| Linalool is one of the most important tea aroma components with high content. It has two enantiomeric forms, namely R-(-)-Linalool and S-(+)-Linalool, and R-(-)-Linalool and S-(+)-Linalool have the entirely different aroma quality. However, little investigation has been carried out on the enantiomeric analysis of linalool in teas, and the enantiomeric composition of linalool in tea has not been clearly elucidated to date. In this study, a method using chiral chromatography and based on headspace solid-phase micro-extraction(HS-SPME) was developed to analyze R-(-)-Linalool and S-(+)-Linalool in tea samples. The enantiomeric distribution of linalool was analyzed in different teas and fresh tea leaves. It was found that tea germplasm and processing steps were important factors in determining the enantiomeric distribution of linalool and levers in tea products. The main results were listed as follows:1. Optimal extraction conditions were: CAR-DVB-PDMS fiber, 60 min at 60 °C, tea: water ratio of 1:6(w/v). Using the optimized HS-SPME method with GC-FID analysis, both R-(-)-linalool and S-(+)-linalool showed good linearity in the range of 0.0625–10μg/g with correlation coefficients(R2) of 0.9954 and 0.9964, respectively. The limits of detection for R-(-)-linalool and S-(+)-linalool(signal/noise = 3) were 3.98 and 4.93ng/g, respectively. The repeatability was 9.91 % for R-(-)-linalool and 7.74 % for S-(+)-linalool.2. There was wide variation in R-(-)-linalool and S-(+)-linalool distribution among the different teas. S-(+)-linalool was detected in all the tea samples, ranging from 0.067 μg/g for green tea to 4.75 μg/g for white tea. In contrast, R-(-)-linalool was found at higher levels only in black tea samples in a concentration range of 0.13–2.78μg/g. It was present at lower levels in three green tea samples(0.07–0.083μg/g), but was not detected in white tea, Oolong tea, dark tea, or in some green tea samples. The highest level of S-(+)-linalool was found in white tea in a concentration range of 1.60–4.75μg/g with a mean value of 2.99μg/g(p < 0.001 compared with the other four teas). The ranking order of mean concentrations of S-(+)-linalool was: white tea(2.99μg/g) > black tea(0.83μg/g) > green tea(0.46μg/g) > Oolong tea(0.22μg/g) > dark tea(0.079μg/g).3. There was wide variation in S-(+)-linalool and R-(-)-linalool levels among the 14 tea cultivars. S-(+)-linalool was detected in all the tea cultivars, ranging from 2.15μg/g for Fuyun 6# to 9.35μg/g for Huang-guan-yin. In contrast, R-(-)-linalool was detected in only 6 samples at levels ranging from 0.32μg/g for Wannong 95# to 2.57μg/g for Gaoyaqi. In addition, it was found that the levels of S-(+)-linalool declined drastically in the prepared green tea samples; they were reduced by 48–97%(average 73%) when compared with the corresponding freeze-dried samples of the 14 different cultivars.4. The levels of the linalool isomers declined drastically during green tea processing, especially during fixation and dry process. During the process of black tea, the changes for the two compounds were similar, with the levels showing a distinct increase from fresh tea leaves to the rolling process, after which they declined drastically through the fermentation and drying processes. Both compounds reached their highest levels during the rolling process. The changes for S-(+)-Linalool were similar dring the process of Oolong tea. |
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