Comparison of Potent Odorants in Raw and Ripened Pu-Erh Tea Infusions Based on Odor Activity Value Calculation and Multivariate Analysis: Understanding the Role of Pile Fermentation

Improving the flavor of summer green tea (Camellia sinensis L.) using the yellowing process

Summer green tea (SGT) has poor flavor due to its high levels of bitterness and astringency. The present study aimed to improve the flavor of SGT using the yellowing process. The results showed that after the yellowing process, the sweetness and overall acceptability increased, and the content of gallated catechins and flavonol glycosides decreased by 30.2% and 27.4%, respectively, as did the bitterness and astringency of SGT. Yellowing caused a decrease in the concentration of some aroma compounds, such as (z)-3-hexen-1-ol, 1-hexanol, pentanal, heptanal and 1-octanol, which caused grassy, floral and fruity aromas. In contrast, the concentrations of 1-octen-3-ol, benzene acetaldehyde and β-ionone increased, which have mushroom and sweet aromas. Meanwhile, the sweetness and umami of SGT were enhanced by the addition of selected aroma compounds (1-octen-3-ol, benzene acetaldehyde and β-ionone), demonstrating that the yellowing process improves the flavor of SGT through odor-taste interactions.

Effects of electrostatic spray drying on the sensory qualities, aroma profile and microstructural features of instant Pu-erh tea

The sensory qualities, aroma profile, and microstructural features of instant Pu-erh teas (IPTs) produced by electrostatic spray drying (ESD) were evaluated by sensory and instrumental analyses and compared with those produced by other drying methods (freeze-drying [FD], vacuum drying [VD], and conventional spray drying [CSD]). The sensory qualities of ESDIPT were similar to those of FDIPT, and better than those of VDIPT and CSDIPT. Eighty-eight volatiles were detected in all IPTs, and 45 odor-active compounds were captured. Most of their OAVs were higher in ESDIPT than in VDIPT and CSDIPT but were lower than those in FDIPT. Dihydro-β-ionone had the highest OAV. Aroma recombination experiments were performed to verify the identification results. ESDIPT was present in the shape of microspheres with many regular concave surfaces, which was different from those treated by other drying methods. In terms of sensory quality and productivity, ESD would be a potential method for IPT production.

Characterization and Quantitative Comparison of Key Aroma Volatiles in Fresh and 1-Year-Stored Keemun Black Tea Infusions: Insights to Aroma Transformation during Storage

The aroma of Keemun black tea (KBT) changes during storage. We investigated key aroma volatiles of fresh KBT (FKBT) and KBT stored for 1 year. Through gas chromatography−olfactometry−mass spectrometry/aroma extract dilution analysis (GC-O-MS/AEDA), 27 aroma volatiles with a flavor dilution (FD) value ≥16 were quantitated. In odor activity value (OAV) analysis, the two samples had nearly the same key aroma volatiles; (Z)-methyl epijasmonate was the exception. Dimethyl sulfide, 3-methylbutanal, 2-methylpropanal, and linalool had especially high OAVs. Except for β-damascenone, volatiles with OAVs > 1 had higher concentrations in FKBT, which revealed that most key aroma compounds were lost during storage. Sweet, malty, floral, and green/grassy aromas corresponded directly to certain compounds. Lastly, the addition test indicated that the addition of several key aroma volatiles decreasing during storage could enhance the freshness of KBT aroma, which may be a potential to control the aroma style of KBT or other teas in industry.

Comparative investigation on aroma profiles of five different mint (Mentha) species using a combined sensory, spectroscopic and chemometric study

Mint is a widely used aromatic plant, and the aroma varies among different species. The aroma of five mint species, Mentha citrata L. (MC), Mentha piperita L. (MPI), Mentha spicata L. (MSP), Mentha persicaria L. (MPE), and Mentha suaveolens L. (MSU), were comparatively studied on the sensorial and molecular level. Quantitative descriptive analysis revealed that MC presented a pronounced lemon-like note, MSU is dominated by citrus and floral aromas, MPI has a prominent minty flavor, MSP and MPE have a similar scent, both of which are flavored with a spearmint-like note. Forty-one odorants with odor activity values (OAVs) ≥1 were characterized. Principal component analysis and orthogonal partial least squares discrimination analysis based on OAVs indicated that α-citral, menthofuran, isomenthone, menthol, carvone, and linalool were potential odor-active markers for five mint species discrimination. This study herein will provide guidance for mint resources utilization and also aid mint breeding with better flavor.

Unraveling the acetals as ageing markers of Chinese Highland Qingke Baijiu using comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry combined with metabolomics approach

Objectives

The ageing process has a significant impact on the aroma of Chinese Baijiu, which could strengthen the desirable flavor characteristics and reduce the undesirable ones. The aim of this study was to observe the initiation of meaningful changes in volatile fraction and locate the ageing markers during ageing storage of Chinese Highland Qingke Baijiu.

