ReviewAnalysis of organic sulfur compounds in wine aroma
Introduction
Flavour is one of the most important determinants of food and beverage quality, since the interaction of aromatic substances with the senses of smell and taste leads to consumer acceptance or rejection. In some cases, the presence of a single compound is sufficient to give the characteristic aroma of a product (key or character impact substance) but, in general, the aroma of foods is influenced by several different compounds. Among these, sulfur-containing structures are an important class both due to their abundance and aromatic impact. In fact, about 10% of the volatile components detected in foods and beverages are sulfur compounds [1] and their character impact is so noticeable as to give characteristic notes to different products [1], [2].
When degraded, these structures, such as cystine, cysteine, methionine, glutathione and some vitamins, become a source of different kinds of sulfur compounds. Their degradation may occur by enzymatic [3], [4], [5] or non-enzymatic routes in which the rate can be greatly influenced by temperature and light [6], [7], [8], [9], [10], [11]. However, this natural source of sulfur may be influenced by addition of additives to foods and beverages. In fact, their degradation may originate further sulfurated compounds.
From organoleptic perspectives, these compounds have different olfactory qualities depending on the position of the sulfur atom in the molecule [1]. In addition, their concentration has a great influence on sensory properties, often being strongly dependent on threshold values, normally low. Therefore low concentrations may give high odour intensities. Some of these aromas have been identified as favourable character-impact substances in different foods such as meat [1], [7], coffee [1], [12], corn [13], oranges [14], yellow passion fruit [1], [15], truffles [1], [16], allium species [1], [5] and beer [17], [18], [19], [20]. On the contrary, these compounds are sometimes responsible for offensive odours (off-flavours), and their presence indicates incorrect conditions of preparation and storage [21]. From an oenological point of view, both aspects have to be taken into account since off-flavours caused by sulfur-containing compounds cause some of the major defects in the quality of wine aroma [22], [23], [24], [25], [26] while other favourable ones may exalt the typical notes of some varietal wines [27], [28], [29], [30], [31], [32].
Section snippets
Sulfur compounds in wines
Wine is a hydroalcoholic solution containing hundreds of compounds that come from grapes or result during winemaking and storage. Several of these compounds affect wine aroma [33], [34], [35], [36], [37], [38], [39] which, besides being a parameter of quality, act as a «finger-print» for each wine variety. In fact, some of these odour compounds are characteristic of certain varieties whereas the concentration of other compounds, although present in all wines, varies according to the type of
Analysis of sulfur compounds in wines
Three of the main problems encountered in analysis of sulfur compounds in wine are the complexity of the sample matrix, the low concentration levels that must be determined, and the highly reactive nature of these compounds.
The literature reports the application of several analytical techniques. Thiols may be determined with the sulfur specific ion electrode [58], [59], [60], but more practical spectrophotometric methods are also available. In fact, they can be detected colorimetrically using
Conclusions
The literature describes several analytical methods for analysing sulfur compounds in wines, mostly involving the gas chromatographic technique. However, established methods have not paid enough attention to some relevant problems encountered in the determination of these compounds.
The first is inherent to a correct quantitative evaluation of the reference standards. In fact, several sulfur compounds, being volatile and/or oxidizable, demonstrated to be unstable and therefore difficult to
Acknowledgements
The authors wish to thank CICYT (project ALI97-0765) for the financial support given.
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