Characterization of volatile metabolites in Pu-erh teas with different storage years by combining GC-E-Nose, GC-MS, and GC-IMS.

Storage time is one of the important factors affecting the aroma quality of Pu-erh tea. In this study, the dynamic changes of volatile profiles of Pu-erh teas stored for different years were investigated by combining gas chromatography electronic nose (GC-E-Nose), gas chromatography-mass spectrometry (GC-MS), and gas chromatography-ion mobility spectrometry (GC-IMS). GC-E-Nose combined with partial least squares-discriminant analysis (PLS-DA) realized the rapid discrimination of Pu-erh tea with different storage time (R2Y = 0.992, Q2 = 0.968). There were 43 and 91 volatile compounds identified by GC-MS and GC-IMS, respectively. A satisfactory discrimination (R2Y = 0.991, and Q2 = 0.966) was achieved by using PLS-DA based on the volatile fingerprints of GC-IMS. Moreover, according to the multivariate analysis of VIP > 1.2 and univariate analysis of p < 0.05, 9 volatile components such as linalool and (E)-2-hexenal were selected as key variables to distinguish Pu-erh teas with different storage years. The results provide theoretical support for the quality control of Pu-erh tea.

Lipid metabolic characteristics and marker compounds of ripened Pu-erh tea during pile fermentation revealed by LC-MS-based lipidomics.

Ripened Pu-erh tea (RPT) is a unique microbial fermented tea. Herein, we investigated the lipid composition of RPT and its metabolic changes during pile fermentation, by nontargeted lipidomics profiling and quantitative analysis using liquid chromatography-mass spectrometry (LC-MS). A total of 485 individual lipid species covering 26 subclasses were detected, and fatty acid ester of hydroxy fatty acid (FAHFA) was detected in tea for the first time. Among them, 362 species were significantly altered during fermentation. Chlorophylls decomposition, phospholipids degradation (especially phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine), formation of free fatty acid (FFA) (especially FFA18:3, FFA18:2), and formation of FAHFA, were annotated as the key pathways. Particularly, FAHFAs were undetected in raw tea and gradually enriched to 227.0 ± 9.6 nmol/g after fermentation (p < 0.001), which could serve as marker compounds of RPT associated with microbial fermentation. This study will advance understanding the lipid metabolic fate in microbial fermentation and its role in RPT quality. Chemical compounds studied in this article: Linolenic acid (PubChem CID: 5280934); Linoleic acid (PubChem CID: 5280450); Oleic acid (PubChem CID: 445639); PS(22:0/18:2) (PubChem CID: 52925820); PS(20:0/18:3) (PubChem CID: 52925629); Pheophytin a (PubChem CID: 135398712); Pheophorbide a (PubChem CID: 253193).

Rapid characterization of the volatile profiles in Pu-erh tea by gas phase electronic nose and microchamber/thermal extractor combined with TD-GC-MS.

Aroma plays an important role in the quality of Pu-erh tea. However, the quality evaluation of Pu-erh tea aroma is heavily relied on the experience of sensory evaluation, and the theoretical research is relatively scarce. In the present work, the volatile compounds in Pu-erh tea were characterized by using gas phase electronic nose (e-nose) and microchamber/thermal extractor (µ-CTE) combined with thermal desorption coupled to gas chromatography-mass spectrometry (TD-GC-MS). A satisfactory discrimination model (R2 Y = 0.95, Q2  = 0.807) was obtained by using orthogonal partial least squares discriminant analysis (OPLS-DA) based on the odor fingerprint of different brands of Pu-erh tea. In addition, based on the double criterion of multivariate analysis with VIP >1.0 and univariate analysis with p ≤ 0.001, 39 volatile components were identified to contribute greatly to the discrimination of five brands of Pu-erh tea. The results suggested that gas phase e-nose and µ-CTE combined with TD-GC/MS were simple, rapid techniques to characterize the volatile compounds in Pu-erh tea and were allowed to effectively distinguish different brands of Pu-erh tea, which would provide an important reference on the quality assessment of Pu-erh tea. PRACTICAL APPLICATION: This work demonstrates that the volatile compounds in Pu-erh tea are simply and rapidly characterized by using µ-CTE/TD-GC/MS and gas phase e-nose, allowing to effectively distinguish different brands of Pu-erh tea, which can provide an important reference for the quality assessment and authentication of Pu-erh tea.