Optimization of Gluten-Free Bread Formulation with Quercus rotundifolia Acorn Flour Using Response Surface Modelling, Digital Image Analysis, and Instrumental Texture Assessment

This study aimed to optimize the formulation of gluten-free bread (GFB) based on rice flour (RF) and Quercus rotundifolia acorn flour (AF) by evaluating the combined effects of flour substitution (0%, 50%, and 100%) and water addition (90%, 100%, and 110%) on technological, textural, colorimetric, structural, and sensory properties. A three-level full factorial design (32) combined with response surface methodology (RSM) was used to model and optimize product quality. The developed models showed high predictive performance (R2 = 0.714–0.999; non-significant lack of fit), confirming their suitability for describing complex interactions in gluten-free systems. Water addition was the dominant factor influencing moisture, crumb structure, and textural softness, while AF mainly affected color, structure, and sensory attributes. Increasing acorn content significantly decreased lightness (L*) and increased redness (a*) and darkness index (DI), reflecting higher phenolic compound content and more intense Maillard reactions. Specific volume (1.85–2.41 cm3/g) was maximized at higher hydration levels, especially when combined with intermediate to high acorn substitution, indicating a synergistic interaction between fiber-rich flour and water availability. Texture analysis showed that AF increased hardness and reduced cohesiveness, while water addition significantly improved softness, elasticity, and overall mouthfeel. Image analysis of crumb structure demonstrated that higher hydration promoted larger pore size and porosity, whereas AF increased cell density, resulting in a finer crumb structure under low hydration conditions. Sensory evaluation confirmed that breads with high acorn content were well accepted due to their characteristic nutty flavor. Multi-response desirability optimization yielded an optimal formulation with approximately 83% AF and 108% water, representing the best achievable compromise among the evaluated quality criteria. The results demonstrate that AF can serve as a key functional ingredient in GFB, provided that hydration is carefully adjusted. This study highlights the effectiveness of RSM combined with image-based analysis as a robust approach for developing high-quality gluten-free bakery products.