Integrated HS-GC–IMS and UPLC-Q-Orbitrap HRMS-based metabolomics revealed the characteristics and differential volatile and nonvolatile metabolites of different citrus peels

Citrus is an important genus in the Rutaceae family, and citrus peels can be used in both food and herbal medicine. However, the bulk of citrus peels are discarded as waste by the fruit processing industry, causing environmental pollution. This study aimed to provide guidelines for the rational and effective use of citrus peels by elucidating the volatile and nonvolatile metabolites within them using metabolomics based on headspace-gas chromatography–ion mobility spectrometry and ultra-high-performance liquid chromatography–Q-Orbitrap high-resolution mass spectrometry. In addition, the antioxidant activities of the citrus peels were evaluated using DPPH radical scavenging, ABTS radical scavenging, and ferric reducing antioxidant power. In total, 103 volatile and 53 nonvolatile metabolites were identified and characterized. Alcohols, aldehydes, and terpenes constituted 87.36% of the volatile metabolites, while flavonoids and carboxylic acids accounted for 85.46% of the nonvolatile metabolites. Furthermore, (Z)-2-penten-1-ol, L-pipecolinic acid, and limonin were identified as characteristic components of Citrus reticulata Blanco cv. Ponkan (PK), C. reticulata 'Unshiu' (CLU), and C. reticulata 'Wo Gan' (WG), respectively. Principal component analysis and partial least squares discriminant analysis indicated that C. reticulata Blanco 'Chun Jian' (CJ), PK, CLU, and C. reticulata 'Dahongpao' (DHP) were clustered together. DHP is a traditional Chinese medicine documented in the Chinese Pharmacopoeia, suggesting that the chemical compositions of CJ, PK, and CLU may also have medicinal values similar to those of DHP. Moreover, DHP, PK, C. reticulata 'Ai Yuan 38'(AY38), CJ, C. reticulata 'Gan Ping'(GP), and C. reticulata 'Qing Jian'(QJ) displayed better antioxidant activities, recommending their use as additives in cosmetics and food. Correlation analysis suggested that some polyphenols including tangeritin, nobiletin, skullcapflavone II, genistein, caffeic acid, and isokaempferide were potential antioxidant compounds in citrus peel. The results of this study deepen our understanding of the differences in metabolites and antioxidant activities of different citrus peel varieties and ultimately provide guidance for the full and rational use of citrus peels.