Identification of 4-O-p-coumaroylquinic acid as astringent compound of Keemun black tea by efficient integrated approaches of mass spectrometry, turbidity analysis and sensory evaluation
Introduction
Keemun congou black tea (KBT) is a typical type of congou black tea produced in Qimen, Shitai, Dongzhi County, Anhui Province, China, and is listed alongside Assam, Darjeeling and Ceylon black teas as the four most recognized black teas in the world. Previous studies have primarily focused on its health benefits (anti-oxidation, anti-cancer and anti-bacterial properties), secondary metabolites transformation during processing, and geographical origin identification (Fang et al., 2019a, Fang et al., 2019b; Ning et al., 2015; Zhang et al., 2019a). KBT is well-known in the international black tea market for its unique “Keemun aroma”, which has attracted the interest of researchers. For example, the effect of 1–20 years of storage on volatiles and aroma of KBT was investigated, and several volatile compounds were proposed as storage indicators for KBT stored for more than 5 years (Tao et al., 2021). Headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry was also used to examine critical odorants during the KBT withering process (Hou et al., 2020). However, there is still a scarcity of research on the key taste compounds, particularly the key astringent compounds.
There have been many reports about the flavor chemistry of black tea, and some critical volatile compounds in different types of black teas have been identified (Alasalvar et al., 2012, Kraujalytė et al., 2016, Chen et al., 2020, Liu et al., 2021). Tea’s tastes including astringency are mainly derived from the combined effect of its complex non-volatile composition. Astringent compounds with a wide range of threshold concentrations include galloylated catechins, flavan-3-ol oligomers, and flavonoid glycosides (Xu, Zhang, Chen, Wang, Du, & Yin, 2018). Caffeine and non-galloylated catechins typically taste bitter, but a portion of them also contribute to tea’s sweet aftertaste (Zhang et al., 2016). l-theanine is a typical umami component, but its threshold concentration is about 24 mM, which is very high compared to other tea taste compounds (Kaneko, Kumazawa, Masuda, Henze, & Hofmann, 2006). The known taste compounds of tea have been summarized in our recent review article (Zhang, Cao, Granato, Xu, & Ho, 2020). However, compared to the taste properties of known catechins, the taste properties of phenolic acids, which might be related to astringency, are still unknown (Drynan et al., 2010, Zhuang et al., 2020) .
To find astringent compounds in black tea, bio-response and chromatographic methods were used (Scharbert, Holzmann, & Hofmann, 2004a). It has an obvious advantage of separating, identifying and tasting the main astringent compounds, meanwhile sensory test with panelists guide the entire study process. High performance liquid chromatography (HPLC) and tandem mass spectrometry (HPLC-MS) was used to analyze the main and trace discernible non-volatile taste compounds qualitatively and quantitatively (Zhuang et al., 2020). The taste of tea infusion is one of the critical indicators in classifying tea grade, with astringency being an essential perception. It was suggested that the formation of astringency was related to physical, physiological and psychological factors, with salivary proteins aggregation being one of the most important astringency formation mechanisms (Bajec and Pickering, 2008, Canon et al., 2021, Fabian et al., 2015).
Turbidity measurement is a useful technique to investigate the interaction of astringent compounds that aggregate with proteins, such as polyphenols. Because mucin is a major component of human saliva, it is commonly used in turbidity analysis models to interact with potential astringent contributors. Turbidity analysis would provide a highly effective tool for identifying other astringent compounds in tea. The aim of this study was to investigate various astringent compounds using sensory-guided separation and turbidity analysis.
Section snippets
Samples and chemicals
Eight KBT samples of various grades were collected for sensory and chemical analysis from the leaves of Camellia sinensis var. sinensis cv. Zhuye in Qimen County, Anhui Province. The samples were processed as described by Guo et al. (Guo, Long, Meng, Ho, & Zhang, 2018). Before analysis, all samples were stored at 4 °C.
(-)-Epigallocatechin gallate (EGCG), (-)-gallocatechin (GC), (+)-catechin (C), (-)-epicatechin (EC), theaflavin (TF1), theaflavin-3-gallate (TF2a), theaflavin-3́-gallate (TF2b),
Determination of the contents and dose over threshold (DoT) values of main taste compounds in different KBT samples
In previous studies, we determined the contents of the main compounds of KBT with various grades (Guo et al., 2018). There was no significant correlation between the main compounds and tea grades, according to the findings. The contents of flavan-3-ols, purine alkaloids, GA and theaflavins in the KBT infusions of eight grades were determined in this study. The results also showed that there was no significant relationship between main compounds and tea grades.
As shown in Fig. 1, the
Conclusion
The present study presented a method for identifying potential astringent compounds with the aid of a combined LC-MS analysis, sensory evaluation, turbidity analysis and sensory-guided separation. Trans-4-O-pCoQA was discovered to be an astringent contributing compound in KBT. The identification method of turbidity analysis combined with sensory evaluation was shown to be effective; however, there are still limitations that should be further considered. It is challenging to conduct a turbidity
CRediT authorship contribution statement
Mingchun Wen: Investigation, Formal analysis, Data curation, Methodology, Writing - original draft. Zisheng Han: Investigation, Data curation. Yuqing Cui: Investigation, Formal analysis. Chi-Tang Ho: Writing - review & editing. Xiaochun Wan: Project administration, Funding acquisition. Liang Zhang: Conceptualization, Writing - original draft, Project administration, Funding acquisition, Project administration, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by Natural Science Foundation of China (32072633, 32072634, and 31201335) and Young Elite Scientist Sponsorship Program by National CAST (2016QNRC001).
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