Differentiation of two types of pu-erh teas by using an electronic nose and ultrasound-assisted extraction-dispersive liquid–liquid microextraction-gas chromatography-mass spectrometry

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Abstract
It is a challenging task to discriminate raw pu-erh tea, notably aged raw tea, from ripened pu-erh tea, both of which are the two primary types of pu-erh teas, only based on the taster's sensory evaluation. In the current study, a workflow was proposed to differentiate those two clusters of pu-erh teas, as well as to point out and verify the markers responsible for the discrimination. Initially, an electronic nose was utilized for the rapid discrimination. Then, an efficient method based on ultrasound-assisted extraction-dispersive liquid–liquid microextraction-gas chromatography-mass spectrometry (UAE-DLLME-GC-MS) coupled with chemometric methods was developed to disclose the metabolic profiles and pinpoint the markers for discrimination. Afterwards seven methoxyphenolic derivatives were simultaneously determined in both pu-erh teas. The role of volatile components in the classification of pu-erh teas was proved using the electronic nose (E-nose). Diverse parameters were optimized for UAE-DLLME-GC-MS, and a total of 84 volatile constituents were detected and identified. The methoxyphenolic derivatives as well as some alcohol derivatives were screened out as the primary markers by principle component analysis, and significant differences were revealed for the contents of methoxyphenolic compounds in these two types of pu-erh teas. Taken together, methoxyphenolic compounds as well as alcohol derivatives were found and verified as the markers for the differentiation between raw and ripened pu-erh teas, and either an E-nose or UAE-DLLME-GC-MS could be applied as a reliable tool to achieve the discrimination.

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