Yellow tea (Camellia sinensis L.), a promising Chinese tea: Processing, chemical constituents and health benefits

Yellow tea, also known as huángchá in Chinese, is a lightly fermented tea unique to China. As a rare and precious variety of tea, it has gained increasing popularity in recent years because of its pleasant mellow taste and known health benefits such as anti-oxidation, anti-inflammation and anti-cancer properties. Yellow tea is similar to green tea in many ways. The initial production process of both teas is the same, but the production of yellow tea requires additional steps. A unique procedure called "sealed yellowing" is always involved in yellow tea processing to increase the oxidation level and remove the characteristic grassy smell associated with green tea while still preserving its health benefits. Compared to other types of teas, yellow tea is much less well-known and studied. In this review, the history and classification of yellow tea are introduced. The processing procedures, including detailed information about "sealed yellowing", are presented. The bioactive chemical compounds common in various types of teas or unique to yellow tea are discussed. Finally, future needs in research and development of yellow tea are proposed.

Antioxidative potential, nutritional value and sensory profiles of confectionery fortified with green and yellow tea leaves (Camellia sinensis)

This paper presents a study on development of functional food products containing green and yellow tea leaves. The results indicated that green and yellow tea are significant tools in the creation of the nutritional value, antioxidative potential and stability of the lipid fraction of cookies. Tea-fortified cookies showed considerably higher contents of dietary fiber, especially hemicellulose and insoluble fractions, and were characterized by significantly higher antioxidant potential associated with their phenolics content. Results of ABTS, DPPH, ORACFL and PCL assay showed significantly higher antioxidant potential of tea cookies, highest for yellow tea. The antioxidative potential of applied teas was significant in terms of the inhibition of hydroperoxide content, while formation of secondary lipid oxidation products was less spectacular. It is concluded that tea leaves could be widely used as a source of polyphenols with high antioxidative potential, as well as fiber; thus introducing numerous health benefits for the consumer.

Analysis of taste characteristics and identification of key chemical components of fifteen Chinese yellow tea samples

In order to explore the taste characteristics and molecular sensory basis of Chinese yellow tea, in this study, quantitative descriptive analysis (QDA) and partial least squares regression (PLSR) were used to analyze the sensory characteristics and chemical components of 15 yellow tea samples from different regions of China. The results showed that: 11 sensory descriptors and their definitions were obtained by QDA, namely, sweet, umami, bitter, sour, astringent, sweet after taste, mellow, neutral, after-taste, thick and tainted taste. The results of variance indicated that there were significant variation in taste sub-attributes of different samples (p <0.05). Principal component analysis indicated that there was a positive correlation between bitter and astringent, between sweet, umami and sour, and between mellow, thick, after-taste and neutral. All yellow tea samples were divided into four categories according to cluster analysis. The results of PLSR showed that there were 22 chemical components that had an important contribution to the taste characteristics of yellow tea, and the chemical components that had an important influence on each taste component were obtained. The identification of key contribution components of taste characteristics in yellow teas will provide a theoretical basis for further research on the directional adjustment and control of tea taste quality.

Aroma formation and transformation during sealed yellowing process of Pingyang yellow tea

Yellow tea, a unique type of tea in China which is characterized with yellow color, has gained increasing popularity due to its pleasant taste. However, transformation of aroma compounds during sealed yellowing has been poorly understood. Results of sensory evaluation exhibited that yellowing time was the key factor for flavor and fragrance formation. A total of 52 volatile components during sealed yellowing process of Pingyang yellow soup were further collected and analyzed. The results demonstrated that the sealed yellowing process significantly increased the ratio of alcohol and aldehyde compounds in the aroma volatiles of yellow tea, which were primarily composed of geraniol, linalool, phenylacetaldehyde, linalool oxide and cis-3-hexenol, and their proportion increased with the prolongation of sealed yellowing. Mechanistic speculation revealed that the sealed yellowing process promoted release of alcoholic aroma compounds from their glycoside precursors and enhanced Strecker and oxidative degradation. This study revealed the transformation mechanism of aroma profile during the sealed yellowing process, which would facilitate processing of yellow tea.