Materials and Methods

Samples of Chinese Qingke Baijiu were aged for 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11 months before analysis. The samples were isolated by liquid–liquid extraction and then analyzed by comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry. The acquired data were processed by untargeted and targeted metabolomics approach to locate the ageing markers.

Results

The untargeted metabolomics analysis (hierarchical clustering analysis, HCA) shows that the chemical composition of Qingke Baijiu presents a statistically significant deviation from the reference scenario after 5 months. Subsequently, supervised statistics analysis (orthogonal partial least squares discrimination analysis) was performed to locate the markers, which changed significantly during ageing. Fifteen markers were located, and seven of them were acetals. Notably, 1,1-diethoxy-propane, 1,1-diethoxy-butane, and 1,1-diethoxy-3-methyl-butane are important contributors to the flavor of Chinese Baijiu. The identified markers were applied for the untargeted metabolomics (HCA), and the results revealed that these markers could divide the Qingke Baijiu into two ageing stages, 0–5 months and 6–11 months.

Conclusion

The results suggest that it is a valuable tool for monitoring the changes of volatile compounds and locating the age markers in Chinese Baijiu.

Evolution analysis of flavor-active compounds during artificial fermentation of Pu-erh tea

For the urgent need for fermentation control and product quality improvement of Pu-erh tea, gas chromatography-mass spectrometry and odor activity value (OAV) were used to comprehensively investigate the flavor-active compounds during artificial fermentation of Pu-erh tea. A flavor wheel was constructed to expound the sensory attributes evolution during fermentation. With an increased total volatiles content, 43 were significantly up-regulated and 30 were down-regulated among 131 detected volatiles. Key active compounds of three aroma types, namely fresh fragrance, fruit-fungus fragrance and stale-Qu fragrance, were analyzed based on OAV. β-damascenone was firstly found contributing most to the aroma of Pu-erh tea, followed by 1,2,3-methoxybenzene and (E,E)-2,4-nonadienal. γ-terpinene, linalool, 1,2,4-trimethoxybenzene, 1,2,3-trimethoxybenzene, and 4-ethylveratrol were identified as the potential markers responsible for aroma differences among three fermentation stages. Finally the metabolic evolution of key flavor-active compounds were systematically summarized. This study provides significant guidance in fermentation control and new product development of Pu-erh tea.

Region identification of Xinyang Maojian tea using UHPLC-Q-TOF/MS-based metabolomics coupled with multivariate statistical analyses

Xinyang Maojian tea is a kind of famous roasted green tea produced in the middle of China. Ultra-high performance liquid chromatography-quadrupole time of flight-mass spectrometry (UHPLC-Q-TOF/MS)-based metabolomics coupled with multivariate statistical analyses, including principal component analysis (PCA) and hierarchical cluster analysis (HCA), were carried out in XMMJTs collected from Luoshan, Shangcheng, and Shihe Counties, respectively. Additionally, seven catechins, four flavonoids, two purine alkaloids, and gallic acid contents were determined by HPLC. Differential metabolites were selected by p-value <0.05, and fold change >1.50 or < 0.66 among 745 detected metabolites in metabolomics analysis. The results showed significant (p < 0.05) differences of three catechins including (-)-epicatechin, (-)-epicatechin gallate, and (-)-gallocatechin gallate, four flavonoids (i.e. quercetin, kaempferol, myricetin, and rutin), and theobromine among three various regions, and significant (p < 0.05) differences of (-)-epicatechin gallate, (-)-epigallocatechin, (+)-catechin, gallic acid, and kaempferol between Shuchazao and Group cultivar. The HCA showed that, except for two samples (i.e. LS 2 and SH 2) of Shuchazao cultivar clustered together, others could be clustered completely according to production place. The 63 relevant differential metabolites could achieve the purpose of region identification through PCA. Kyoto encyclopedia of genes and genomes (KEGG) metabolic pathway analysis elaborated the impact of geographical origin and tea cultivar on physiological metabolism in tea tree. PRACTICAL APPLICATION: Ultra-high performance liquid chromatography-quadrupole time of flight-mass spectrometry (UHPLC-Q-TOF/MS)-based liquid chromatography-tendem mass spectrometry (LC-MS/MS) metabolomics coupled with multivariate statistical analyses, such as principal component analysis (PCA) and hierarchical cluster analysis (HCA), revealed 63 differential metabolites related to production place, which contributed to the region identification of Xinyang Maojian teas.

HS-SPME and GC/MS volatile component analysis of Yinghong No. 9 dark tea during the pile fermentation process

Each type of tea has a unique volatile profile due to its variety, processing technologies and origin. Using HS-SPME and GC/MS, we analyzed the changes of volatile components in cultivar Yinghong No. 9 during pile-fermentation every 10 days. A total of 94 compounds showed significant differences during a total of 60 days mainly including alkanes, ketones, esters, terpenes, aromatics and heterocyclic compounds. Interestingly, 13 metabolites were progressively reduced during the first 20 days and remained unchanged in subsequent procedures, while 17 metabolites remained unchanged in the early stage and progressively increased during the last 20 days of pile fermentation, indicating that they are characteristic volatile compounds of raw material sun-dried green tea and dark tea, respectively. β-ionone, phenylethyl alcohol, and a-ionone could be the top three contributed aroma compounds in the final dark tea. Our study provides a theoretical basis for process and quality improvement of Yinghong No. 9.

