The increased incidence of vegetation fires near vineyards in the Western Cape, South Africa has led to growing concern over the appearance of smoke taint in the affected grapes. This study focused on the effect of smoke exposure on the volatile phenol (VP) composition of wines made from affected grapes over two vintages (2012 and 2013). Cabernet Sauvignon grapes were exposed, 10 days post-véraison, to a single, hour-long treatment with smoke derived from burning a mixture of vegetation (including fynbos) under controlled conditions in enclosures. Enclosures were sealed for 24 h after exposure, and then removed. Grapes were allowed to ripen and wines were then produced. Wines were analysed by headspace gas chromatography mass spectrometry (HS-SPME GC-MS) for selected volatiles. The results of the investigation show that the exposure of grapes to smoke during ripening leads to the accumulation of VPs that were detected in wines. The detected VPs were guaiacol, phenol and the cresols. Smoke exposure of grapes during ripening may have a negative impact on wine quality due to the accumulation of these compounds.
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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