Impact of Thermal Processing Associated with Essential Oil Recovery on the Phenolic Metabolome and Bioactivity of Lemon, Mandarin, and Orange Peel Wastes

Citrus peel wastes are valuable sources of phenolic compounds (PCs); however, the industrial essential oil extraction exposes the peels to heat, which may significantly affect their concentration, stability, composition, and biological activities. This study aimed to evaluate the impact of thermal processing associated with prior essential oil recovery on the phenolic metabolome and bioactivity of lemon, mandarin, and orange peels. Aqueous-ethanol extracts were prepared from crude peels (CPEs) and heat-treated residues remaining after the essential oil extraction (HPEs) of citrus peel wastes. The molecular-network-assisted LC-MS/MS analysis identified a total of 120 annotated metabolites, predominantly 98 phenolics, and showed a marked reduction in phenolic diversity after heat treatment. In CPEs 59, 55, and 44, phenolics were detected in lemon, mandarin, and orange, whereas phenolics in 43, 49, and 43 were noticed in their corresponding HPEs, respectively. CPEs showed stronger antioxidant activity, with lower IC50 values in the DPPH assay (38.72, 60.38, and 70.77 μg/mL) and NO inhibition assay (61.54, 90.32, and 117.50 μg/mL) compared with HPEs. The extracts also exhibited significant anti-inflammatory activity through selective inhibitory effects against the COX-2 enzymes. Overall, thermal treatment during essential oil extraction markedly altered the phenolic composition and reduced the biological activity of citrus peel extracts. These findings demonstrate that processing conditions critically influence the functional value of citrus peel wastes.