Takatoshi Tominaga

A new type of flavor precursors in Vitis vinifera L. cv. Sauvignon Blanc : S-Cysteine conjugates

Three flavor-active volatile thiols (4-mercapto-4-methylpentan-2-one, 4-mercapto-4-methylpentan-2-ol and 3-mercaptohexan-1-ol) involved in Vitis vinifera L. var. Sauvignon blanc wine aroma can be generated in vitro from nonvolatile must extracts by the enzyme action of a cell-free extract obtained from the gastrointestinal bacterium Eubacterium limosum. The specificity of a cysteine conjugate β-lyase (EC 4.4.1.13) which is contained in the cell-free extract, strongly suggested the existence of precursor for these volatile thiols having a structure of S-cysteine conjugate. Gas-phase chromatography analysis coupled with mass spectrometry of must extract in the form of trimethylsilylated derivatives, verified this hypothesis. The release of flavor-active volatile thiols during alcoholic fermentation of the must is shown to be due to the degradation, by yeast, of the corresponding S-cysteine conjugates. This mechanism explains the amplification of the typical Sauvignon blanc grape aroma during alcoholic ferme...

Identification of new volatile thiols in the aroma of Vitis vinifera L. var. Sauvignon blanc wines

Three new flavour-active mercapto-alcohols have been identified in Sauvignon blanc wines: 4-mercapto-4-methylpentan-2-ol, 3-mercaptohexan-1-ol and 3-mercapto-3-methylbutan-1-ol. The first two have a perception threshold of the order of 60 ng/l in an aqueous alcohol solution and their respective odours are reminiscent of citrus zest and grapefruit. The perception threshold of 3-mercapto-3-methylbutan-1-ol, with its odour of cooked leeks, is higher (1500 ng/l in the same solution). These three compounds may contribute towards the typical aroma of Sauvignon blanc wines.

Impact of Oxygen Dissolved at Bottling and Transmitted through Closures on the Composition and Sensory Properties of a Sauvignon Blanc Wine during Bottle Storage

This work outlines the results from an investigation to determine the effect of the oxygen dissolved at bottling and the specific oxygen barrier properties of commercially available closures on the composition, color and sensory properties of a Bordeaux Sauvignon Blanc wine during two years of storage. The importance of oxygen for wine development after bottling was also assessed using an airtight bottle ampule. Wines were assessed for the antioxidants (SO(2) and ascorbic acid), varietal thiols (4-mercapto-4-methylpentan-2-one, 3-mercaptohexan-1-ol), hydrogen sulfide and sotolon content, and color throughout 24 months of storage. In addition, the aroma and palate properties of wines were also assessed. The combination of oxygen dissolved at bottling and the oxygen transferred through closures has a significant effect on Sauvignon Blanc development after bottling. Wines highly exposed to oxygen at bottling and those sealed with a synthetic, Nomacorc classic closure, highly permeable to oxygen, were relatively oxidized in aroma, brown in color, and low in antioxidants and volatile compounds compared to wines sealed with other closures. Conversely, wines sealed under more airtight conditions, bottle ampule and screw cap Saran-tin, have the slowest rate of browning, and displayed the greatest contents of antioxidants and varietal thiols, but also high levels of H(2)S, which were responsible for the reduced dominating character found in these wines, while wines sealed with cork stoppers and screw cap Saranex presented negligible reduced and oxidized characters.

Nitrogen catabolic repression controls the release of volatile thiols by Saccharomyces cerevisiae during wine fermentation

Volatile thiols such as 4-methyl-4-sulfanylpentan-2-one (4MSP) and 3-sulfanylhexan-1-ol (3SH) are aromatic molecules having an important organoleptic impact on white wines. These components are produced from inodorous nonvolatile cysteinylated precursors by Saccharomyces cerevisiae metabolic activity during alcoholic fermentation. Here we provide a new insight into the genetic determinism of the production of volatile thiols by yeast. Using a gene deletion approach, we investigated the role of three yeast beta-lyases and demonstrate that Irc7p, a putative cystathionine beta-lyase, is one of the main proteins catalyzing the 4MSP and 3SH release under enological conditions. Moreover, we demonstrate that Ure2p/Gln3p proteins mainly control the bioconversion of volatile thiols by the transcriptional regulation of the IRC7 gene through the general mechanism of nitrogen catabolic repression. Finally, our findings suggest that the enantiomer balance of 3SH may be modulated by activating specifically stereoselective enzymes such as Irc7p.

Identifying the Chemical Composition Related to the Distinct Aroma Characteristics of New Zealand Sauvignon blanc Wines

A comprehensive set of aroma compounds was quantified in 79 Sauvignon blanc wines from different international producers. Emphasis was given to understanding the chemical differences that can explain the unique character of Marlborough Sauvignon blanc. Quantification revealed the potential importance of several volatile compounds in addition to the polyfunctional mercaptans and methoxypyrazines already known to be important to the aroma of Sauvignon blanc wines. Multivariate statistical approaches, including canonical variate analysis, classification tree and partial least square analysis, established correlations between the chemical and the sensory profiles of the wines. A significant role of 3-mercaptohexyl acetate and 3-mercaptohexanol in the unique tropical, fruity character of Marlborough Sauvignon blanc wines was demonstrated, together with important variations in their concentrations, pointing to different styles even within the Marlborough wines.

Identification and characteristics of new volatile thiols derived from the hop (Humulus luplus L.) cultivar Nelson Sauvin.