Comparison of characteristic components in tea-leaves fermented by Aspergillus pallidofulvus PT-3, Aspergillus sesamicola PT-4 and Penicillium manginii PT-5 using LC-MS metabolomics and HPLC analysis

Microbiota influenced quality formation of ripened Pu-erh tea. To understand the effect of each tea-derived fungal strain, tea-leaves were fermented by Aspergillus pallidofulvus PT-3 (ApaPT), Aspergillus sesamicola PT-4 (AsePT) and Penicillium manginii PT-5 (PmaPT), respectively. 14 Phenolic compounds, 3 purine alkaloids, 19 free amino acids and γ-aminobutyric acid contents were determined by HPLC and amino acid analyzer analysis. Additionally, UHPLC-Q-TOF/MS method was developed for LC-MS metabolomics analysis. Multivariate statistical analyses, such as PCA and HCA, exhibited that the chemical profile of PmaPT fermentation was similar to biocidal treatment, but had significant differences with ApaPT and AsePT fermentation. The differentiated metabolites (VIP > 1, p < 0.05 and FC > 1.50 or < 0.66) and one-way ANOVA revealed the impact of three fungal strains in tea-leaves fermentation. APaPT and AsePT contributed to biosynthesis of gallic acid and several flavonoids, such as kaempferol, quercetin and myricetin in the metabolism of phenolic compounds.

Characterizing volatile metabolites in raw Pu'er tea stored in wet-hot or dry-cold environments by performing metabolomic analysis and using the molecular sensory science approach

The aroma profile of raw pu'er tea (RPT) depends on its storage duration (2-10 years) and storage conditions (wet-hot or dry-cold environment). We analyzed the major odorants of RPT samples by performing metabolomic analysis and by using the molecular sensory science approach. Under dry-cold storage conditions, tea leaves had more carotenoid derivatives, glycoside-derived volatiles, and phenolic volatiles, resulting in "fresh," "floral," and "sweet" aroma. Under wet-hot storage conditions, tea leaves had more methoxybenzenes, which contributed considerably to their "stale" and "woody" aroma. We identified 11 and 4 compounds as the odor markers of RPTs when stored in dry-cold and wet-hot environments, respectively. Our findings provide a scientific basis for optimal storage that yields the desired aroma profile.

Influence of Eurotium cristatum and Aspergillus niger individual and collaborative inoculation on volatile profile in liquid-state fermentation of instant dark teas

The three instant dark teas were produced from instant green tea (IGT) by liquid-state fermentations using the microorganisms Eurotium cristatum (EFT), Aspergillus niger (AFT), and sequential inoculation of E. cristatum/A. niger (EAFT), respectively. The volatile compounds of four tea samples were extracted by headspace-solid phase microextraction (HS-SPME) and analyzed using gas chromatography-mass spectrometry (GC-MS) coupled with chemometrics. A total of 97 volatile compounds were tentatively identified to distinguish three fermented instant dark from IGT. Alcohols, acids, esters, ketones, aldehydes, and heterocyclics could be clearly distinguished by principal component analysis (PCA), venn diagram, heatmap analysis and hierarchical cluster analysis (HCA). Descriptive sensory analysis revealed that AFT had a moldy, woody and herbal aroma; EFT showed woody and herbal aroma; and EAFT smelled an herbal, sweet, minty and floral aroma. This study indicates that fermentation using different microorganisms is critical in forming unique aroma characteristics of instant dark teas.

Distinct Changes of Metabolic Profile and Sensory Quality during Qingzhuan Tea Processing Revealed by LC-MS-Based Metabolomics

Qingzhuan tea (QZT) is a unique type of dark tea exclusively produced in Hubei Province of China. In the current study, liquid chromatography-mass spectrometry (LC-MS) coupled with multivariate analysis was applied to characterize the chemical composition of QZT and investigate the effect of QZT processing on its metabolic profile and sensory quality. The contents of polyphenols and flavonoids decreased significantly while the polysaccharides content remained stable, while the theabrownin content inversely increased during QZT processing. LC-MS-based metabolomics analyses revealed that the tea sample after microbial fermentation (MFT) was dramatically different from the sample before microbial fermentation (UFT), while MFT was very similar to QZT. A total of 102 compounds were identified as critical metabolites responsible for metabolic changes caused by QZT processing, with the contents of catechins and flavonoids significantly decreased, and some novel phenolic acids and catechin derivatives were formed. The sensory quality of QZT was mainly formed during microbial fermentation, which greatly reduced the astringency and bitterness of raw tea leaves and produced its characteristic woody and stale aroma as well as mellow taste. These results suggested that microbial fermentation is the critical process in changing the metabolic profile of raw tea leaves and forming the sensory quality of QZT.