Dynamic changes of key metabolites in Longjing green tea during processing revealed by widely targeted metabolomic profiling and sensory experiments.

In this study, widely targeted metabolomics and chemometrics were utilized to comprehensively analyse the formation of taste compounds in Longjing green tea. A total of 580 non-volatile metabolites were identified by using ultra-performance liquid chromatography-electrospray ionization-tandem mass spectrometry, and alterations in three metabolic pathways were investigated. Notably, the fixation process reduced phosphatidic acid levels, resulting in the formation of lyso-phosphatidylcholine and lyso-phosphatidylethanolamine, as well as the release of esterified polyunsaturated fatty acids. Baiye No.1 had high levels of L-glutamic acid and l-glutamine, while Longjing 43 showed elevated levels of flavones. Correlation analysis and sensory verification indicated that the specific concentration of L-leucine could decrease the umami of the tea. These findings advance our understanding of Longjing green tea quality improvement and cultivar development.

Exogenous stimulation-induced biosynthesis of volatile compounds: Aroma formation of oolong tea at postharvest stage.

Volatile organic compounds (VOCs) are produced by plants responding to biotic and abiotic stresses. According to their biosynthetic sources, induced VOCs are divided into three major classes: terpenoids, phenylpropanoid/benzenoid, and fatty acid derivatives. These compounds with specific aroma characteristics importantly contribute to the aroma quality of oolong tea. Shaking and rocking is the crucial procedure for the aroma formation of oolong tea by exerting mechanical damage to fresh tea leaves. Abundant studies have been carried out to investigate the formation mechanisms of VOCs during oolong tea processing in recent years. This review systematically introduces the biosynthesis of VOCs in plants, and the volatile changes due to biotic and abiotic stresses are summarized and expatiated, using oolong tea as an example.

Effects of baking treatment on the sensory quality and physicochemical properties of green tea with different processing methods.

Seven batches of raw tea leaves, processed by different methods (steaming, pan-frying) and from two different harvesting seasons (spring, autumn), were used to investigate the effect of baking treatment on changes in the composition and content of nonvolatile and volatile compounds. The results showed that baking had a greater impact on sensory and flavor quality, which chemically modified some of taste and aroma components. The aroma concentrations of steamed teas (4,168-10,706 µg/L) were significantly higher than those of pan-fried teas (959-2,608 µg/L), and the aroma concentrations of baked green teas (2,608-10,706 µg/L) were significantly higher than those of unbaked teas (959-4,213 µg/L). Based on VIP > 1 and ACI > 1, (E, E)-3,5-octadien-2-one, hexanal, ß-ionone, 5-methylfurfural, ß-cyclocitral, and linalool were identified as the main aroma compounds. Chemical changes resulting from Maillard reaction were greater during baking of steamed, than pan-fried green tea. These results help improve the quality of green tea with baking.

A targeted and nontargeted metabolomics study on the oral processing of epicatechins from green tea.

Oral processing (OP), referring to the whole process of food digestion in human mouth, has a major influence on food flavor perception. This study focused on the compositional changes of the four green tea epicatechins (viz., EC, EGC, ECG, EGCG) during OP, based on targeted and nontargeted metabolomics. It was found that the four epicatechins were all extensively lost through transformation undergoing OP, among which EC was the most stable one, whereas EGCG the least. EGCG was further revealed to be susceptible to human oral cavity in the simulated OP in vitro. It could be converted physically by precipitating with mucin in saliva, and chemically through hydrolysis and dimerization, mediated mainly by the neutral pH condition. The OP of epicatechins also caused salivary composition changes possibly involving health benefits of green tea. These findings could raise awareness of the interactions between epicatechins, or any other food materials, with human mouth.

Sensory and chemical characteristics of Tieguanyin oolong tea after roasting.

