Response Surface Optimization of Bioactive, Nutritional, and Color Properties in Carrot and Celery Peel-Based Formulations

The valorization of vegetable processing by-products as functional food ingredients represents a sustainable approach to waste reduction and nutritional enhancement. This study investigated the potential of carrot (Daucus carota L.) and celery (Apium graveolens L.) peel waste for the development of nutritionally enriched formulations. Response surface methodology (RSM) combined with a three-level factorial design was used to evaluate the effects of peel incorporation on total polyphenol content (TPC), antioxidant activity (DPPH), crude fiber, ash content, and color characteristics. Carrot peel significantly increased TPC, whereas increasing carrot and celery peel substitution levels were associated with higher antioxidant activity within the investigated experimental range. Celery peel significantly increased crude fiber content, while ash content decreased with increasing celery peel incorporation. The selected models provided an adequate fit for the investigated responses. The highest predicted functional properties reached 3704.15 mgGAE/100 g TPC, 26.94% DPPH inhibition, and 8.35% crude fiber. Three optimization scenarios with increasing complexity were evaluated using the desirability function approach. The highest desirability (D = 0.869) was achieved when only functional properties were considered. Inclusion of ash content and color change reduced desirability values to 0.675 and 0.555, respectively, indicating trade-offs among functional, nutritional, and color-related attributes. The optimized formulation corresponded to 75.08% substitution of the carrot fraction with carrot peel waste and 44.23% substitution of the celery fraction with celery peel waste, providing a balanced combination of enhanced functionality, nutritional quality, and acceptable appearance.