Combining Abiotic Stresses as a Low-Cost Strategy for Increasing the Phenolic Content in Apple Agro-Industrial By-Products.

The circular economy approach offers innovative solutions for valorizing apple by-products through biofortification strategies transforming waste into high-value products and reducing environmental impact. This study evaluates innovative solutions for valorizing Granny Smith apple peel (RM) through biofortification in phenolic compounds using individual or combined abiotic stresses, like wounding stress and ultraviolet A (UVA) radiation. The effects of cutting type (Ct) [whole (C1), 5 mm (C2), 1.5 mm (C3)], storage temperature (ST) [20, 15, 10, 5 °C], and storage time (TM) [0, 12, 24, 48, 72 h] on phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO) activity, total phenolic content (TPC), and phenolic profiles were studied first. The results show that higher stress intensity (C3, 15 °C, 48 h) significantly enhanced secondary metabolism, leading to notable increases in PAL activity (1201%), PPO activity (308%), TPC levels (108-118%), and Procyanidin B2 (PACB2, 22%), the predominant phenolic compound. These changes were critical for improving the bioactive properties and antioxidant potential of RM. The second assay combined wounding stress (same levels of Ct and ST of previous assay, TM: 56 h) with UVA radiation (UVA-D) [0, 86.4 KJ m, 172.8 KJ m], determining the optimal conditions (C3, UVA-D 66 KJ m, 17 °C) for maximizing PAL activity (0.12-0.20 ΔA h⁻ mg⁻), and TPC (3.3 g GAE kg⁻). This study demonstrates the potential of combined abiotic stresses as cost-effective scalable tools to biofortify RM, promoting the sustainable and value-added utilization of agro-industrial by-products.