Extraction and Characterization of Antioxidant Compounds in Almond (Prunus amygdalus) Shell Residues for Food Packaging Applications

This work proposes the revalorization of almond shell (AS) wastes as an active additive for food packaging applications. A new microwave-assisted extraction (MAE) method to obtain extracts rich in polyphenolic compounds with high antioxidant capacity was optimized. An experimental design to optimize the MAE procedure through response surface methodology (RSM) using a Box–Behnken design was proposed. The effects of extraction temperature, irradiation time, ethanol:water concentration, and solvent pH at three levels were evaluated in terms of total phenolic content (TPC) and antioxidant activity (DPPH (2,2-diphenyl-1-picrylhydrazyl) and ferric reducing antioxidant power (FRAP) assays). The optimal conditions found were 57 min, 80 °C, pH 8, and 70% (v/v) ethanol. Optimized MAE extracts showed low soluble protein content (0.43 mg BSA g(−1)) and were rich in TPC (5.64 mg GAE g(−1)), flavonoids (1.42 mg CE g(−1)), and polysaccharides (1.59 mg glucose g(−1)), with good antioxidant capacity (2.82 mg AAE acid g(−1)). These results suggest the potential application of these extracts in the food industry as active additives. This strategy opens new pathways to valorize almond shell residues, contributing to the circular economy.