Investigation of Lactone Chiral Enantiomers and Their Contribution to the Aroma of Longjing Tea by Odor Activity Value and S-Curve

Extensive gene mining and facile engineering of a novel carbonyl reductase for asymmetric synthesis of anti-aging (S)-Pro-Xylane from d-xylose

Carbonyl reductases (CRs) are promising biological macromolecules for the asymmetric synthesis of chiral secondary alcohols. A novel CR was evolved for efficient conversion of d-xylose into anti-aging (S)-Pro-Xylane. Through extensive gene mining, an active carbonyl reductase ZmCR was identified from Zygofabospora marxiana with high reductive activity towards d-xylose-derived 1-C-(β-d-xylopyranosyl)-acetone (XPSA) and in situ cofactor regeneration capability. Using a three-round of concise engineering strategy, a hextuple mutant C85V/V128C/S129A/V161S/Y166H/T217A (M6) was identified from merely 157 mutants with specific activity of 46.00 U·mg-1, about 306 folds of WT. The Shannon-Wiener index of M6 was 2.21, much higher than WT, indicating its extended substrate scope. M6 could efficiently catalyze the asymmetric reduction of 190 g·L-1 XPSA into (S)-XPSP. The kcat/KMXPSA of M6 was 203.84 s-1·mM-1, about 910 folds of WT. More non-bonded interactions were monitored in M6, contributing to the stabilization of reactive catalytic conformation. This study provides a promising biocatalyst for the chemoenzymatic synthesis of value-added anti-aging (S)-Pro-Xylane from lignocellulosic d-xylose.

Influence of Different Shaping Techniques on the Aroma Quality and Volatile Metabolites of Green Tea Revealed by Gas Chromatography Electronic Nose and Gas Chromatography-Tandem Mass Spectrometry

The shaping process is recognized as a crucial step in the manufacturing of green tea. However, its influence on aroma quality remains unclear. In this study, the effects of four shaping techniques, including flat green tea (FGT), straight green tea (SGT), phoenix green tea (PGT), and curled green tea (CGT), on the aroma quality and volatile metabolites of green tea were investigated by gas chromatography electronic nose (GC-E-Nose) and gas chromatography-tandem mass spectrometry (GC-MS/MS). The findings indicated that distinct shaping processes significantly influenced the development of the aroma quality and aroma components of green tea. The PGT processing facilitated the attainment of superior aroma quality of green tea. In total, 60 volatile components were identified by GC-MS/MS, with 54 of these compounds being consistently detected across four different shaping techniques. In particular, the PGT processing method was effective in yielding elevated levels of alcohols, esters and ketones. Moreover, 20 key odorants were screened out, with (E,E)-2,4-decadienal, (E,E)-2,4-nonadienal, phenylethyl alcohol, and benzeneacetaldehyde proven to be substantial contributors to the overall aromas of green tea under diverse shaping procedures. These key odorants were primarily derived from lipid degradation and the Maillard reaction. GC-E-Nose served as a significant adjunct to sensory evaluation, enabling the swift differentiation of green tea samples that have undergone various shaping processes. These findings offer both theoretical and technical perspectives that may guide the creation of innovative green tea products distinguished by their unique shapes.

Nontarget analysis of natural flavor metabolites in tobacco extracts

Large-scale production of tobacco generates significant amounts of waste, making the reuse and recycling of tobacco waste crucial for sustainability. However, the high diversity of tobacco metabolites requires a comprehensive analysis of tobacco waste extracts, so as to optimize extraction selectivity and recovery. In this study, we thoroughly analyzed the metabolic composition of 168 tobacco extracts using nontarget analysis. The extracts were derived from 56 tobacco wastes covering all four major tobacco types: flue-cured, dark air-cured, Burley, and Oriental tobaccos. From each sample, we obtained three types of extracts, such as concretes, absolutes, and supercritical fluid extraction (SFE) oils, through various extraction protocols. By matching deconvolved experimental MS/MS spectra with databases, 587 metabolites were putatively annotated, which were categorized into 24 groups, including alkaloids, indoles, and flavor compounds. The metabolic compositions of extracts from dark air-cured and Burley tobaccos were found to be similar, whereas flue-cured and Oriental tobaccos contained more diverse flavor components. Notably, concretes of various tobaccos prepared with 70 %-ethanol contained similar amounts of addictive alkaloids. However, the absolutes obtained by sequential extraction of these concretes with 95 %-ethanol effectively removed alkaloids while modestly maintaining the flavor compounds. Conversely, extraction protocols incorporating supercritical fluid extraction followed by 95 %-ethanol extraction resulted in the loss of most flavor compounds, likely due to the incompatibility between the solvents used in the two extraction steps. In conclusion, our study demonstrates that absolutes derived through secondary extraction with varying ethanol-water ratios holds significant potential for utilization.

