Rapid determination of volatile compounds in grapes by HS-SPME coupled with GC–MS
Introduction
Grape volatile compounds are the main contributor to the fresh and fruity note of wines. Compounds responsible for this aroma (terpenes, C13-norisoprenoids, benzene derivatives, and aliphatic alcohols) are present in grapes, mainly in the skin [1]. Concentration of these volatile compounds is different depending on the grape variety, cultural practiques, and climatic or biological factors [2], [3].
On the other hand, grape volatile composition can greatly vary during ripening [4]. Traditionally grape maturity corresponds to an optimal sugar/acid ratio, however production of quality white wines requires grapes whose aromatic substances are at a maximal concentration [5]. Monitoring aromatic grape maturation needs a simple and rapid method to analyse volatile compounds in grape using as little amount of sample as possible to enable an adequate sampling.
Volatile compounds of grapes are generally present in trace amounts and require a previous step of isolation and concentration for the subsequent gas chromatography analysis. Most of the proposed methods are based on obtaining must by previously pressing the grapes, and the skin composition is not taken into account. Different methods have been used for isolating volatile compounds from must such as simultaneous distillation-extraction [6] or solid-phase extraction [7], [8]. Berry fractions were analysed after homogenisation and centrifugation of 500 g of grapes [9], [10].
These methods are based on the use of solvents and present some drawbacks, such as the possibility of sample contamination and the loss of analytes during the concentration step. Additionally they are time-consuming and in general require high temperatures and large amounts of sample.
Dynamic headspace technique has been used to analyse the volatile composition of grapes in order to discriminate different varieties [11].
Solid-phase microextraction (SPME) is a fast, simple and solvent-free technique that, thanks to the appearance of different types of adsorbents with a wide range of polarity, makes it possible to isolate trace compounds of different substrates [12]. SPME has been used in a range of fields including studies of flavours and taints especially for quick screening of the volatile composition of a wide range of products [13].
Traditional fibres (PDMS and PA) have been used to determine terpenes and fermentation compounds in wines [14], [15], [16], [17], [18], fruits and juices [19], [20], [21], [22], however these fibres present a poor sensitivity for polar compounds.
Mixed coating fibres containing divinylbenzene (DVD), PDMS and carboxen (CAR) or polyethylene glycol (CW), increase the tramping ability of the fibre due to the synergic effect of adsorption and distribution within the stationary phase, producing higher sensitivity than PDMS and PA fibres [23].
In this work, we propose a method for a rapid analysis of volatile compounds of grape (pulp and skin) using HS-SPME-MS. These method can be useful for monitoring grape maturation and to determine grape composition before winemaking. Optimisation of operating conditions and selection of the best fibre were evaluated.
Section snippets
Samples
Commercial samples of Muscat grapes were purchased from a local store, and frozen at −20 °C until their analysis. Before isolating the volatile compounds, the grapes were unfrozen at 5 °C under nitrogen.
HS-SPME conditions
The grapes were manually pealed and the skins were separated from the pulp. Both fractions (skin and pulp) were individually homogenised and analysed.
Three SPME fibre coatings were tested and used: 65 μm PDMS/DVB, 50/30 μm CAR/DVB/PDMS, and 70 μm CW/DVB. Fibres were exposed to the headspace of a
HS-SPME performance
HS-SPME is an equilibrium technique that requires a previous optimisation step of the sampling conditions, in order to obtain high recoveries of volatiles and a good precision of the method. Some of these sampling conditions are extraction temperature and time, and fibre adsorbent phase.
To select the optimum extraction temperature and time, a 50/30 μm CAR/DVB/PDMS fibre was used, since this is one of the last fibres that has been introduced in the market and, due to its nature, it is very
Conclusions
HS-SPME using a PDMS/DVB fibre is a fast and useful method for quantification of volatile compounds in skins and pulp, allowing a rapid screening of aroma compounds in grapes of different varieties or cultivars. This method can also be used to determine the aromatic maturity in grapes in order to decide the optimal harvest date.
Acknowledgements
The authors thank the INIA (Ministerio de Ciencia y Tecnología) for the financial support under the project (VIN02-007). Eva Sánchez-Palomo would like to thank the Ministerio de Educación Cultura y Deporte for the award of a grant.
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