Search

Volatile terpenoids play important roles in the formation of tea aroma quality due to their pleasant scents and low odor thresholds. Most volatile terpenoids contain stereogenic centers, which results in various stereo distributions of their enantiomers and diastereoisomers in different types of tea. However, the distribution characteristics of terpenoid enantiomers in teas were still unclear, which poses an obstacle to the scientific understanding of tea aroma. In this work, a new and efficient analysis approach based on headspace solid phase microextraction (HS-SPME)-chiral gas chromatography-mass spectrometry (GC–MS) was established to analyze 12 pairs of familiar terpenoid enantiomers in different teas. The extraction efficiency of the HS-SPME method to extract volatile terpenoids in teas was the greatest when using CAR-DVB-PDMS (50/30 μm) fibers and 1:10 proportions between tea and boiling water at a 50 °C extraction temperature for 40 min, and the stability observation of enantiomeric ratios of the terpenoids well proved the feasibility of the extraction method. The favorable limits of detection, limits of quantitation, repeatability, linearity, and concentration ranges of each terpenoid enantiomer demonstrated the repeatability and reliability of the analytical approach. The enantiomeric and quantitative analyses indicated that S -limonene, S -linalool, (2 S , 5 S )-linalool oxide A, (2 S , 5 R )-linalool oxide B, R -4-terpineol, (2 S , 5 R )-linalool oxide C, (2 S , 5 S )-linalool oxide D, S -α-terpineol, R -α-ionone, peak 1 of theaspirane A and peak 2 of theaspirane B were the major terpenoid components in most Chinese teas; instead, higher proportions of the opposite enantiomers of the above terpenoids were frequently detected in black teas with large leaf origin and Indonesia white teas. Besides, great diversities of enantiomeric ratios and concentrations among different teas were observed. Furthermore, partial least-squares discriminant analyses were performed to distinguish the concentration differences of the terpenoid enantiomers among different teas; the analysis results indicated that highly significant concentration differences existed between large and small leaf origins of black teas, and significant differences of the concentrations of linalool oxides A–C were observed between green, white and dark teas. The successful application of this chiral analysis technique of tea aroma will lay a scientific foundation for further quality assessment, botanical origin determination and authenticity assessment of teas. [ABSTRACT FROM AUTHOR]

• Volatiles in baked green teas with different tenderness were examined thoroughly. • cis -Linalool oxide (furanoid) and methyl salicylate were abundant in mature leaves. • Most chiral odorants had significantly higher contents in mature tea samples. • Enantiomeric ratios of most odorants were unaffected by tenderness and processing. • Evident variation on ERs of limonene, α-terpineol etc. occurred after processing. Suitable picking tenderness is an essential prerequisite for manufacturing tea. However, the influence of picking tenderness of fresh tea leaves on the aromatic components is still unclear. In this study, aromatic profiles and chiral odorants in fresh tea leaves and corresponding baked green teas with five levels of tenderness of two representative cultivars were analysed using stir bar sorptive extraction-gas chromatography-mass spectrometry. cis -Linalool oxide (furanoid) and methyl salicylate exhibited significantly increasing trends as samples of all series matured. The content of most chiral odorants was significantly high in the mature samples, and significant content variations of all enantiomers during baked green tea processing could be observed with different trends according to their precursors. In particular, the enantiomeric ratios of most chiral odorants were less influenced by the picking tenderness and processing, while drying (limonene), spreading and fixation (α-terpineol), and spreading (dihydroactinidiolide) influenced the chiral distribution of the aforementioned odorants. [ABSTRACT FROM AUTHOR]

A chestnut-like aroma is widely considered an important indicator of an excellent-quality green tea; however, the key odorants responsible for chestnut-like aroma have never been systematically studied and remain unknown. In this study, the aroma components of green teas and Chinese chestnuts were analyzed using comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC × GC-TOFMS), and 58 compounds were identified as common aroma components among green teas, boiled Chinese chestnuts, roasted Chinese chestnuts and raw Chinese chestnuts. Subsequently, 17 volatiles, including 3-methylbutanal, ( E )-3-penten-2-one, ethylbenzene, heptanal, benzaldehyde, 2-pentylfuran, octanal, benzeneacetaldehyde, ( E )-2-octenal, ( E,E )-3,5-octadien-2-one, linalool, nonanal, ( E )-2-nonenal, decanal, ( Z )-hex-3-en-1-yl hexanoate, trans -β-ionone and ( E )-nerolidol, were identified as the key odorants responsible for chestnut-like aroma based on the odor activity value (OAV) calculation method. Besides, the comparison of OAVs of key odorants between fresh tea leaves and finished teas indicated that all key odorants were present in fresh tea leaves and that their contents increased or decreased during tea processing. Moreover, the comparison between results of OAV and gas chromatography-olfactometry (GC-O) methods showed that ethylbenzene, heptanal, benzaldehyde, 2-pentylfuran, ( E,E )-3,5-octadien-2-one, linalool, ( Z )-hex-3-en-1-yl hexanoate and trans -β-ionone were the common identified compounds between the two methods. The identification of chestnut-like aroma in green teas will provide a theoretical basis for further research on the directional adjustment and control of tea aroma quality. [ABSTRACT FROM AUTHOR]

