Analysis of volatile substances in peels of four citron varieties

Abstract:【Objective】Systematic comparative analysis of the volatile components in the peel of four main cultivated citron, screening of the characteristic volatile components in the four citrons and analyzing their differences in aroma substances from the perspective of metabolism are of great significance for better utilization of the germplasm resource of citron citrus and promoting the development of citrus essential oil industry.【Methods】The volatile components of YN 1 lemon, Red lemon, Xiangshui lemon and Citron peels were detected by low temperature ultrasonic solvent extraction combined with GCMS, and compounds were identified with the Xcalibur software and NIST mass spectrometry database for compound matching, combined with artificial spectrum analysis and comparison with relevant literature. The content of volatile components was determined by the internal standard method, and the data were statistically analyzed by WPS Office 2020 software. SPSS 26.0 software was used to analyze the variance and difference significance of means. Origin 2021 software was used for constructing graphs with the data, and Simca14.1 for multivariate statistical analysis to screen the characteristic volatile components. Partial least square discriminant analysis (PLS-DA) was used to predict the stability and re-liability of the data.【Results】There were significant differences in the types and contents of volatile components in the peel of the four citrons. A total of 85 volatile components were detected in the peels of YN 1 lemon, Red lemon, Xiangshui lemon and Citron, and 72, 57, 58 and 58 were detected respectively, which fell into 11 classes, including monothenes, sesquiterpenes, monoterpene alcohols, sesquiterpene alcohols, monothene aldehydes, aldehydes, esters, alcohols, etc. Monothene substances accumulated significantly, accounting for more than 80% of the total volatile components, among which D-limonene was the highest volatile substance in the peels of the four citrons. Among the 85 volatile components detected, 34 were common to the four varieties and they were the typical volatile components of citrons. The peel of YN 1 lemon contained 9 unique volatile components, and the peel of Red lemon contained 7 unique volatile components, while no unique volatile components were detected in the peels of Xiangshui lemon and Citron. YN 1 lemon peel had the highest total volatile component content (24 554.42 µg·g-1 ), followed by Xiangshui lemon peel with 22 004.48 µg·g-1 , Red lemon peel with 9 677.24 µg·g-1 , and Citron peel with the lowest content (6 671.86 µg·g-1 ). Further, by partial least square discriminant analysis (PLS-DA), the established PLS-DA model could effectively distinguish the volatile components of the four citron varieties. The largest contribution to principal component 1 (PC1) was D-limonene, followed by citronellal, citral, neral, etc., and the largest contribution to principal component 2 (PC2) was β-pinene, followed by γ-perpinene, α-pinene, β-myrcene, etc. With VIP≥1, p＜0.05 as the basis, 12 volatile components with different characteristics were screened. Among them, D-limonene, γ-terpinene, β-pinene, citral, β-myrcene, α-pinene and neral were significantly accumulated in the peel of YN 1 lemon. (Z)-α-bergamotene and α- farnesene accumulated significantly in the peel of Red lemon, in which (Z)-α-bergamotene was 61.14 times that of YN 1 lemon, 5.11 times that of Xiangshui lemon and 281.27 times that of Citron, and α-farnesene was unique to the peel of Red lemon. D-limonene and citronellal accumulated significantly in the peel of Xiangshui lemon, especially citronellal, which was 111.77 times that of YN 1 lemon, 37.35 times that of Red lemon and 814.44 times that of Citron. Geraniol and perillal accumulated significantly in Citron peel, in which geraniol was 10.57 times that of YN 1 lemon, 6.17 times that of Xiangshui lemon, and perillal was 3.66 times that of YN 1 lemon, and 16.01 times that of Xiangshui lemon. The contents of D-limonene, citral and neral in the peel of YN 1 lemon and Xiangshui lemon were significantly higher than those of Red lemon and Citron.【Conclusion】There were significant differences in the types and contents of volatile components in different citron citrus peel. Monopetene compounds were the most important volatile components in the peel of the four kinds of citron varieties. The content and types of volatile components in the peel of YN 1 lemon were the highest, and the content and types of volatile components in the peel of citron were the least. Based on the PLS-DA result, γ-terpinene, β-pinene, α-pinene and β-myrcene might be the characteristic volatile components of YN 1 lemon peel; (Z)-α-bergamotene and α-farnesene were characteristic volatile components of Red lemon peel; citronellal might be used as the characteristic volatile component of Xiangshui lemon peel, and geraniol and perillal might be used as the distinctive volatile components of Citron peel. YN 1 lemon and Xiangshui lemon had rich aroma components and might be excellent citrus aroma resources.