Study on the mechanism of glucose-lowering and sweetening of key sweet aroma compounds in sweet orange

Rising living standards heighten the demand for healthier sugar-reduced foods. This study used Headspace Solid-Phase Microextraction (HS-SPME) and Gas Chromatography-Olfactometry-Mass Spectrometry (GC/O-AT) to analyze volatile components in sweet orange juice, identifying 12 key sweet aroma compounds. Sensory and electronic tongue evaluations indicated that seven sweetness-related odor substances- (E)-citral, (E)-β-farnesene, β-myrcene, tallo-ocimene, nonanal, citronellyl formate, and tallo-ocimene-significantly enhanced the sweetness of a 5 % sucrose solution. In contrast, while nonanol was found to have no sugar-reducing sweetness-enhancing effect. Furthermore, molecular docking analysis was employed to examine the regions, binding energies, and interaction forces between eight sweet and fragrant aroma compounds from sweet orange and the T1R2-T1R3 sweet taste receptor-sucrose ternary system. The average binding energies with the receptor were -3.2 kcal/mol, -1.2 kcal/mol, -3.0 kcal/mol, -1.6 kcal/mol, -5.9 kcal/mol, -5.8 kcal/mol, -3.6 kcal/mol, and -6.0 kcal/mol, respectively. However, it should be noted that binding energy alone is not the sole criterion for judging the sweetening effect. Molecular dynamics (MD) results further demonstrated that the stability of the binding between sucrose and the sweet taste receptor was improved under the influence of (E)-citral, with the interaction between the two relying on hydrogen bonds, water bridges, and hydrophobic forces. This provides a theoretical basis for validating the sweetness-enhancing effects of aroma substances and insights into novel sweetener development.

Exploration of carbonyl compounds in red-fleshed kiwifruit wine and perceptual interactions among non-volatile organic acids

Carbonyl compounds are vital constituents that contribute to the flavor profile of alcoholic beverages. We examined 3-nitrophenylhydrazine as a derivatizing reagent for the measurement of 34 carbonyl compounds using UPLC-MS/MS. Adding formic acid and sodium acetate to the mobile phase significantly enhanced the detection limit of carbonyl compounds. The technique exhibited a notable extraction efficiency, yielding recovery percentages ranging from 83.6% to 117.1%, coupled with exceptional sensitivity, as evidenced by detection limits spanning from 0.07 μg/L to 4.80 μg/L. The relative standard deviation was <6.9%, indicating the precision and reliability of the analytical methodology. The method was verified by analyzing carbonyl compounds from red-fleshed kiwifruit wine. Furthermore, sensory assessment revealed that the amalgamation of tartaric acid, malic acid, and citric acid contributes to sour taste perception at sub-threshold concentrations through an additive interaction with supra-threshold non-volatile organic acids such as lactic acid and acetic acid.

Study on the saltiness-enhancing mechanism of chicken-derived umami peptides by sensory evaluation and molecular docking to transmembrane channel-like protein 4 (TMC4)

The previously obtained chicken-derived umami peptides in the laboratory were evaluated for their saltiness-enhancing effect by sensory evaluation and S-curve, and the results revealed that peptides TPPKID, PKESEKPN, TEDWGR, LPLQDAH, NEFGYSNR, and LPLQD had significant saltiness-enhancing effects. In the binary solution system with salt, the ratio of the experimental detection threshold (129.17 mg/L) to the theoretical detection threshold (274.43 mg/L) of NEFGYSNR was 0.47, which had a synergistic saltiness-enhancing effect with salt. The model of transmembrane channel-like protein 4 (TMC4) channel protein was constructed by homology modeling, which had a 10-fold transmembrane structure and was well evaluated. Molecular docking and frontier molecular orbitals showed that the main active sites of TMC4 were Lys 471, Met 379, Cys 475, Gln 377, and Pro 380, and the main active sites of NEFGYSNR were Tyr, Ser and Asn. This study may provide a theoretical reference for low-sodium diets.

Mining and tailor-made engineering of a novel keto reductase for asymmetric synthesis of structurally hindered γ- and δ-lactones

A novel carbonyl reductase from Hyphopichia burtoni (HbKR) was discovered by gene mining. HbKR is a NADPH-dependent dual function enzyme with reduction and oxidation activity belonging to SDR superfamily. HbKR strictly follows Prelog priority in the reduction of long-chain aliphatic keto acids/esters containing remote carbonyl groups, such as 4-oxodecanoic acid and 5-oxodecanoic acid, producing (S)-γ-decalactone and (S)-δ-decalactone in >99 % e.e. Tailor-made engineering of HbKR was conducted to improve its catalytic efficiency. Variant F207A/F86M was obtained with specific activity of 8.37 U/mg toward 5-oxodecanoic acid, which was 9.7-fold of its parent. Employing F207A/F86M, 100 mM 5-oxodecanoic acid could be reduced into optically pure (S)-δ-decalactone. Molecular docking analysis indicates that substitution of aromatic Phe with smaller residues renders sufficient space for accommodating substrates in a more stable conformation. This study offers an efficient biocatalyst for the biosynthesis of (S)-lactones, and provides guidance for engineering carbonyl reductases toward structurally hindered substrates.