Characterization of the aroma profile of Madeira wine by sorptive extraction techniques

Abstract

The characterization of the aroma profile of 33 samples of Madeira wine from five monovarieties (Sercial, Verdelho, Boal, Malvasia and Tinta Negra Mole) having different type and categories is presented, using solid phase microextraction and stir bar sorptive extraction techniques (SPME and SBSE) followed by capillary gas chromatography and mass spectrometry detection (GC–MS).

Headspace SPME/GC–MS provided effectiveness to identify the major constituents of the aroma profile of Madeira wine, where no remarkable differences occur among the samples studied. The volatile compounds are mainly constituted by ethyl octanoate (11.3–256.9 μg L⁻¹), ethyl decanoate (21.5–210.5 μg L⁻¹), ethyl decenoate (0.1–112.8 μg L⁻¹), diethyl succinate (0.9–65.6 μg L⁻¹), ethyl dodecanoate (1.2–6.5 μg L⁻¹), ethyl nonanoate (0.6–5.2 μg L⁻¹), ethyl hexanoate (0.2–3.7 μg L⁻¹) and isoamyl octanoate (0–2.2 μg L⁻¹). C₁₃ norisoprenoids such as vitispirane (0.9–7.0 μg L⁻¹) and 1,1,6-trimethyl 1,2-dihydro naphthalene (0.7–12.5 μg L⁻¹), as well as phenyl ethanol (0–8.1 μg L⁻¹), were also found in Madeira wine samples.

The powerful capabilities of SBSE followed thermal desorption and GC–MS analysis allowed higher ability for profiling traces and ultra traces of compounds in Madeira wine samples, including esters (80.7–89.7%), carboxylic acids (1.6–4.2%), alcohols (3.5–8.2%), aldehydes (0.9–3.7%), pyrans (0.2–1.7%), lactones (<3%), monoterpenes (0.1–1.4%), sesquiterpenes (0.1–0.8%) and C₁₃ norisoprenoids (1.7–6.5%), which some of them play a remarkable impact on the aroma complexity. C₁₃ norisoprenoids in particular, seem to play an important role on Madeira wine bouquet since presenting very low sensorial threshold limits. Excellent correlation between Madeira wine ageing and the abundance of cis-oak lactone was attained showing to be an important chemical descriptor to characterize reserves and Vintages as well as a contributor to wine flavour.

The differentiation between reserves, dry/medium dry and sweet/medium sweet young wines could be well established by means of chemometric analysis, using particular aroma compounds such as diethyl succinate, cis-oak lactone and ethyl octanoate as discriminating variables.

Introduction

The aroma profile of wine is determined through the combined effects of several hundreds of chemically different compounds where the amounts in some cases can be lower than parts-per-billion (μg L⁻¹). The aroma depends on several factors such as the environment (climate and soil), ripeness and grape variety, winemaking as fermentation conditions and ageing. Several classes of compounds mainly alcohols, aldehydes, esters, acids, monoterpenes and other minor components, usually constitute the volatile fraction, which some of them are already present in the grapes or being formed during the fermentation and maturation process. In last years, emphasis have been done to particular groups of new compounds, e.g. norisoprenoids, due the important impact contribution on the aroma of some wines, where the sensorial detection threshold is sometimes typically in parts-per-trillion (ng L⁻¹) range [1].

The low concentration of the most volatile aroma compounds makes enrichment as a basis for identification and quantification, where liquid–liquid extraction using organic solvents is a suitable method to characterize the bouquet of wines, prior to analysis usually by capillary gas chromatography coupled to mass spectrometry detection (GC–MS). In the meantime, liquid–liquid extraction is a time consuming and labour-intensive technique, involving multi-step procedures prone to analyte loss and usually requires toxic organic solvents.

Solid phase microextraction (SPME) developed by Pawliszyn and co-workers [2], [3] is an alternative to liquid–liquid extraction, which is growing in popularity due to its ease of use, high sensitivity and reproducibility and low cost. It requires neither solvents nor previous sample preparation is easy to automate and has successfully been applied for the characterization of wines and beverages [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. The SPME process has two steps: partitioning of analytes between the coatings, e.g. polydimethylsiloxane (PDMS), and the sample matrix, followed by desorption of concentrated extracts into the GC injector port. In the first step, the fiber is exposed either to direct or to headspace sampling modes, which the last one has shorter equilibration times and the fibber presents a longer lifetime. Another advantage is that the high content of ethanol as well as other compounds usually present in wine matrices could decrease the sensitivity of the minor constituents in the direct sampling mode. Nevertheless, in many cases only the main components, i.e. fermentation bouquet, are found and important compounds that have a significative impact in the aroma profile are not detected.

Stir bar sorptive extraction (SBSE) is a novel sample preparation method [29], [30] that has been also applied for the enrichment of organic solutes from wine and beverages samples [31], [32], [33], [34], [35]. In the SBSE, a stir bar coated with a PDMS layer is stirred for a given time in the sample. After sampling, the stir bar is placed in a thermal desorption unit coupled on-line to capillary GC–MS. SBSE had been shown a much higher sensitivity than SPME by a factor within 50 and 250 due the higher content of PDMS (24–126 μL), in which the amount of analyte extracted is proportional to the coating thickness, increasing the limit of detection of ultra trace compounds during sampling. The theory of SBSE is quite similar to that of SPME, which the efficiency of analytes partitioning into the PDMS phase on the stir bar at the equilibrium, can rugged be predicted by the octanol–water partition coefficients [29].

Madeira wines in particular, produced from Sercial, Verdelho, Boal, Malvasia and Tinta Negra Mole cultivars, are very famous all over the world due the pleasant organoleptic complexity. Although physicochemical and sensorial characterization of Madeira wines had been previously reported [36], the bouquet has never been studied.

The aim of the present work is to characterize for the first time the main chemical composition which constitute the free aroma compounds that could have relevance on the organoleptic profile of Madeira wine, using for such purpose sorptive extraction techniques (SPME and SBSE) followed by capillary GC–MS analysis. The evaluation of particular compounds was established with accurate criteria of differentiation using multivariate techniques of data analysis.