Guaiacol and 4-methylguaiacol accumulate in wines made from smoke-affected fruit because of hydrolysis of their conjugates

Abstract

Background and Aims: Taint in smoke-exposed grapes have been associated with elevated levels of guaiacol and 4-methylguaiacol. Previous research has reported guaiacol and 4-methylguaiacol in both fruits and wines. In some cases, these compounds were not detected, or were detected at low levels in the fruit while high levels were subsequently identified during or after winemaking. Later research indicated that this was due to the presence of glycosidic conjugates. Here we report a method for the routine analysis of guaiacol and 4-methylguaiacol released after acid hydrolysis of glycoside precursors. Methods and Results: Chardonnay, Merlot, Shiraz, Sangiovese and Cabernet Sauvignon fruits were collected following bushfire events in 2006-2007 in the King Valley wine region of NE Victoria, Australia. Gas chromatography-mass spectrometry (GC-MS) was used to detect free guaiacol and 4-methylguaiacol in both fruits and wines. Low levels of free and bound forms were present in fruit not exposed to smoke. Substantial levels of free guaiacol and 4-methylguaiacol were detected in the wines made from the smoke-affected fruits. These compounds increased during bottle storage. Acid hydrolysis of wines and berries resulted in a several-fold increase in free guaiacol and 4-methylguaiacol. Conclusions:The validated GC-MS method is suitable for monitoring free and glycosidically bound guaiacol and 4-methylguaiacol after acid hydrolysis in both fruits and wines. Acid hydrolysis of wines provided evidence that bound volatiles, most probably glycosidically, act as reserve for guaiacol and 4-methylguaiacol, which are released during ageing of wines. Significance of the Study: This is the first study published in a refereed journal to demonstrate that smoke taint-associated volatiles increase during ageing of wine and bound forms of guaiacol and 4-methylguaiacol represent an aroma reserve for smoke taint in ageing/bottled wines.

Induced Changes in Phenolic Composition of Wines After Foliar Application of Eugenol and Guaiacol Solutions to Grapevines

“…However, it can be observed that wines from treatments had lower pH values than control wine, meaning that treatments improved the microbiological stability of wines. On the other hand, wines from G treatment had the lowest alcoholic degree, which could be related with the negative effect in the sugar accumulation of grapes observed by Kennison, Wilkinson, Pollnitz, Williams, and Gibberd (2009) which studied grapevine exposure, being the guaiacol a component of the smoke and present at elevated concentration in wines from smoke-affected grapes (Singh et al, 2011). Anyway, G treatment seems to affect the primary metabolism of the plant because the wines obtained had also less acidity than wines from the other treatments.…”

“…The aroma glycosidic precursors are a diverse group of odorless compounds which can suffer hydrolysis by acids or enzymes, giving the odour-active aglycones (Gunata, Bayonove, Baumes, & Cordonnier, 1985). An increase of glycosylated forms of guaiacol and 4-methylguaiacol were observed when grapevines were exposed to smoke from the fire forests (Singh et al, 2011). On the other hand, when both volatiles were applied together, higher volatile concentration of each compound was produced in wines than when they were applied independently.…”

The Impact of Smoke From Vegetation Fires on Sensory Characteristics of Cabernet Sauvignon Wines Made From Affected Grapes

“…However, glycosylated precursors may also be present in the flesh of severely exposed grapes (Singh et al, 2011), and even the free-run juice of smoke-tainted grapes can contain high levels of 4-MG and guaiacol (Kennison et al, 2008).…”

“…The effect of burning fynbos, and the sensory properties of the smoke produced, have not yet been assessed in any context. Investigations in Australia have established that wine produced from grapes exposed to smoke from bushfires develop objectionable smoky, ashy and burnt characters (Kennison et al, 2008;Sheppard et al, 2009;Hayasaka et al, 2010;Singh et al, 2011). Guaiacol and 4-methylguaiacol (4-MG) have been used as key indicators of smoke exposure and potential taint formation (Kennison et al, 2007;Sheppard et al, 2009).…”

“…Once VPs are taken up by grapes, they are rapidly metabolised into their more stable glycosidic forms (Hayasaka et al, 2010;Singh et al, 2011;Parker et al, 2012). These glycoconjugated precursors may hydrolyse during the fermentation, ageing and storage of wine to release free volatile phenols (Kennison et al, 2008;Hayasaka et al, 2013).…”