The volatile phenols most often found in wine are 4- ethylguaiacol (4- EG), 4- ethylphenol (4- EP), 4-methylguaiacol, vinylphenols, guaiacol, eugenol, and vanillin (Kennison et al 2008, Spillman et al 1997, Chatonnet et al 1997). Each of these compounds has a distinct aroma ranging from sweaty saddle to cloves (Chatonnet et al. 1992). The potential sensory impact of these compounds is dramatic due to their low sensory thresholds. For instance, the microbial derived volatile phenols, 4- EP and 4- EG, require only 770 µg/L and 436 µg/L, respectively, to be recognized (Chatonnet et al. 1992). With such low thresholds, it is essential to understand the origins and methods mitigation of these compounds. These potent aroma compounds can be traced back to one of three sources in wine; microbial, oak maturation, and smoke-taint.
Of the three sources of volatile phenols in wine, the microbial component is best studied and understood. It has been determined that Brettanomyces/Dekkera sp. (specifically B. bruxellensis) are the only microorganisms capable of synthesizing high quantities of ethylphenols in red wine conditions (Chatonnet et al 1997). Brettanomyces typically produces a ratio of 4- EP and 4- EG of 10:1 and this is predetermined by the precursor ratio of p-coumaric acid and ferulic acid (Chatonnet et al. 1992, Romano et al. 2009). While this ratio holds constant, different strains of Brettanomyces can vary greatly in their overall production of volatile phenols (Conterno et al. 2006).
Oak maturation can affect the volatile phenol profile of a wine in two ways. The first is that its porous nature is impossible to sanitize in a winery setting and is a source for Brettanomyces inoculum in dry red wines (Garde-Cerdán, and Ancín-Azpilicueta 2006). The second is the presence of volatile phenols in new oak barrels. The number of volatile phenols, increases in both quantity and variety from degredation of lignin, cellulose and hemicellulose during toasting. The extraction of volatile phenols into wine is dependent on two factors. The first is the original concentrations available in the oak as determined by the level of toastin. The second is length of time wine is exposed to the barrel.
Kennison et al. (2008) found only trace levels (< 1 µg/L) of volatile phenols are present in the free run juice of smoke tainted grapes. These numbers had risen dramatically by the time the wine was finished; guaiacol – 388 µg/L, 4-methylguaiacol- 93 µg/L, 4- EP 58 µg/L, and 4- EG 16 µg/L. By comparison, the levels of these compounds were not detectable in wines made from the smoke-free grapes. It has been shown that the volatile phenols of smoke tainted grapes are located in the skin and do not penetrate to the pulp (Hoj et al. 2003). Therefore, the maceration program of grapes is a critical consideration when working with smoke exposed fruit. This presents a danger to the winemaker as smoke taint may be underestimated until the grapes are paid for and maceration has begun (Kennison et al. 2008).
Volatile phenol mitigation
Certain strains of Saccharomyces cerevisiae are known to participate in the conversion of hydroxycinnamate acids, the precursors to volatile phenols, through the production of hydroxycinnamate decarboxylase. As such, Morata et al. (2012) recently evaluated the possibility of selecting these strains and coupling them with cinnamyl esterase to effectively reducing the substrate available to Brettanomyces, should a wine become contaminated. In trials, the treated wine, intentionally infected with Brettanomyces bruxellensis, resulted in an overall reduction of ethylphenol production when compared to a control (Morata et al. 2012).
Another strategy being developed is the active removal of 4- EP / 4- EG from wines by using reverse osmosis to create a permeant that can then be processed through column containing hydrophobic adsorptive resin (Ugarte et al. 2005). While still in the exploratory stage, this method aims to reclaim wines spoiled by Brettanomyces and smoke-taint contamination.
Chatonnet, Pascal, Coralie Viala, and Denis Dubourdieu. 1997. Influence of polyphenolic components of red wines on the microbial synthesis of volatile phenols. American Journal of Enology and Viticulture 48:4:443-448.
Chatonnet, P., Dubourdie, D., Boidron, J. N., & Pons, M. 1992. The origin of ethylphenols in wines. Journal of the Science of Food and Agriculture, 602, 165-178.
Conterno, L., Joseph, C. L., Arvik, T. J., Henick-Kling, T., & Bisson, L. F. 2006. Genetic and physiological characterization of Brettanomyces bruxellensis strains isolated from wines. American Journal of Enology and Viticulture, 572, 139-147.
Garde-Cerdán, T., & Ancín-Azpilicueta, C. 2006. Review of quality factors on wine ageing in oak barrels. Trends in food science & technology, 178, 438-447.
Hoj, P., I. Pretorius, ] and R. Blair. 2003. The Australian Wine Research Institute, Annual Report. The Australian Wine Research Institute: Adelaide, Australia. Vol. 3, pp 7-38.
Kennison, K. R., Gibberd, M. R., Pollnitz, A. P., & Wilkinson, K. L. 2008. Smoke-derived taint in wine: the release of smoke-derived volatile phenols during fermentation of Merlot juice following grapevine exposure to smoke. Journal of agricultural and food chemistry, 5616, 7379-7383.