Quantification approach for assessment of sparkling wine volatiles from different soils, ripening stages, and varieties by stir bar sorptive extraction with liquid desorption
Introduction
According to the Champagnois method, sparkling wine is a double fermented wine. After the first alcoholic fermentation, it is submitted to a second fermentation inside the bottle by addition of the tirage liquor (suspension of yeasts and sugar). Thus, its aroma is influenced by the varietal components (monoterpenoids, sesquiterpenoids, and C13 norisoprenoids), as well as by those produced during the fermentation processes. Alcohols, esters, acids, aldehydes, ketones, lactones, terpenoids, and phenols, representing more than 800 volatile compounds, have already been identified in grapes and wines [1]. The volatile composition of the wine is one of the most important factors to determine its aroma character and quality. The monoterpenoids, sesquiterpenoids, and C13 norisoprenoids play an important role in the wine varietal character, contributing to their differentiation.
‘Baga’ (BG) is the main variety in Bairrada Appellation, an ancient winemaking region in Portugal. This variety represents 92% of the red grape vineyard, and 80% of the overall Bairrada vineyard. Volatile composition of BG monovarietal wine studied by liquid–liquid dichloromethane continuous extraction includes aliphatic and aromatic alcohols, aliphatic acids, esters, phenols, lactones, and amides [2]. Sesquiterpenoids, which have been detected for the first time by the use of solid phase microextraction (SPME), represent 56–80% of varietal compounds of BG grapes at maturity [3]. ‘Fernão-Pires’ (FP) is the main white grape variety harvested in Bairrada Appellation, representing 80% of the white vineyard and 10% of the overall Bairrada vineyard. Studies carried out on grapes and table wine of FP variety showed that monoterpenoids, aromatic alcohols, and C13 norisoprenoids were the main chemical compounds that contribute to the varietal volatile characteristics of this variety [4], [5], [6], [7].
Environmental factors (topographical, agro-pedological, climatic, etc.), usually described by the French term “terroir”, influence grape and wine composition and quality [8]. However, the dependence of grape berry attributes on environmental conditions, as well as the possible effects of the soil types still remains uncertain. Soil may affect water and nutrient availability to the plant by its retaining capacity, may affect the microclimate by its heat-retaining and light reflecting capacity, and may affect the root growth by its penetrability [8]. Deficit water status imposed by same types of soils and climate parameters have been shown to increase the oenological potential of Agiorgitiko red grape variety [9].
The low concentration of the majority of the volatile compounds present in a wine matrix makes enrichment as a basis for identification and quantification. Nowadays, the solventless approaches such as SPME and, more recently, stir bar sorptive extraction (SBSE), showed to be environmental friendly alternatives due to its ease of use, high selectivity, high sensitivity and reproducibility, and less time consuming than the usually used solvent extraction techniques [10], [11]. SBSE has been widely used in several types of applications, especially associated with thermal desorption (TD) systems on-line with gas chromatography–mass spectrometry (GC–MS). However, the TD units are expensive devices and, although this approach presents a remarkable sensitivity, it is not the most indicated to analyse thermolabile compounds due the very high desorption temperatures of operation (up than 200 °C) which can contribute to the occurrence of artefacts. In addition, TD does not offer the opportunity of reanalysis, which is an important issue in many studies for validation purposes. Several studies have already been published using the SBSE-TD/LVI-GC–MS methodology to characterize the volatile fraction of wine [10], [11], [12], [13].
The volatile composition of sparkling wines has already been characterized by HS-SPME/GC, in order to evaluate the development of volatiles during second fermentation and aging [14], [15], [16]. The volatile fraction of these wines was composed mainly by esters, alcohols, and acids. The C13 norisoprenoids and terpenoids were absent in all studies. The volatile fraction of Spanish sparkling wine (Cava) has been studied by three different techniques: simultaneous distillation extraction (SDE), closed-loop stripping analysis, and HS-SPME. Beyond the esters, alcohols, and acids, terpenoids such as limonene, α-terpineol, and lilial were also detected by SDE [17]. SBSE combined with LD was recently proposed for the enrichment of volatile compounds in sparkling and table wines, which can be easily performed by trapping the analytes followed by removal through back-extraction with a small volume (200 μL) of a convenient organic solvent such as pentane, combined with large volume injection [18].
The volatile composition of natural products is usually composed by several compounds corresponding to different chemical structures and presenting very different relative amounts. The corresponding chromatograms present several co-elutions and/or interferences, even if it was used relatively long chromatographic programmes. Thus, the aim of this manuscript is to develop a new approach for the determination of volatiles in sparkling wines based on the SBSE-LD/LVI-GC–qMS methodology previously developed [18]. The quantification of individual components was achieved by linear regression using structurally related standards and suitable ion extraction chromatography (IEC). This methodology was applied to BG and FP sparkling wines produced from grapes obtained from different ripening stages and soils.
Section snippets
Samples and reagents
Sparkling wines from two grape varieties, i.e. ‘Fernão-Pires’ (FP) and ‘Baga’ (BG), having different ripening stages and soils were used. To produce FP wines grapes were picked up at three harvest moments: (1) at the adequate harvest maturity to produce sparkling wines, determined by the physical–chemical parameters berry texture and colour, sugar content, and titratable acidity (FPHC); (2) at an early harvest moment, one week before maturity harvest (FPEarlyHC); and (3) at a late harvest
Quantification approach for assessment of sparkling wine volatiles
As shown in Fig. 1a, the total ion chromatogram (TIC) from the sparkling wines is very complex, combining major compounds, having very high signal intensity, with minor compounds, most of them presenting co-elution. This is a chromatographic profile typical of natural products, corresponding to several chemical structures showing very different relative amounts. Generally, in these cases, the accurate quantification, as well as the corresponding identification, is always a very hard step. The
Conclusion
SBSE-LD/LVI-GC–qMS methodology allowed, in a single run, the quantification of 71 wine volatiles (major compounds – fermentative and minor compounds – varietals) at levels lower than their respective olfactory thresholds. This methodology, in combination with IEC mode, allowed profiling volatiles, with special emphasis to terpenoids that are minor compounds but greatly influence the wine aroma. Fermentative compounds comprise 94–99% of the sparkling wine volatile compounds. Low detection limits
Acknowledgements
This work was financially supported by AGRO N° 38 and Fundação para a Ciência e a Tecnologia (SFRH/BD/25336/2005). The authors thank Estação Vitivinícola da Bairrada, Eng. A. Dias Cardoso for providing the sparkling wines and MSc Nuno Neng for technical assistance.
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