Applied GC-MS: Analysis of Wine Samples

by Mar 19, 2019Analytical

Free and Bound Smoke Taint Chemicals in Pinot Gris Samples by seq-SBSE-GC-TOFMS

Application Summary

GC-MS analyses of wine samples were conducted by a modified sequential-SBSE-GC-TOFMS method to compare concentrations of free and glycosidically bound smoke taint chemicals. Free volatile phenolics were determined by seq-SBSE-GC-TOFMS as described below. Samples were then retested after adjustment to pH 1.5, sonicated, and heated to 100°C to release the gylcosidically bound smoke taint volatiles (cresols, methyl guaiacols, guaiacol, and syringol). Results are reported as free phenolic smoke taint volatiles and (glycosidically) bound smoke taint phenolics.

The results provide an indication of the extent of smoke taint odor/flavor actively present in the wine and the potential for the release of additional smoke taint chemicals over shelf life. While fermentation tends to free glycosidically bound chemicals, not all of these malodor chemicals may be released. Over time, the smoke taint chemicals that remain bound to sugars in wine may be released, increasing the smoky malodor of the wine during shelf life storage.

INSTRUMENTATION

GERSTEL MPS 2 robotic sampler with TDU option

GERSTEL 1 cm x 0.5 mm PDMS Twisters

SAMPLES ANALYZED

Five Pinot Gris samples were analyzed; Incoming, Pass 2, Pass 6, Pass 10 and Final Blend.

ANALYSIS CONDITIONS

  • Column: Agilent 30 m x 0.25 mm i.d., df = 1.4 µm DB-624.
  • Pneumatics: CIS4
  • Glass wool liner
    0.05 min solvent vent; splitless transfer 1.5 min Vent flow: 50 mL/min
  • GC Column: Constant flow; 1.0 mL/min
  • CIS 4 -100°C; 12°C/sec; 270°C (3.0 min)
  • GC 40°C (4 min), 15°C/min (5 min); 250°C (5 min)
  • TDU 40°C (0.4 min), 60°C/min; 270°C (4.0 min)
  • Solvent vent flow: 50 mL/min
  • TOFMS: 40-300 amu; 30 spectra/sec acquisition rate; S/N (data processing) 50.0

DETAILS

Sample Preparation

Free smoke taint phenolics: Two grams of wine + 8 mLs DI water + 5L 0.025 g/L 2-undecanone (internal standard) were added to a 20 mL GC vial and stirred with two GERSTEL PDMS Twisters (1 cm x 0.5 mm) for 1 hr at 1000 rpm (one Twister used for stirring and the other was immobilized on the side of the vial below the surface of the liquid). Two grams NaCl were added, and the sample was stirred an additional hour.

Gylcosidically bound smoke taint phenolics: Hydrolysis of wine to release smoke taint chemicals that are glycosidically bound was performed. Two grams of wine were adjusted to pH 1.5 by the addition of 1.5 mL of 1 N HCl. The sample was then tightly closed and incubated at 100°C for four hrs. The sample was allowed to cool and then placed in a water bath at 50°C and sonicated for 10 min. After cooling to room temperature, the sample was extracted by seq-SBSE as described above for free smoke taint chemicals. This procedure is slightly modified from the published procedure by Matthew Noestheden et al.2.
Sample introduction.
TDU tubes were placed on the MPS in a TDU VT98 tray. seq-SBSE samples were desorbed in the splitless mode with the solvent vent flow rate of 50 mL/min helium flow at 270°C with a 4 min hold time.

The desorbed analytes were trapped in the CIS 4 inlet at -100°C on a glass wool liner. When desorption was complete, analytes were transferred to the column in splitless mode by heating the inlet rapidly to 270°C with a 3.0 min hold time.

RESultS

Figure 1 shows an example chromatogram for the Incoming Pinot Gris sample using the free volatile phenolics method. Figure 2 shows an example chromatogram for the same sample using the bound volatile phenolics method.
Nine different volatile phenolic compounds associated with smoke taint in wines were detected in samples. Most were below the odor detection thresholds. However, when bound guaiacol is added to amounts of free guaiacol present, smoke sensory problems would be detectable in the samples.
Four-point calibration curves were developed for the volatile phenolic compounds. The curves were all linear with R2 values >0.99. A typical curve for p-creosol (4-methylguaiacol) is shown in Figure 3. The curves were used to quantify the bound and free phenolic compounds in the samples. Table 1 shows the results for the free phenolics, Table 2 shows the results for the bound phenolics. Table 3 gives the odor threshold for these compounds.

Table 1: Amount (ppb) for Free Phenolics in the Samples

From the Australian Wine Research, Leffingwell & Associates

Table 2: Amount (ppb) for Bound Phenolics in the Samples

From the Australian Wine Research, Leffingwell & Associates

Table 3: Odor Descriptors and Thresholds for Phenolics

 From the Australian Wine Research, Leffingwell & Associates

Figure 1: Chromatogram for Incoming Pinot Gris Sample using Free Volatile Phenolics Method

Figure 2: Chromatogram for Incoming Pinot Gris Sample using Bound Volatile Phenolics Method

Figure 3: Calibration Curve for p-Creosol (4-MethylGuaiacol)

4-Methoxy-3-methyl phenol was detected in samples for the first time by our laboratory. It was
present at the highest level in both free and bound forms of any of the volatile phenolics. We were
unable to see references to this specific chemical in smoke tainted wine. While the odor threshold
of this chemical is unknown, it is likely similar to the 64-ppb level reported for 4-methylguaiacol
and is, therefore, likely the most significant smoky odor contributor of any of the volatile
phenolics. The contribution of glycosidically bound phenolics to smoky malodor is significant.
The results provide an indication of the extent of smoke taint odor/flavor actively present in the
wine and the potential for the release of additional smoke taint chemicals over shelf life. While
fermentation tends to free glycosidically bound chemicals, not all of these malodor chemicals may
be released. Over time, the smoke taint chemicals that remain bound to sugars in wine may be
released, increasing the smoky malodor of the wine during shelf life storage.
The sequential SBSE-GC-TOFMS provides an easy, solvent free method for determining both
free and bound phenolics.

References

  1. “Sequential Stir Bar Sorptive Extraction for Uniform Enrichment of Trace Amounts of Organic Pollutants in Water Samples”, N. Ochiai, K. Sasamoto, H. Kanda and E. Pfannkoch, J. Chrom. A, 1200 (2008) 72-791. “Sequential Stir Bar Sorptive Extraction for Uniform Enrichment of Trace Amounts of Organic Pollutants in Water Samples”, N. Ochiai, K. Sasamoto, H. Kanda and E. Pfannkoch, J. Chrom. A, 1200 (2008) 72-792.
  2. “Quantitating Organoleptic Volatile Phenols in Smoke-Exposed Vitis vinifere Berries”, M. Noestheden, K. Thiessen, E. Dennis, B. Tiet, and W. Zandberg, J. Agric. Food Chem., 2017, 65, 8418−8425. This method was recommended by Susan Ebler, UC Davis Enology Department.

AVAILABLE ON GSA THROUGH GOVERNMENT SCIENTIFIC SOURCE

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