Upcycling Melon Peel Powder as a Value-Added Ingredient for Biscuits with Improved Nutritional and Antioxidant...

In recent years, the food industry has increasingly focused on reintegrating nutrient- and phytochemical-rich processing by-products into high-consumption food matrixes. Aligned with this trend, our study investigated melon peel powder (MPP) as a value-added, upcycled ingredient in biscuit formulations with 0, 5, 10, 15, and 20% (w/w) MPP as a partial replacement for wheat flour (WF). The effects of this substitution on nutritional profile, colour, physical properties, total phenolic content (TPC), total flavonoid content (TFC), DPPH radical scavenging activity, ferric reducing antioxidant power (FRAP), structural characteristics, and post-baking retention were evaluated. MPP exhibited higher TPC (1531.76 mg GAE/100 g DM), TFC (681.42 mg QE/100 g DM), FRAP (104.37 µM Fe²⁺/g DM) and DPPH (125.01 µM TE/g DM) than WF. Its incorporation improved nutritional quality by increasing dietary fiber and ash, while reducing available carbohydrates and energy value. Substantial enhancements were observed, with up to 6.72- and 6.93-fold increases in FRAP and DPPH, alongside 3.82- and 3.92-fold increases in TPC and TFC at the highest substitution level. Although baking reduced these levels, retention remained at 57-62% (TPC), 55-59% (TFC), 60-65% (FRAP), and 63-70% (DPPH), indicating partial thermal stability and possible contribution of heat-induced antioxidant compounds. MPP addition also affected technological properties, increasing baking yield and spread ratio while decreasing lightness (L*) and increasing yellowness (b*) and browning index (BI), resulting in yellow-brown tones. Fourier-transform infrared spectroscopy (FTIR) and wide-angle X-ray scattering (WAXS) analyses confirmed structural compatibility between WF and MPP, evidenced by the absence of new functional groups, preservation of the A-type crystalline structure of wheat starch without polymorphic transitions, and no indication of phase separation, whereas baking promoted starch gelatinization and integration of fibrous and pectic components. These findings support a potential valorisation pathway for melon by-products through their conversion into a value-added ingredient for biscuit production.