Metabolic Flow of C6 Volatile Compounds From LOX-HPL Pathway Based on Airflow During the Post-harvest Process of Oolong Tea

Abstract

Aroma is an essential quality indicator of oolong tea, a tea derived from the Camellia sinensis L. plant. Carboxylic 6 (C6) acids and their derivative esters are important components of fatty acid (FA)-derived volatiles in oolong tea. However, the formation and regulation mechanism of C6 acid during postharvest processing of oolong tea remains unclear. To gain better insight into the molecular and biochemical mechanisms of C6 compounds in oolong tea, a combined analysis of alcohol dehydrogenase (ADH) activity, CsADH2 key gene expression, and the FA-derived metabolome during postharvest processing of oolong tea was performed for the first time, complemented by CsHIP (hypoxia-induced protein conserved region) gene expression analysis. Volatile fatty acid derivative (VFAD)-targeted metabolomics analysis using headspace solid-phase microextraction–gas chromatography time-of-flight mass spectrometry (HS-SPEM-GC-TOF-MS) showed that the (Z)-3-hexen-1-ol content increased after each turnover, while the hexanoic acid content showed the opposite trend. The results further showed that both the ADH activity and CsADH gene expression level in oxygen-deficit-turnover tea leaves (ODT) were higher than those of oxygen-turnover tea leaves (OT). The C6-alcohol-derived ester content of OT was significantly higher than that of ODT, while C6-acid-derived ester content showed the opposite trend. Furthermore, the HIP gene family was screened and analyzed, showing that ODT treatment significantly promoted the upregulation of CsHIG4 and CsHIG6 gene expression. These results showed that the formation mechanism of oolong tea aroma quality is mediated by airflow in the lipoxygenase–hydroperoxide lyase (LOX-HPL) pathway, which provided a theoretical reference for future quality control in the postharvest processing of oolong tea.

Aroma characteristics of Wuyi rock tea prepared from 16 different tea plant varieties

“…Additionally, linalools, which have been identified as key volatile compounds contributing to oolong tea aroma ( Zeng et al, 2020 ), were also not listed since their rOAVs were less than one because the manufacturing processing and the quality characteristics of WRTs were different from those of Minnan oolong tea (Tieguanyin) or Guangdong oolong tea (Fenghuangdancong). Interestingly, the manufacture processing of sun withering, shaking, and fixing can promote the accumulation of volatile compounds, such as linalool, (E)-nerolidol, indole and jasmine lactone ( Hu et al, 2018 , Wu et al, 2022 , Zeng et al, 2016 , Zeng et al, 2018 , Zeng et al, 2019 , Zhou et al, 2017 , Zhou et al, 2021 , Zhu et al, 2019 , Zhu et al, 2020 ); drying and baking also contributed to aroma compound formation; however, the contents of certain key floral odor volatiles, including linalool, geraniol, hexanal, and phenethyl isovalerate, were reduced after drying and baking ( Guo et al, 2021 , Guo et al, 2022 , Liu et al, 2022 , Yang et al, 2021 , Yang et al, 2022 ), suggesting that baking is the key and fundamental processing for the quality formation of WRTs. Similar to other types of oolong tea, indole, benzoic acid methyl ester, methyl salicylate, methyl jasmonate, (E)-nerolidol, and alpha-farnesene were key contributors to the WRT aroma.…”

Chromatin accessibility mediated transcriptome changes contribute to flavor substance alterations and jasmonic acid hyperaccumulation during oolong tea withering process

“…In addition to JA, the LOX has been also confirmed as an upstream enzyme of long‐chain fatty acid catabolism pathway, responsible for the biosynthesis of short‐chain C5/C6 volatiles in various plant species (Chen et al., 2004; Shen et al., 2014; Zhang et al., 2017; Zhou et al., 2021). In tea plant, C5/C6 volatiles are important components of fatty acid‐derived volatiles, contributing to aroma of oolong tea (Zhou et al., 2021).…”

Metabolomic analysis reveals the quality characteristics of Yi Gong tea leaves at different harvesting periods

“…As an essential quality indicator of tea, the aroma of tea leaves can be classified into four categories based on its source: terpenes, fatty acids and their derivatives, phenylpropanoids and isoprenoids (Zhou et al, 2021). Among them, fatty acids and their volatile oxidation products contributed to the formation of flavor in tea beverages (Pignitter et al, 2015).…”