Nelson Sauvin (NS) is a unique hop cultivar that was bred and grown in New Zealand. This hop gives a specific flavor (exotic fruit-like, white wine-like) to finished beers. However, the key compounds of this flavor have not yet been identified. We have attempted to identify the specific flavor compounds derived from NS. We focused on certain volatile thiols that are well-known to contribute to wine flavors, especially Sauvignon Blanc. The product made from NS (NS product) lost its specific flavor by contact with copper. Copper is well-known as an absorber of thiols in the field of wine flavor investigations. Therefore, it might point to the existence of thiols. We analyzed the NS product by GC-FPD, GC-olfactometry and GC-MS, and identified two new volatile thiols, 3-sulfanyl-4-methylpentan-1-ol (3S4MP), and 3-sulfanyl-4-methylpentyl acetate (3S4MPA). These compounds have a grapefruit-like and/or rhubarb-like odor, similar to that of Sauvignon Blanc. We quantified these compounds in the NS products and determined their thresholds. As a result, 3S4MP contained about 2-fold of its threshold in beers, and 3S4MPA was included below its threshold. However, it was confirmed that 3S4MP enhanced the flavors of 3S4MPA by synergy. Therefore, we concluded that both of the new volatile thiols would contribute to the specific odor of beers produced with NS.

Aromatic potential of botrytized white wine grapes: Identification and quantification of new cysteine-S-conjugate flavor precursors

Sweet wines made from botrytized grapes contain much higher concentrations of volatile thiols, especially 3-sulfanylhexan-1-ol (3SH), than dry white wines. Three new specific volatile thiols (3-sulfanylpentan-1-ol (3SP), 3-sulfanylheptan-1-ol (3SHp), and 2-methyl-3-sulfanylbutan-1-ol (2M3SB) were recently identified in Sauternes wines. Like most volatile thiols, these compounds were almost totally absent from must, mainly being formed during alcoholic fermentation. In this work, we describe the identification and quantification of three new cysteine-S-conjugate precursors in must made from Botrytis-infected grapes. S-3-(pentan-1-ol)-L-cysteine (P-3SP), S-3-(heptan-1-ol)-L-cysteine (P-3SHp), and S-3-(2-methylbutan-1-ol)-L-cysteine (P-2M3SB) were identified by direct GC-MS analysis of their derivative forms obtained by silylation of an enriched fraction, isolated from must by affinity chromatography. Concentrations were considerably higher when Botrytis cinerea had developed on the grapes. In botrytized must, the mean levels of P-3SP, P-3SHp, and P-2M3SB were in the vicinity of 700, 50, and 500 nM, respectively, whereas concentrations in healthy must ranged from 0 to 50 nM. This indicated that these three new sulfanyl alcohols, responsible for the characteristic aroma of botrytized wines, were formed by the yeast metabolism during alcoholic fermentation from the corresponding non-volatile cysteine-S-conjugate precursors. Moreover, these results highlighted the predominant role of botrytization in developing grape aroma potential.

Surprising Structural Lability of a Cysteine‐S‐Conjugate Precursor of 4‐Methyl‐4‐sulfanylpentan‐2‐one, a Varietal Aroma in Wine of Vitis vinifera L. cv. Sauvignon Blanc

4‐Methyl‐4‐sulfanylpentan‐2‐one (1; 4MSP) provides a characteristic aroma compound of wines made from Vitis vinifera L. cv. Sauvignon blanc. 4MSP has a strong box‐tree odor with a very low perception threshold and is derived from the cysteinylated precursor S‐(1,1‐dimethyl‐3‐oxobutyl)cysteine (4; P‐4MSP). P‐4MSP is transformed into 4MSP during alcoholic fermentation and is an excellent marker of varietal aroma potential. An improved synthesis of P‐4MSP as well as of its deuterium‐labeled analogue [D6]‐P‐4MSP is described. Several analytical methods (NMR, IR, LSI‐MS, GC/MS, ESI‐MSn) were combined to elucidate spontaneous reversible structural changes of P‐4MSP at different pH values. At low pH, P‐4MSP has a linear keto form. The keto–enol tautomerism was observed at neutral pH. At pH 8, the formation of N‐substituted intramolecular hemiaminal was characterized by ESI‐MS and ESI‐MSn experiments. The hemiaminal loses H2O at high pH to produce a cycloimine, which is easily opened by acid hydrolysis. The keto–enol tautomerism explained the incorporation of only six D‐atoms during the preparation of the P‐4MSP deuterated standard even if [D10]mesityl oxide was used. Derivatization conditions for GC/MS analysis strongly affected the ratio of the monosilylated intramolecular cyclic form and the disilylated linear form of P‐4MSP. The structural changes of P‐4MSP may have a considerable impact on the development of methods of measuring varietal aroma potential.

Identification of 4-mercapto-4-methylpentan-2-one from the box tree (Buxus sempervirens L.) and broom (Sarothamnus scoparius (L.) Koch.)

A very odorous compound, 4-mercapto-4-methylpentan-2-one has been identified in the box tree, Buxus sempervirens L., and in broom, Sarothamnus scoparius (L.) Koch by a selective extraction of volatile thiol compounds. Although the concentration varied depending on weather and the places where samples were collected, the maximum concentration of the thiol from these plants reached ca. 22 ng/g fresh leaves (box tree) and 16 ng/g fresh stem (broom), respectively. Considering the very low perception threshold of this thiol, the characteristic aroma of these plants is due to 4-mercapto-4-methylpentan-2-one. A plant that contains this thiol abundantly has not been reported before.