Browsing by Author "Mokwena, L."

Now showing 1 - 3 of 3
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    Investigation of thiol levels in young commercial South African Sauvignon Blanc and Chenin Blanc wines using propiolate derivatization and GC-MS/MS
    (South African Society for Enology and Viticulture, 2018) Coetzee, C.; Schulze, A.; Mokwena, L.; Du Toit, W. J.; Buica, A. S.
    In this work, the ethyl propiolate method for analysing thiols in white wine by GC-MS, originally proposed by Herbst-Johnstone et al.) (2013), has been adapted to GC-MS/MS and has been validated. The method performance has shown improvement in terms of sensitivity (limit of detection, LOD) and of the number of compounds measured. In addition to 3-mercaptohexanol (3MH), 3-mercaptohexyl acetate (3MHA), and 4-mercapto-4-methylpentan-2-one (4MMP), the adapted method can also measure 2-furanmethanethiol (FMT) and makes use of a commercially-available internal standard (IS), 4-methoxy-2-methyl-2-butanethiol (4M2M2B, IS). The proposed method was applied to determine thiol levels in young commercial South African Sauvignon and Chenin Blanc wines. The samples (n=20 for each cultivar) were chosen according to a high frequency of the typical descriptors associated with this class of impact compounds. 3MH was found at 178-904 ng/L and 99-1124 ng/L, and 3MHA at 23-151 ng/L and 5-253 ng/L in Sauvignon and Chenin Blanc respectively. 4MMP was present in Sauvignon Blanc in concentrations up to 21.9 ng/L, but in none of the Chenin Blanc samples.
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    Potentially smoke tainted red wines: volatile phenols and aroma attributes
    (South African Society for Enology and Viticulture, 2019) McKay, M.; Bauer, Florian; Panzeri, V.; Mokwena, L.; Buica, A. S.
    Malodourous compounds, including volatile phenols (VPs) are frequently found at concentrations below their odour thresholds in wine, and may therefore be considered to present no threat to wine quality. Most investigations into smoke taint quantify compounds by chemical/analytical means, or investigate sensory effects of supra- and peri-threshold contamination in model wine. In this project, twelve wines submitted by the South African industry as ‘faulty’ and/or smoke tainted were screened for VPs using GC-MS, and characterized using Descriptive Analysis (DA) by a sensory panel highly trained in smoke taint evaluation. Results were compared statistically to elucidate relationships between chemical and sensory characteristics. It was demonstrated, using the combined dataset, that concentration and composition of VPs in the wines correlated well with certain sensory attributes. Guaiacol was present in most samples at peri- or supra-threshold levels, but did not cause taint unless in combination with other phenols, when it was associated with ‘smoky’, ‘ashy’ and ‘herbaceous’ attributes. Wines with very low levels of VPs showed more sweet-associated aroma characteristics. Wines with supra-threshold levels of VPs showed negative attributes (‘chemical/plastic’, ‘tar/BR’ and ‘medicinal/Elastoplast™’). In some cases, sensory effects (‘earthy/dusty/potato skin’, ‘mouldy/musty’ and ‘cooked veg’) could not be attributed to peri- or supra-threshold concentrations, but seemed to be due to combinations of volatile phenols at subthreshold levels. Associations between negative attributes and historical bushfire events prior to harvest were found for a number of the wines. This study emphasizes the importance of understanding effects of VPs on wine aroma, and escalating awareness and sensitivity to these issues in the wine industry.
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    Profiling potentially smoke tainted red wines : volatile phenols and aroma attributes
    (South African Society for Enology and Viticulture, 2019) McKay, M.; Bauer, F. F.; Panzeri, V.; Mokwena, L.; Buica, A.
    Malodourous compounds, including volatile phenols (VPs) are frequently found at concentrations below their odour thresholds in wine, and may therefore be considered to present no threat to wine quality. Most investigations into smoke taint quantify compounds by chemical/analytical means, or investigate sensory effects of supra- and peri-threshold contamination in model wine. In this project, twelve wines (submitted by the South African industry as potentially smoke tainted) were screened for VPs using GC-MS, and characterized using descriptive analysis (DA) by a sensory panel highly trained in smoke taint evaluation.  Results were compared statistically to elucidate relationships between chemical and sensory characteristics.  It was demonstrated, using the combined dataset that concentration and composition of VPs in the wines correlated well with certain sensory attributes. Guaiacol was present in most samples at peri- or suprathresholdlevels, but was not correlated with taint unless in combination with other phenols, in which case it was associated with ‘smoky’, ‘ashy’ and ‘herbaceous’ attributes. Wines with supra-threshold levels of VPs showed negative attributes (‘chemical / plastic’, ‘tar / BR’ and ‘medicinal / Elastoplast™’). In some cases, sensory effects (‘earthy / dusty / potato skin’, ‘mouldy / musty’ and ‘cooked vegetables (veg.)’) could not be attributed to supra-threshold VP contamination, and therefore seemed to be due to combinations of VPs at subthreshold levels. Associations between negative attributes and historical bushfire events prior to harvest were found for a number of the wines. This study emphasizes the importance of understanding effects of VPs on wine aroma, and escalating awareness and sensitivity to these issues in the wine industry.