Directed Accumulation of Nitrogen Metabolites through Processing Endows Wuyi Rock Tea with Singular Qualities

Abstract

The execution of specific processing protocols endows Wuyi rock tea with distinctive qualities produced through signature metabolic processes. In this work, tea leaves were collected before and after each of three processing stages for both targeted and untargeted metabolomic analysis. Metabolic profiles of processing stages through each processing stage of rotation, pan-firing and roasting were studied. Overall, 614 metabolites were significantly altered, predominantly through nitrogen- enriching (N) pathways. Roasting led to the enrichment of 342 N metabolites, including 34 lipids, 17 organic acids, 32 alkaloids and 25 amino acids, as well as secondary derivatives beneficial for tea quality. This distinctive shift towards enrichment of N metabolites strongly supports concluding that this directed accumulation of N metabolites is how each of the three processing stages endows Wuyi rock tea with singular quality.

Keywords: N-contained metabolites; Wuyi rock tea; metabolome; process; tea quality.

Figure 1

Flowchart of the main processing stages utilized to produce Wuyi rock tea (A) and PCA analysis between corresponding tea samples (B). The processing stages of Wuyi rock tea include rotation, pan-firing and roasting. The data used for PCA analysis includes the concentrations of conventional quality index compounds and main secondary metabolites measured in the samples of fresh leaves (FL), spread leaves (SL), raw tea (RT) and commercial tea (CT).

Figure 2

Dynamic changes in the concentrations of conventional quality index compound categories (A–E) and main secondary metabolites (F–L) through the processing stages of Wuyi rock tea. Conventional quality index compound categories include total flavonoids (TF), water extracts (WE), soluble sugars (SS), free amino acids (FAA) and tea polyphenols (TP). The main secondary metabolites include caffeine, theanine, epigallocatechin gallate (EGCG), epicatechin gallate (ECG), epigallocatechin (EGC), gallocatechin (GC) and gallocatechin gallate (GCG). FL: fresh leaves; SL: spread leaves; RT: raw tea; CT: commercial tea. The values are shown as means ± standard errors (SEs), (n = 10). Different letter(s) indicate significant differences among process stages at the p = 0.05 threshold level.

Figure 3

Analysis of metabolites significantly altered through processing of Wuyi rock tea as identified in untargeted metabolomics: (A) PCA plots; (B) heat map and cluster analysis; (C) volcano plot maps; (D) trends in the abundance of significant metabolite clusters across processing stages, with metabolite clusters delimited in K-means cluster analysis of metabolites displaying significant variation in concentration across processing stages; (E) Diagrams of significant metabolite subclasses grouped according to K-means clustering of metabolites altered across processing stages. FL: fresh leaves; SL: spread leaves; RT: raw tea; CT: commercial tea.

Figure 4

KEGG enrichment analysis of untargeted metabolites that were significantly altered between FL and CT. Green triangles indicate pathways associated with N metabolism.

Figure 5

Dynamic changes in the abundance of N-containing metabolites across the processing stages of Wuyi rock tea production: (A) heat map of all altered N metabolites; (B,C) dynamic profiles of N metabolites that were significantly increased in the rotation and roasting stages, respectively; (D) changes of key metabolites that significantly increased in the roasting stage from (C). The values are shown as means ± standard errors (SEs), (n = 3). Yellow and blue triangles indicate the rotation and roasting stages, respectively. Different letters indicate significant differences among different processing stages at the p = 0.05 threshold level. FL: fresh leaves; SL: spread leaves; RT: raw tea; CT: commercial tea.

Figure 6

Model of Wuyi rock tea processing stages and mechanisms proposed to underlie the accumulation of N metabolites. In the rotation stage, proteins can be degraded to various amino acids and their derivatives through mechanical collisions spurred in leaf rotation. Then, pan-firing promotes hydrolysis of polysaccharides in high temperature and moisture conditions. Subsequent Maillard reactions occurring in the roasting stage promote a large increases in N-contained metabolites via the combination of amidogen and carbonyl compounds. The resulting N metabolites increase the quality of Wuyi rock tea by imparting specific aromas, flavors and health-promoting characteristics. ω: moisture percentage of samples.