Volatile sulphur compounds in wines related to yeast metabolism and nitrogen composition of grape musts
Introduction
In recent years, the composition in sulphur compounds of wines has become a subject of many studies concerning their identification and origin, as well as their character and impact on wine quality. Generally, sulphur compounds are classified as detrimental to wine quality; however, new developments in wine research allowed the differentiation of a family of sulphur compounds responsible for a varietal aroma of wines [1]. Further investigation is required to better understand the origin of these compounds and their impact on sensory properties of wines.
Volatile sulphur compounds in wines are classified into two categories according to their bp: the highly volatile compounds (bp <90 °C) and the less volatile compounds (bp >90 °C) [1], [2], [3]. The highly volatile compounds, particularly hydrogen sulphide, ethanethiol and methanethiol, are present in reduced wines and, at concentrations higher than their perception threshold, they are sensed by rotten eggs, garlic, onion and cabbage smells; however, they do not pose a serious problem to wine, as their boiling point is relatively low and, therefore, most of them are volatilised by simple racking and aeration. Other highly volatile sulphur compounds, such as carbonyl sulphide and dimethyl sulphide, are less significant in reduction defects. The latter has been considered by some authors to contribute to the bouquet of wines [4].
Among the less volatile sulphur compounds, 3-(methylthio)-1-propanol (methionol) is present in wines at concentrations up to 5 mg l−1. When this compound occurs at concentrations above its threshold value (1.2 mg l−1), it contributes a cauliflower aroma [1], [2]. Among the many other less volatile sulphur compounds identified in wine, with low detection limits, most of them are usually found at levels below their threshold value; these include 2-mercaptoethanol (poultry-like aroma), 2-methyltetrahydrothiophen-3-one (metallic, natural gas odour), 2-methylthioethanol (French bean), ethyl-3-methylthiopropionate (metallic, sulphur aroma), acetic acid-3-(methylthio)propyl ester (cooked potatoes) and 4-methylthiobutanol (chive-garlic aroma) [1], [2], [5].
The mechanism of production of many sulphur compounds is still not well-known. Beside enzymatic formation resulting from yeast metabolism, other non-enzymatic reactions may occur during vinification through the influence of temperature, light and chemical transformation. Moreover, sulphur-containing pesticides or other breakdown products can be used as precursors in biochemical reactions arising from grape juice enzymes or yeast metabolism [5].
The reduction of sulphate and sulphite by yeasts results in the production of hydrogen sulphide [6], [7], [8], [9]. Hydrogen sulphide is an essential metabolic intermediate in the biosynthesis of cysteine and methionine necessary for protein synthesis and cellular metabolism. Hydrogen sulphide may also be produced from cysteine through its degradation directly to sulphide by the cysteine desulphydrase of yeast [5], [6], [7], [8], [9], [10], [11]. Dimethyl sulphide is synthesised by yeast from cysteine, cystine and glutathione [1] or it can be produced from dimethyl sulphoxide by yeast reductase [3], [11].
Sulphur amino acids and precursors mediate the production of organic sulphur compounds in wines, followed by the Ehrlich reaction. The production of 3-(methylthio)-1-propanol, for instance, is well-known; this compound is produced by yeast from methionine, via deamination, followed by decarboxylation (Ehrlich reaction). The aldehyde thus formed (methional) is reduced by an enzymatic reaction into an alcohol. The biosynthesis of 4-(methylthio)-1-butanol from homomethionine and 2-mercaptoethanol from cysteine was also proposed [1], [5], [12].
The aim of this study is to determine the influence of nitrogen compounds in grape musts and yeast metabolism on the production of sulphur compounds in wines. Six grape musts, with different nitrogen composition, were used. Different vinifications were also performed with the addition of methionine and/or cysteine to grape must before alcoholic fermentation.
Section snippets
Grape musts and fermentations
Experiments were carried out on white musts obtained from grapes of the Vitis vinifera varieties from the Vinho Verde Region in Portugal (Alvarinho, Loureiro, Trajadura, Pedernã, Azal branco and Avesso), in the 1999 vintage. Duplicate vinifications were performed with the addition of methionine (20 mg l−1) and/or cysteine (40 mg l−1) to grape must before alcoholic fermentation. All fermentations were conducted in 10 l tanks, at 18 °C, with inoculation of a rehydrated strain of Saccharomyces cerevisiae
Results and discussion
The variation of hydrogen sulphide concentration in the control wines (no amino acid supplementation) and in wines produced with the addition of cysteine and/or methionine to grape musts is presented in Fig. 2. The addition of 40 mg l−1 cysteine to grape musts resulted in an increase of the hydrogen sulphide content in all wines. This increase can be explained by an overproduction from cysteine, by the action of cysteine desulphydrase, or by the repression of cysteine synthesis from sulphide
Conclusions
The content in sulphur compounds of wine is associated with the yeast metabolism of nitrogen compounds in the media, but further work is needed to better understand the origin of such compounds. This work showed that wines produced from grape musts of low amino acid concentration present a high total sulphur compounds content. Addition of methionine to grape musts led to a higher production of 3-(methylthio)-1-propanol and, consequently, of 3-(methylthio)propionic acid and acetic
Acknowledgements
The authors gratefully acknowledge the financial support from FCT and PAMAF (INIA, Project 2025).
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