Linalool is a major volatile component of tea aroma. It occurs naturally as R -(−)- and S -(+)-linalool enantiomers, which exhibit entirely different sensorial properties. Using headspace solid-phase microextraction with chiral gas chromatography, we quantitated R -(−)- and S -(+)-linalool in teas. Optimal extraction conditions were as follows: CAR–DVB–PDMS fiber, 60 min at 60 °C, and tea/water ratio of 1:6 (w(g)/v(mL)). We measured linalool levels in five different teas, in fresh leaves of 14 different cultivars, and in samples collected during processing of green and black teas. Enantiomeric distributions of linalool were significantly different among the different teas and among the different tea cultivars. R -(−)-Linalool and S -(+)-linalool reached maximum levels during the rolling of black tea, with the levels declining drastically during green tea processing. Although tea germplasm was an important factor in determining the enantiomeric distribution of linalool in tea products, the processing steps also had a large impact on linalool levels. [ABSTRACT FROM AUTHOR]

[Display omitted] • The deficient inorganic N, P, and C sources induced decline in S0AD efficiency. • Specific thresholds for inorganic N, P, and C to robust S0AD were established. • Substantial reduction in SOBs occurred under deficient inorganic nutrients. • External supplying mitigated nutrient constraints in a pilot-scale S0PB bioreactor. Elemental sulfur packed-bed (S0PB) bioreactors are increasingly employed to treat secondary effluents from wastewater treatment plants. Limited availability of inorganic nutrients, such as ammonium, phosphate, and inorganic carbon, significantly determines the growth of autotrophic microorganisms and the denitrification efficiency. This study investigated the constraining thresholds of inorganic nutrients on the elemental sulfur autotrophic denitrification process. In the absence of ammonium, phosphate, and bicarbonate, the denitrification rate decreased. Specifically, the ammonium assimilation process could be replaced by nitrate assimilation pathways, resulting in a 29.5 % decline in denitrification efficiency when the ammonium-N feed was reduced from 2.0 to 0 mg-N/L. However, the absence of phosphate-P and bicarbonate-C led to substantial drops in denitrification efficiency by 66.7 % and 77.5 %, respectively. This was primarily attributed to diminished biomass yield, as evidenced by reductions of 77.8–82.6 % in ATP and 35.7–44.3 % in protein content, along with a 12.4 %–14.7 % decrease in the relative abundances of dominant sulfur-oxidizing bacteria. Correlation analyses established specific thresholds for desired nitrate removal amounts: 0.131 for ammonium-N, 0.053 for phosphate-P, and 0.597 for bicarbonate-C. These thresholds provided valuable guidance for adjusting real-life wastewater through external supplementation when employing a pilot-scale S0PB bioreactor, leading to a significant enhancement of denitrification efficiency. This study emphasizes the significance of considering the concentrations of inorganic nutrients in wastewater when implementing S0PB bioreactors, which is a context that frequently neglects potential negative consequences. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel Es-GC×GC-TOFMS approach to separate chiral volatiles in tea was established. • Volatile enantiomers in Wuyi rock teas from four cultivars were firstly quantitated. • Limits of quantitation of linalool oxides A−B in teas were as low as 0.2 pg/mL. • R -α-Ionone, S -(E)-nerolidol etc. were the potential markers to distinguish cultivar. • 7 Volatile enantiomers were identified as aroma-active compounds in Wuyi rock teas. Chiral volatiles play important roles in the formation of aroma quality of foods. To date, enantiomeric characteristics of chiral volatiles in Wuyi rock tea (WRT) and their aroma contributions are still unclear. In this study, an efficient enantioselective comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (Es-GC × GC-TOFMS) approach to separate and precisely quantitate 24 pairs of chiral volatiles in WRTs was established, and the enantiomeric distribution and aroma contribution of chiral volatiles among WRTs from four representative cultivars were investigated. Enantiomeric ratio (ER) of R -α-ionone (80%) in Dahongpao (DHP), ER of S -α-terpineol (57%) in Jinfo (JF), ERs of R -γ-heptanolactone (69%), S -γ-nonanolactone (55%), (2 R , 5 S)-theaspirane B (91%), concentration of S -(E)-nerolidol (313.37 ng/mL) in Rougui (RG) and concentration of R -α-ionone (33.01 ng/mL) in Shuixian (SX) were unique from other types of WRTs, which were considered as the potential chemical markers to distinguish WRT cultivars. The OAV assessment determined 7 volatile enantiomers as the aroma-active compounds, especially R -α-ionone and R -δ-octanolactone in SX, as well as S -(E)-nerolidol and (1 R , 2 R)-methyl jasmonate in RG contribute much to aroma formation of the corresponding WRTs. The above results provide scientific references for discrimination of tea cultivars and directed improvement of the aroma quality of WRT. [ABSTRACT FROM AUTHOR]

Aroma is one of the most important determinants of tea quality; thus, the precise characterization of the chemical composition of tea aroma is of significant value. In this study, aroma components of the most famous Chinese green tea, Xihu Longjing tea, were first systematically analyzed using comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC × GC-TOFMS) combined with simultaneous distillation extraction (SDE). 478 volatile compounds were identified as common constituents of fifty-five Xihu Longjing tea samples based on standard mass spectra in available databases, retention times, retention indices, structured chromatograms and standards. Subsequently, the aroma qualities of Xihu Longjing tea samples were discriminated by sensory evaluation combined with multivariate analysis. The study results indicated that 37 compounds were positively correlated with aroma quality, and 65 compounds were negatively correlated with aroma quality. The above compounds with various odor characteristics were considered as the key components to affect the aroma quality of Xihu Longjing tea. [ABSTRACT FROM AUTHOR]