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.
Highlights
Oolong tea, which is derived from the Camellia sinensis L. plant, is a well-known traditional Chinese tea
Targeted detection of Carboxylic 6 (C6) acids and C6 alcohols during the postharvest processing of oolong tea showed that three types of C6 alcohol were detected, namely, hexanol (CAS: 111-27-3), (Z)-3-hexen-1-ol (CAS: 92896-1), and (E)-2-hexen-1-ol (CAS: 928-95-0), with hexanoic acid (CAS: 142-62-1) as the only C6 acid detected
The turnover stage is conducive to increasing the C6 alcohol content, while the spreading stage promotes C6 acid accumulation
Summary
Oolong tea, which is derived from the Camellia sinensis L. plant, is a well-known traditional Chinese tea. About 80 types of volatile aroma compounds have been identified in fresh tea leaves, while more than 300 have been found in oolong tea (Yang et al, 2013). Volatile compounds that constitute oolong aroma are divided into four classes based on their origin, namely, fatty acid derivatives, phenylpropanoids/benzenoids, terpenoids, and norisoprenoids (Fu et al, 2017). Fatty acid (FA) derivatives of carboxylic 6 (C6) volatile compounds are considerably accumulated and transformed during the postharvest processing of oolong tea (Zhou et al, 2020). Postharvest processing imparts the oolong tea with a rich and varied natural floral and fruity aroma. Volatile fatty acid derivatives (VFADs) of C6 compounds, such as hexanal, (Z)-3-hexen-1-ol, and hexanoic acid, are favorable indicators for tea technicians during oolong tea processing and precursors of dispersive fruit esters and boiling-point aroma substances
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