The formation mechanism of aroma quality of green and yellow teas based on GC-MS/MS metabolomics

Aroma is a crucial determinant of tea quality. While some studies have examined the aroma of yellow tea, there are no reports of the difference and formation mechanism of aroma quality between yellow and green teas from the same tea tree variety. This study employed gas chromatography-mass spectrometry to investigate the difference and formation mechanism of the aroma of yellow and green tea at the omics level, based on sensory evaluation. The sensory evaluation revealed that green tea has a distinct faint scent and bean aroma, while yellow tea, which was yellowed for 48 h, has a noticeable corn aroma and sweet fragrance. A total of 79 volatile metabolites were detected in the processing of yellow and green tea, covering 11 subclasses and 27 were differential volatile metabolites. Benzoic acid, 2-(methylamino-), methyl ester, terpinen-4-ol ethanone, 1-(1H-pyrrol-2-yl-), 3-penten-2-one, 4-methyl- and benzaldehyde were characteristic components of the difference in aroma quality between green and yellow teas. Eleven volatile metabolites significantly contributed to the aroma quality of green and yellow teas, especially acetic acid, 2-phenylethyl ester, with rose and fruity aromas. KEGG enrichment analysis showed that the arginine and proline metabolism might be the key mechanism of aroma formation during green and yellow teas' processing. These finding provide a theoretical basis way for the aroma formation of green and yellow teas.

Metabolomics and electronic-tongue analysis reveal differences in color and taste quality of large-leaf yellow tea under different roasting methods

Roasting is a key process in the production of large-leaf yellow tea (LYT). In this study, we synthesized metabolomics and electronic-tongue analysis to compare the quality of charcoal-roasted, electric-roasted and drum-roasted LYT. Charcoal-roasted LYT had the highest yellowness and redness, drum-roasted LYT had a more prominent umami and brightness, and electric roasting reduced astringency. A total of 48 metabolites were identified by metabolomics. Among these, leucocyanidin, kaempferol, luteolin-7-lactate, and apigenin-7-O-neohesperidoside might affect the brightness and yellowness. Theanine, aspartic acid, and glutamic acid contents significantly and positively correlated with umami levels, and the high retention of flavonoid glycosides and catechins in drum-roasted LYT contributed to its astringency. These findings elucidate the contribution of the roasting method to the quality of LYT and provide a theoretical basis for LYT production.

Chemical profile and in-vitro bioactivities of three types of yellow teas processed from different tenderness of young shoots of Huoshanjinjizhong (Camellia sinensis var. sinensis)

In the present study, bud yellow tea (BYT), small-leaf yellow tea (SYT) and large-leaf yellow tea (LYT) were produced from the same local "population" variety Huoshanjinjizhong (Camellia sinensis var. sinensis), and the effects of raw material tenderness on the chemical profile and bioactivities of these teas were investigated. The results showed that 11 crucial compounds were screened by headspace solid-phase microextraction-gas chromatography-mass spectrometry from 64 volatiles in these yellow teas, among which the heterocyclic compounds showed the greatest variations. In addition, 43 key compounds including organic acids, flavan-3-ols, amino acids, saccharides, glycosides and other compounds were screened by liquid chromatography-mass spectrometry from 1781 non-volatile compounds. BYT showed the best α-glucosidase inhibitory activity and antioxidant capacity among the selected yellow teas, which might be contributed by the higher content of galloylated catechins. These findings provided a better understanding of the chemical profile and bioactivities of yellow teas.

Integration of non-targeted/targeted metabolomics and electronic sensor technology reveals the chemical and sensor variation in 12 representative yellow teas

Yellow tea is a lightly fermented tea with unique sensory qualities and health benefits. However, chemical composition and sensory quality of yellow tea products have rarely been studied. 12 representative yellow teas, which were basically covered the main products of yellow tea, were chosen in this study. Combined analysis of non-targeted/targeted metabolomics and electronic sensor technologies (E-eye, E-nose, E-tongue) revealed the chemical and sensor variation. The results showed that yellow big tea differed greatly from yellow bud teas and yellow little teas, but yellow bud teas could not be effectively distinguished from yellow little teas based on chemical constituents and electronic sensory characteristics. Sensor variation of yellow teas might be attributed to some compounds related to bitterness and aftertaste-bitterness (4'-dehydroxylated gallocatechin-3-O-gallate, dehydrotheasinensin C, myricitin 3-O-galactoside, phloroglucinol), aftertaste-astringency (methyl gallate, 1,5-digalloylglucose, 2,6-digalloylglucose), and sweetness (maltotriose). This study provided a comprehensive understanding of yellow tea on chemical composition and sensory quality.

