Aroma Quality Evaluation of High-Quality and Quality-Deficient Black Tea by Electronic Nose Coupled with Gas Chromatography-Mass Spectrometry

According to the results of sensory evaluation performed by experts, 14 black tea samples were divided into two groups based on their aroma quality: high-quality and quality-deficient black tea. Using fast gas chromatography-electronic-nose (GC-E-Nose) and gas chromatography-mass spectrometry (GC-MS) combined with multivariate statistical analysis, discriminant analysis of the two groups were carried out, and the key differential components between these groups were selected. The results showed that 117-dimensional dataset was obtained by the fusion of the GC-E-Nose (44-dimensional) and GC-MS (73-dimensional) data and used to establish a model for accurate classification of the two types of black tea employing orthogonal partial least squares-discriminant analysis (OPLS-DA). The model’s explanatory and predictive capacity (R2Y = 0.976, Q2 = 0.959) were better than those of the model established based on the GC-E-Nose or GC-MS data. Based on variable important in projection (VIP) scores > 1.6 and P < 0.05, eight key aroma components including dimethyl sulfide (B3 and B25), β-ionone (A59), (3E)-4,8-dimethylnon-1,3,7-triene (A20), dihydroactinidiolide (A64), linalool (A17), phenylethyl alcohol (A19), δ-octyl lactone (A41) and γ-nonalatone (A45) were selected, which played an important role in the classification. These results showed that GC-E-Nose combined with GC-MS allows rapid and accurate discrimination between quality-deficient and high-quality black tea, which can be used as a supplement to traditional sensory evaluation, providing technical support for quality control and improvement of black tea

Changes in Aroma Compounds and Selected Precursors of Sun-Dried Green Tea during Processing

In order to investigate the changes in the characteristic aroma components of sun-dried green tea during processing, the volatile substances in fresh, fixated, rolled and sun-dried leaves of Yunnan large leaf tea were determined using solvent flavor-assisted evaporation extraction (SAFE) combined with gas chromatography-mass spectrometry (GC-MS), and the aroma active components of sun-dried green tea were identified by odor activity value (OAV). The results showed that totally 67 volatile compounds were detected, the major ones being alcohols, esters and aldehydes, and the total amount of volatile substances increased from fresh leaves to the end of fixation. The total amount of volatile substances slightly decreased in sun-dried leaves compared to fresh leaves, but the number of volatile substances increased. A total of 23 aroma active compounds with OAV > 1 were found in sun-dried green tea, with the major ones being those contributing to floral and green aromas such as linalool, geraniol and nonanal. Aldehydes were the most prominent component in each processing stage. Using gas chromatography-flame ionization detector (GC-FID) and high performance liquid chromatography-photo-diode array (HPLC-PDA), a total of 7 fatty acids were detected, with saturated fatty acids being the major components; 13 carotenoids were detected, including lutein, whose content was the highest in all processing stages. During the processing of sun-dried green tea, the total amounts of fatty acids and carotenoids decreased continuously, especially linoleic acid and linolenic acid, and the content of hexanal as their major degradation product showed the opposite trend. The contents of terpene ketones, ionone and its oxidized derivatives such as β-ionone and 5,6-epoxy-β-ionone significantly increased as a result of the degradation and transformation of carotenoids. The trends of fatty acids and carotenoid precursors during the processing of sun-dried green tea were correlated with their associated aroma products. In conclusion, the results of this study may provide theoretical guidance for the targeted aroma improvement of sun-dried green tea

Effect of blister blight disease caused by Exobasidium on tea quality

Blister blight, as one of the most threatening and damaging disease worldwide, mainly infects young organs and tissues seriously affecting tea growth and quality. In this study, the spread of pathogen on tea leaves were examined by toluidine blue staining, scanning electron microscope and transmission electron microscope analysis. The composition and abundance of fungal community on leaf tissues were firstly analyzed. Sensory evaluation and metabolites analysis indicated that diseased tea leaves had strong sweet taste and soluble sugars contributed significantly to the taste, while metabolites showing bitter and astringent taste (caffeine, catechins) were significantly decreased. According to the biological functions of differential metabolites, sugars including 7 monosaccharides (d-xylose, d-arabinose, d-mannose, d-glucuronic acid, glucose, d-galactose and d-fructose), 2 disaccharide (sucrose and maltose) and 1 trisaccharide (raffinose) were the main differential sugars increased in content (>2 fold change), which was of great significance to sweet taste of diseased tea

Characterization of triterpenoids as possible bitter-tasting compounds in teas infected with bird's eye spot disease

Tea infected with bird's eye spot disease generally imparts a long-lasting bitter taste, which is unacceptable to most consumers. This study has comprehensively evaluated the taste profiles of infected and healthy teas and investigated their known bitter compounds previously reported in tea. Quantification analyses and calculation of dose-over-threshold (DoT) factors revealed that no obvious difference was visualized in catechins, caffeine, bitter amino acids, and flavonols and their glycosides between infected and healthy tea samples, which was also verified by principal component analysis (PCA) and hierarchical cluster analysis (HCA). Therefore, these known bitter compounds have been ruled out as critical contributors to the long-lasting bitterness of infected teas. Furthermore, Gel permeation chromatography, sensory analysis, and UPLC-Q-TOF-MS were employed and identified 13 substances from the target bitter fractions, including caffeine, ten triterpenoids, and two oxylipins. The higher triterpenoid levels were supposed to be the reason causing the long-lasting bitterness. This study has provided a research direction for the molecular basis of the long-lasting bitterness of infected tea leaves with bird's eye spot disease

Palladium-Catalyzed Amide-Directed Enantioselective Hydrocarbofunctionalization of Unactivated Alkenes Using a Chiral Monodentate Oxazoline Ligand

A Pd-catalyzed amide-directed enantioselective hydrocarbofunctionalization of unactivated alkenes with C–H nucleophiles has been developed using a chiral monodentate oxazoline (MOXin) ligand. Various indoles react at C3 position with aminoquinoline-coupled 3-alkenamides to give γ addition products in good to excellent yield and enantioselectivity. This study represents an important advance of the development of chiral monodentate oxazoline ligands, which have been underexplored for asymmetric catalysis

Palladium-Catalyzed Amide-Directed Enantioselective Hydrocarbofunctionalization of Unactivated Alkenes Using a Chiral Monodentate Oxazoline Ligand

A Pd-catalyzed amide-directed enantioselective hydrocarbofunctionalization of unactivated alkenes with C–H nucleophiles has been developed using a chiral monodentate oxazoline (MOXin) ligand. Various indoles react at C3 position with aminoquinoline-coupled 3-alkenamides to give γ addition products in good to excellent yield and enantioselectivity. This study represents an important advance of the development of chiral monodentate oxazoline ligands, which have been underexplored for asymmetric catalysis