Chapter 1

Chemical Components of Grapes and Wine

Volatile Phenols in Grapes and Wine

Volatile phenols are aromatic compounds and one of the key molecules responsible for olfactory defects in wine. The volatile phenols most often found in wine are 4-ethylguaiacol (4-EG), 4-ethylphenol (4-EP), 4-methylguaiacol, 4-ethylcatechol (4-EC), vinylphenols, lactone, guaiacol, eugenol, and vanillin. The potential sensory impact of these compounds is dramatic due to their low sensory thresholds. 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; smoke taint, microbial, and oak maturation.

Smoke Taint

When vineyards and grapes are exposed to smoke from wildfires it can result in wines with undesirable sensory characters referred to as smoke taint (Section 16.3). Of all of the compounds present in smoke, the volatile phenols guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4-ethylphenol, eugenol, cresol, furfural, and syringol are thought to have the largest impact on wine sensory qualities.


Of the three sources of volatile phenols in wine, the microbial component is best studied and understood. It has been determined that Brettanomyces spp. (specifically B. bruxellensis) are the only microorganisms capable of producing negative aromatic compounds: volatile phenols (4-ethylphenol, 4-ethylguaiacol and 4-ethylcatechol), characterized by animal type (horse, barnyard, etc.), pharmaceutical (Band-Aid, medicine, etc.), and ink aromas (Section 15.1).

Oak Maturation

During barrel maturation, the volatile compounds extracted from oak wood can contribute to a wine's overall aroma and flavor, enhancing its character and complexity (Section 21.1). These compounds include, but are not limited to, cis- and trans-oak lactone, guaiacol, 4-methylguaiacol, eugenol, vanillin, furfural, and 5-methylfurfural.

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