Roasting, a critical process for oolong tea, has been applied to Tieguanyin tea to improve flavor attributes. To investigate the effects of the roasting on the flavor of Tieguanyin, the global metabolomics analysis on the non-volatile and volatile components were proceeded. The weakening of bitterness and astringency, caused by roasting, may be attributed to the decreasing of flavonoids glycosides and procyanidins, whereas the enhancing of sweet aftertaste to the increasing of gallic acid. Besides, l-theanine flavan-3-ols adducts (N-ehtyl-2-pyrrolidinone substituted flavan-3-ols) increased dramatically at 130 °C compared with 105 °C, with the reduction of l-theanine and flavan-3-ols. Meanwhile, high temperature hampered the volatiles' diversity and intensity, resulting from the lowering of floral volatiles, i.e., ß-ionone, jasmine, and nerolidol, yet the nitrogen-containing heterocyclic compounds increased, e.g., pyrroles and pyrazines. The results can help to comprehensively understand the influences of roasting technology on the flavor and chemistry of oolong tea.

Zijuan tea- based kombucha: Physicochemical, sensorial, and antioxidant profile.

Zijuan tea is a representative anthocyanin-rich tea cultivar in China. In this study, Zijuan tea was used to produce a novel kombucha beverage (ZTK). The physicochemical, sensory properties, and antioxidant activity of ZTK were compared with that of black tea kombucha (BTK) and green tea kombucha (GTK). Results indicated that after fermentation, the color of ZTK changed from yellowish-brown to salmon-pink, because its anthocyanins (4.5 mg/L) appeared red in acidic conditions. Meanwhile no significant changes of color were observed in BTK and GTK. The dynamic changes of pH, biomass, and concentrations of sugars, amino acids, and main organic acids were similar in three kombucha beverages, except catechins showing different trends. Moreover, ZTK showed the highest overall acceptability score, antioxidant activity, and concentration of volatiles among the three kombucha beverages. Therefore, Zijuan tea is suitable for the preparation of kombucha beverage with attractive color and health benefits.

Effect of chemical composition of black tea infusion on the color of milky tea.

Milky tea is popular in many countries and its color is an important sensory property. The effects of black tea infusion on the color of milky tea prepared with non-dairy creamer were investigated. The results showed that the redder black tea infusion produced milky tea with more redness, and the color of milky tea was a pleasant pink when the a* value (redness indicator) was in the range of 6.0-7.0. Correlation analysis revealed that the respective theaflavins (TFs), thearubigins (TRs), thearubigins (TBs), (-)-epigallocatechin-3-gallate (EGCG) and chlorogenic acid contents significantly correlated with the a* values of milky tea. A series of complementary experiments were performed to elucidate that TFs and EGCG contributed to the redness of milky tea. The color formation was mainly associated with the binding of phenols to the proteins in the non-dairy creamer. These results contribute to understand the mechanism of color formation in milky tea.

Effect of baking on the flavor stability of green tea beverages.

Flavor stability is important for the quality of tea beverages. Baking is a typical processing technology to improve the flavor of tea leaves. In present study, seven raw tea materials, including steamed spring and autumn tea leaves, pan-fired spring tea leaves, and their corresponding baked tea leaves, were used to investigate the effect of baking on flavor stability of green tea beverages. The results showed that tea beverages prepared with baked tea had better flavor stability. The baking process obviously changed the concentrations of some important flavor substances, especially the aromatic pyrrole substances from 0 (unbaked) to 338.13 µg/L (baked) in tea beverages. Heat treatment had little influence on the flavor of tea beverages prepared from baked tea, but caused great changes in non-volatile and volatile components in those prepared from unbaked leaves. These results could help guide the processing of tea beverages which would improve their flavor quality stability.

Chemical composition, sensory properties and bioactivities of Castanopsis lamontii buds and mature leaves.

Castanopsis lamontii is used as functional herbal tea in southwest China. Usually, only buds rather than mature leaves are applied. To figure out whether mature leaves were suitable for producing herbal tea, chemical composition, sensory properties and bioactivities of Castanopsis lamontii bud infusion (CLB) and mature leaf infusion (CLM) were investigated. According to the results, CLB and CLM had similar non-volatile composition, but in different proportion. Meanwhile, CLB contained more types of volatiles than CLM, leading to distinguishable volatile profiles between them. Sensory assessment showed that CLB had sweet aftertaste and floral aroma. CLM tasted astringent and smelled grassy. Bioactivity evaluation indicated that CLB exhibited higher activities in scavenging free radicals and suppressing lipopolysaccharide-induced inflammation. Taken together, CLB had better overall acceptability in sensory quality and higher bioactivity, implying that Castanopsis lamontii buds were more suitable for producing herbal tea.