Comparison of methods for the analysis of smoke related phenols and their conjugates in grapes and wine

Abstract

Background and Aims: Australian grape growers and winemakers have typically relied on guaiacol and 4-methylguaiacol measurements to determine smoke exposure of grapes following bushfires or prescribed burns. However, the guaiacol and 4-methylguaiacol content of grapes does not always correlate with the extent of taint in resultant wines. This study compared several methods for the analysis of smoke related phenols and their conjugates in grapes and wine, to determine their capacity as diagnostic assays for smoke exposure. Methods and Results: Smoke-affected grapes were sourced from commercial vineyards exposed to bushfire smoke and from experimental field trials involving the application of smoke to grapevines, and small-scale wines were made from a number of these samples. Several analytical methods were applied to grapes and wine to determine the concentration of smoke related phenols and their conjugates. Strong correlations were observed between the glycoconjugate content of smoke-affected grapes and the concentration of guaiacol and 4-methylguaiacol released following acid hydrolysis of juice. Conclusions: Where smoke-affected grapes contain low or non-detectable levels of guaiacol and 4-methylguaiacol, analytical methods that quantify their glycoconjugate forms (either directly or indirectly) provide a better indication of the extent of smoke exposure. Significance of the Study: This is the first study to compare different methods for assessing smoke exposure in grapes and wine, through analysis of free and bound guaiacol and 4-methylguaiacol. These methods will allow grape growers and winemakers to more reliably assess smoke exposure of grapes, enabling better informed decisions to be made with regards to harvesting and processing smoke-affected grapes.

Exposure of grapes to smoke of vegetation with varying lignin composition and accretion of lignin derived putative smoke taint compounds in wine

“…Each fuel type was (1) reconstituted in proportion to biomass components that burn in a decadal fire event and (2) pyrolysed under conditions that reproduce wildfire temperature profiles (Gould et al, 2007). Recent work has shown that, at least in young wines, the putative smoke taint compounds are predominantly sequestered as glycoconjugates and only a small proportion is present as volatile phenols (Hayasaka, Baldock, Parker et al, 2010;Kennison et al, 2008;Singh et al, 2011, in press;Wilkinson et al, 2011). Thus, in this study both volatile and glycoside-bound phenols were considered.…”

Determination of Volatile Phenol in Cabernet Sauvignon Wines, Made from Smoke-affected Grapes, by using HS-SPME GC-MS

“…Wines from the earlier vintage therefore spent more than a year in storage. Other workers have suggested that tests for smoke compounds should include the testing of glycoconjugated VP precursors in order to better predict taint potential (Dungey et al, 2011;Wilkinson et al, 2011;Parker et al, 2012). In this study, differences in guaiacol concentration were not significantly different over the two vintages.…”

“…The smoke was produced by burning fynbos mixed with other vegetation in order to simulate smoke that might be generated by veld fires. Wines made from the smoke-affected grapes were then subjected to VP analysis by HS-SPME GC-MS, using a method adapted from Wilkinson et al (2011) and Singh et al (2012).…”

Smoke taint compounds in wine: nature, origin, measurement and amelioration of affected wines

“…Responding to the need to more reliably quantify the diversity of volatile phenol aglycones and glycoconjugated, and hence aid prediction of the degree of contamination of wine with smoke taint, Wilkinson et al (2011) compared recently published analytical methods. Strong correlations were found between the amounts of guaiacol and 4-methylguaiacol glycoconjugates and the aglycones released through hydrolysis.…”