Metabolomics analysis reveals the mechanism underlying the improvement in the color and taste of yellow tea after optimized yellowing

In this study, an optimized yellowing process for yellow tea (YT) was developed by response surface methodology. The results showed that increasing the yellowing temperature from 20 °C to 34 °C, increasing the relative humidity from 55% to 67%, and reducing the yellowing time from 48 h to 16 h, caused a 40.5% and 43.2% increase in the yellowness and sweetness of YT, respectively, and improved the consumer acceptability by 36.8%. Moreover, metabolomics was used to explore the involved mechanisms that resulted in the improved YT quality. The optimized yellowing promoted the hydrolysis of 5 gallated catechins, 6 flavonoid glycosides, theogallin and digalloylglucose, resulting in the accumulation of 5 soluble sugars and gallic acid. Meanwhile, it promoted the oxidative polymerization of catechins (e.g., theaflagallin, δ-type dehydrodicatechin and theasinensin A), but decelerated the degradation of chlorophylls. Overall, this optimized yellowing process could serve as a guide to a shorter yellowing cycle.

GC-MS, GC-O, and sensomics analysis reveals the key odorants underlying the improvement of yellow tea aroma after optimized yellowing

An optimized yellowing process for yellow tea (YT) was recently developed. The study found that the optimized yellowing process caused a significant increase in sweet and floral aromas by 31.3% and 24.0%, respectively. A total of 21 aroma-active compounds were identified using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-olfactometry (GC-O) combined with sensomics analysis. Quantification of the 15 aroma-active compounds and calculation of odor activity values (OAVs) showed that the OAVs of sweet and floral aroma compounds increased significantly by 986.2% and 46.4%, respectively, after the optimized yellowing process. Sensory-directed aroma reconstitution and omission experiments confirmed that dimethyl sulfide, 3-methylbutanal, β-ionone, β-damascenone, geraniol, phenylacetaldehyde, and linalool were the key odorants in YT after the optimized yellowing process. Odorant addition tests further demonstrated that β-damascenone (OAV 590.4) was the main odorant for YT sweet aroma enhancement, while β-ionone (OAV 884.6) was the main odorant for YT floral aroma enhancement.

Yellow tea polysaccharides protect against non-alcoholic fatty liver disease via regulation of gut microbiota and bile acid metabolism in mice

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) is a major clinical and global public health issue, with no specific pharmacological treatment available. Currently, there is a lack of approved drugs for the clinical treatment of NAFLD. Large-leaf yellow tea polysaccharides (YTP) is a natural biomacromolecule with excellent prebiotic properties and significant therapeutic effects on multiple metabolic diseases. However, the specific mechanisms by which YTP regulates NAFLD remain unclear.

PURPOSE

This study aims to explore the prebiotic effects of YTP and the potential mechanisms by which it inhibits hepatic cholesterol accumulation in NAFLD mice.

METHODS

The effects of YTP on lipid accumulation were evaluated in NAFLD mice through obesity trait analysis and bile acids (BAs) metabolism assessment. Additionally, fecal microbiota transplantation (FMT) was performed, and high-throughput sequencing was employed to investigate the mechanisms underlying YTP's regulatory effects on gut microbiota and BA metabolism.

RESULTS

Our study demonstrated that YTP altered the constitution of colonic BA, particularly increasing the levels of conjugated BA and non-12OH BA, which suppressed ileum FXR receptors and hepatic BA reabsorption, facilitated BA synthesis, and fecal BA excretion. The modifications were characterized by a decrease in the levels of FXR, FGF15, FGFR4, and ASBT proteins, and an increase in the levels of Cyp7a1 and Cyp27a1 proteins. YTP might affect enterohepatic circulation and by the activated the hepatic FXR-SHP pathway. Meanwhile, YTP reshaped the intestinal microbiome structure by decreasing BSH-producing genera and increasing taurine metabolism genera. The correlation analysis implied that Muribaculaceae, Pseudomonas, acterium_coprostanoligenes_group, Clostridiales, Lachnospiraceae_NK4A136_group, Delftia, Dubosiella, and Romboutsia were strongly correlated with specific BA monomers.

CONCLUSIONS

YTP modulates bile salt hydrolase-related microbial genera to activate alternative bile acid synthesis pathways, thereby inhibiting NAFLD progression. These results suggest that YTP may serve as a potential probiotic formulation, offering a feasible dietary intervention for NAFLD.