Analysis of Grape Volatiles

Introduction

Volatile composition is one of the most important factors to determine wine character and quality. Several studies have recognized a relationship between the wine varietal character, and the grape and musts volatile compounds, namely terpenoids (Wilson et al., 1986) and aromatic alcohols (Rocha et al., 2000). Volatile compounds appear in the free and/or glycosidically linked forms. These precursors have been reported as glycosides having the aroma compounds as their aglycons. They may be released during winemaking by acid and/or enzymatic treatments.

The aim of this work was to study the varietal volatile composition of mature crushed grape berries from Fernão Pires (white) and Baga (red) grape varieties, from the Portuguese Bairrada appellation. Baga is the main variety
from the Bairrada appellation, an ancient winemaking region in Portugal. This variety represents 92% of the red vineyard, and 80% of the overall Bairrada vineyard, covering 15,000 ha, with a mean wine production of 450,000 hL. Fernão Pires is a variety that is spread throughout the Portuguese appellations, and represents 70% of the white vineyard in the Bairrada appellation. Due to the considerable importance of volatile monoterpenoids and sesquiterpenoids to flavor and varietal character of Vitus vinifera varieties, particular attention was devoted to these compounds.Grape volatiles represent a very complex matrix. For the purpose of mass spectral identification, good gas chromatographic separation (GC) is crucial. Comprehensive two-dimensional gas chromatography (GCxGC) employs two orthogonal mechanisms on apolar and polar columns to separate the constituents of the sample. Therefore, the separation potential is greatly enhanced compared to one-dimensional GC. Time-of-flight mass spectrometry (TOFMS) provides full mass spectra and identification based on comparison with NIST library spectra.

Experimental Conditions

Samples Vitis vinifera var. healthy-state Fernão Pires (white) and Baga (red) grapes were collected from the experimental vineyard propriety of Estação Vitivinícola da Bairrada (EVB), the Vine and Wine Research Institute of the Bairrada appellation. The grape berries were transported immediately under refrigeration to the laboratory and were stored at -80ºC until they were analyzed. Sample Preparation 50 g of frozen grapes were ground, mixed with 8 g NaCl, and transferred to a 125 ml amber solid phase microextraction (SPME) vial. A Teflon stirrer was placed in the suspension. The sample in the vial was conditioned and stirred in a 40°C warm-water bath for 1 hour. Subsequently, 1 hour headspace SPME sorption was performed on a 65 μm Carbowax/divinylbenzene fiber (Supelco, USA).

Analysis Conditions

GC Parameters: Agilent 6890N Gas chromatograph equipped with a LECO GCxGC Thermal      Modulator and Secondary Oven

Injection: Manual injection of SPME fiber, Splitless 5 min, 250°C

Primary Column: Equity-5 60 m x 0.25 mm x 1 μm (Supelco, USA)

Secondary Column: Supelcowax 2.5 m x 0.1 mm , 0.1 μm (Supelco, USA)

Carrier Gas: Helium, 1 ml/min, constant flow

Primary Oven Program: 40°C, 1 min, 5°C/min to 260°C, 15 min

Secondary Oven Program: 45°C, 1 min, 5°C/min to 265°C, 15 min

Modulator Temp Offset (above primary oven): 30°C

Modulation Time: 4 s

Hot Pulse Time: 0.8 s

Transfer Line Temp: 250°C

Total Run Time: 64 min

MS Parameters: LECO Pegasus^® 4D GCxGC-TOFMS

Ionization: Electron Ionization at 70 eV

Source Temp: 220°C

Stored Mass Range: 33-350 u

Acquisition Rate: 125 spectra/s

Data Processing: LECO ChromaTOF^® software

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