Smoke-Derived Taint in Wine: The Release of Smoke-Derived Volatile Phenols during Fermentation of Merlot Juice following Grapevine Exposure to Smoke

The release of smoke-derived volatile phenols during the fermentation of Merlot grapes, following grapevine exposure to smoke, has been investigated. The concentrations of guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4-ethylphenol, and eugenol were determined by gas chromatography−mass spectrometry and found to increase throughout the winemaking process. Only trace levels (≤1 μg/L) of guaiacol and 4-methylguaiacol could be detected in free run juice derived from the fruit of smoked vines; the highest levels, 388 μg/L and 93 μg/L, respectively, were observed in the finished wine. Control wine (derived from fruit of unsmoked vines) contained 4 μg/L guaiacol, with the volatile phenols either not detected or detected at only trace levels (≤1 μg/L) throughout fermentation. The role of enzyme and acid catalyzed hydrolysis reactions in releasing smoke-derived volatile compounds was also investigated. The volatile phenols were released from smoked free run juice by strong acid hydrolysis (pH 1.0) and enzyme (β-glucosidase) hydrolysis, but not mild acid hydrolysis (juice pH 3.2−3.7). Guaiacol was again the most abundant smoke-derived phenol, present at 431 μg/L and 325 μg/L in strong acid and enzyme hydrolysates, respectively. Only trace levels of each phenol could be detected in each control hydrolysate. This study demonstrates the potential for under-estimation of smoke taint in fruit and juice samples; the implications for the assessment of smoke taint and quantification of volatile phenols are discussed.