Elucidation of dynamic evolution patterns of non-volatile and volatile compounds in peel of Citrus reticulata cv....

Tree age is an important but under-explored factor influencing the chemical quality of Citrus reticulata 'Chachiensis' peel (Chenpi). Peels from 5, 20 and 40-year-old trees were analyzed using UHPLC-MS/MS metabolomics and HS-SPME-GC-MS volatilomics to investigate the variations in chemical and sensory profiles across these age groups. In total, 2188 non-volatile metabolites and 911 volatile organic compounds (VOCs) were identified. PCA and OPLS-DA models clearly separated the three age groups, revealing 183 age-responsive non-volatiles metabolites and 127 differential VOCs. Secondary metabolism showed notable differences among the age groups, with polymethoxylated flavones, hydroxycinnamoyl-quinic acids, and coumarins exhibiting the highest abundances at 20 years, suggesting age-related variations in antioxidant potential. Primary metabolites like amino acids showed variations in concentration, with the highest levels observed at 20 years, reflecting the nutrient dynamics associated with differences in fruit load at various tree ages. Relative odor activity value (ROAV) analysis identified 11 key aroma-active VOCs, including the caramel-like 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone and the green-note (Z,Z)-3,6-nonadienal. These compounds contributed most significantly to the aroma at 20 years, although total VOC levels continued to rise at 40 years. The aroma profiles of the Chenpi samples showed distinct variations: 5-year-old trees had a fresher, more citrusy aroma, while 20-year-old trees exhibited a richer, more complex aroma, with enhanced notes of caramel and green leaf. At 40 years, the aroma became more intense, with an increase in total VOC levels, but with a more subdued contribution from key aroma-active compounds. Collectively, the data highlight significant metabolic and sensory differences across the tree ages, providing insights into the potential for age-based quality grading of Chenpi and other specialty foods.