Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2008. / A method for the detection of adulteration of South African Sauvignon blanc
wines, by enrichment with foreign sources of 3-alkyl-2-methoxypyrazenes, is
described. The levels of three 3-alkyl-2-methoxypyrazenes (3-isobutyl-, 3-
isopropyl- and 3-sec-butyl-2-methoxypyrazine) in South African Sauvignon blanc
wines were measured with liquid chromatography-mass spectrometry. Sample
preparation involved clean-up and pre-concentration by distillation followed by
solvent extraction of the distillate with dichloromethane. Extracts were acidified
and concentrated by evaporation and finally reconstituted to a fixed volume to
affect quantitative pre-concentration of the samples. Sample extracts were
separated with reversed phase liquid chromatography utilizing a phenyl-hexyl
separation column. Residues were measured with liquid chromatography-mass
spectrometry utilizing a tandem quadrupole mass spectrometric detector
operated in multiple reaction monitoring mode for optimal trace level quantitation.
Atmospheric pressure chemical ionization was utilized as electrospray ionization
was found to suffer from quenching effects attributed to the sample matrix.
Qualitative information was obtained from the relevant molecular ions as well as
two secondary ion transitions (and one ion ratio) in each case. Recoveries
obtained by the extraction procedure were better than 90% with coefficient of
variance of better than 10% at concentrations from 1 to 100 ng/L. The limit of
detection of the method was 0.03 ng/L and the limit of quantification 0.10 ng/L for
the three analytes measured. The described LC-MS method is more sensitive for
the determination of 3-alkyl-2-methoxypyrazines in wine than GC methods
reported for the same purpose.
From the experimental data, a set of parameters were established to discriminate
adulterated South African Sauvignon blanc wines. It was demonstrated that the
3-isobutyl-2-methoxypyrazine concentration, despite showing considerable
variance, was confined to a relatively narrow range spanning approximately two
orders of magnitude (0.20 to 22 ng/L). A clear indication of possible maximum values for this compound in South African Sauvignon blanc wines was obtained
from the analysis of a large number of samples (577), spanning most relevant
wine producing regions and representing vintages 2003 to 2006. It was also
demonstrated that South African Sauvignon blanc wines contain the major 3-
alkyl-2-methoxypyrazenes in reasonably distinct relative amounts and that the
said ratios of abundance may be used to elucidate authenticity. The expected
effect of adulteration with green pepper extracts or some synthetic preparations
on the 3-isobutyl-2-methoxypyrazine concentration as well as the relative
abundances were also determined by characterizing the corresponding profiles in
green peppers and some synthetic flavor preparations. Two adulterated samples
in the dataset were identified by both outlined criteria. A limited number of wines
of other cultivars were also analyzed. The results represent the most complete
and accurate data on the 3-alkyl-2-methoxypyrazine content of South African
Sauvignon blanc wines to date.
A publication covering the work presented in this thesis is currently in
preparation.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2961 |
| Date | 03 1900 |
| Creators | Alberts, P. |
| Contributors | De Villiers, A. J., Stander, M. A., University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
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