice Protein Bars with Elderberry and Diospyros lotus Extracts Using FTIR and Multivariate Analysis

Protein bars are widely consumed functional snacks, yet their quality often deteriorates during storage due to excessive hardness, limited moisture retention, and reduced sensory acceptability. This study examined the effects of elderberry extract and Diospyros lotus concentrate on the physicochemical, microbiological, structural, antioxidant, and sensory properties of whey–brown rice protein bars and optimized their formulation using response surface methodology (RSM). The optimized formulation showed a 53.2% reduction in hardness and substantial improvements in antioxidant capacity, including increases of 57.8% in DPPH activity, 144.4% in total phenolic content, 345.2% in total flavonoid content, and a marked rise in FRAP values. Microbial stability improved with increasing D. lotus concentration, associated with reduced water activity and phenolic-driven antimicrobial effects. FTIR analysis revealed that these formulations induced a shift in protein structure from rigid α-helices and intermolecular β-sheets toward more flexible β-turns and intramolecular β-sheets, enhancing matrix flexibility. Sensory evaluation confirmed improvements in flavor, texture, and overall acceptability. RSM identified 0.98 g/100g elderberry extract and 19.10 g/100g D. lotus concentrate as optimal. Partial least squares regression (PLSR) demonstrated strong correlations between FTIR-derived structural features and key quality attributes, supporting the applicability of spectroscopic–multivariate approaches for assessing protein-based snack products. ❖ FTIR revealed protein structural changes induced by added extracts ❖ PLSR predicted texture from FTIR-derived spectral features ❖ Polyphenols modified protein interactions and matrix structure ❖ FTIR–chemometric integration optimized protein bar formulation