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

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.