12
Volatile Phenols

12.1 Introduction

Volatile phenols contribute characteristic odors to various foods and beverages, including wine. They are of particular importance to the flavor and appeal of smoked foods [1] and historically the application of wood smoke was a useful way of preventing food from spoiling.1 In wine and alcoholic beverages, barrels or tanks made from wood (particularly oak) are frequently used for maturation and storage, and volatile phenols are among the most important organoleptic compounds extracted from toasted oak wood [2] (Chapter 25). Volatile phenols in wines may also arise from precursors originating in the grapes that are transformed by microbiological or chemical processes. A number of highly potent, halogenated volatile phenols, which are detrimental to wine aroma are derived from additional sources and are presented elsewhere (Chapter 18).

12.2 Structure and chemical properties

Volatile phenols are low molecular weight, aromatic2 alcohols with some similarities in reactivity to other phenols (Chapter 11). The term encompasses phenol and its derivatives containing alkyl, methoxyl, vinyl, allyl, aldehyde (Table 12.1), and halide substituents (Chapter 18). Volatile phenols have hydrogen bonding capability due to the hydroxyl group, but in general are of intermediate hydrophobicity, with influences from various ring substituents (e.g., guaiacol has log P = 1.3 and water solubility = 15.3 g/L whereas 4‐vinylguaiacol has log P = 2.2 and water solubility = 3.0 g/L). Compared to other phenolic compounds in wine (e.g., polyphenols and hydroxycinnamic acids (HCAs)), volatile phenols are much lower in concentration and simple o‐diphenols (i.e., catechols) are rare, meaning this class of compounds is less likely to play a role in oxidation phenomenon (via quinone formation, Chapter 24). Apart from electrophilic aromatic substitution reactions (Chapter 11), volatile phenols are generally stable molecules, although reactive functional groups can undergo the usual enzymatic transformations (i.e., reduction of aldehyde or alkene), and they can be bound as phenolic O‐glycosides (which may undergo hydrolysis, Chapter ). Based on the extent of hydrophobicity of the different volatiles phenols, they may be adsorbed onto hydrophobic surfaces such as wood [3], yeast cell walls [4] or lees [5], or solid‐phase polymers used for amelioration [6, 7].

Table 12.1 Indicative odor descriptors, detection thresholds, and odor activity values of selected volatile phenols encountered in wine [8, 11–18]

Compounda Structure Odor descriptorb Threshold (μg/L)b OAV (max.) Major sourcec
Phenol Structural formula of phenol. Chemical, ink 30 4 L
Guaiacol
(2‐methoxyphenol)		 Structural formula of guaiacol (2-methoxyphenol). Smoke, sweet 23 3 L
4‐Methylguaiacol
(2‐Methoxy‐4‐methylphenol)		 Structural formula of 4-methylguaiacol (2-methoxy-4-methylphenol). Smoke, ash 21 3 L
Syringol
(2,6‐dimethoxyphenol)		 Structural formula of syringol (2,6-dimethoxyphenol). Smoke, medicinal 57 8 L
Eugenol
(4‐Allyl‐2‐methoxyphenol)		 Structural formula of eugenol (4-allyl-2-methoxyphenol). Spice, clove 6 15 L
Vanillin
(4‐hydroxy‐3‐methoxybenzaldehyde)		 Structural formula of vanillin (4-hydroxy-3-methoxybenzaldehyde). Vanilla 200 5 L
m‐Cresol
(3‐methylphenol)		 Structural formula of m-cresol (3-methylphenol). Leather 20 8 L
4‐Ethylphenol
(4‐EP)		 Structural formula of 4-ethylphenol (4-EP). Leather, horse stable 440 6 B
4‐Vinylphenol
(4‐VP)		 Structural formula of 4-vinylphenol (4-VP). Medicinal, phenolic, 180 27 S
4‐Ethylguaiacol
(4‐EG)		 Structural formula of 4-ethylguaiacol (4-EG). Spice, clove 33 13 B
4‐Vinylguaiacol
(4‐VG)		 Structural formula of 4-vinylguaiacol (4-VG). Smoke, phenolic 40 10 S

a Common names are widely used in the literature for these compounds; systematic names (or common abbreviations) can also be encountered and are given in parentheses.

b Descriptor and threshold data refer to different matrices, including water and red or white wine.

c L, lignin degradation (i.e., oak or smoke); B, Brettanomyces yeast activity; S, Saccharomyces yeast activity.

12.3 Concentrations in wine and sensory effects

Volatile phenols can be found at μg/L–mg/L (i.e., suprathreshold) concentrations in wine and impart odors reminiscent of smoke, medicinal/cleaning products, vanilla, spice, and leather (Table 12.1). The presence of the phenolic hydroxyl group has a major impact on the odor of benzenoid substances; the hydroxylated phenols have lower thresholds compared to the methyl (toluene) analogs. The position of ring substituents relative to the hydroxyl group plays an important role in the aroma characteristics and detection thresholds of these compounds. For instance, monoalkyl groups meta to the hydroxyl (e.g. m‐cresol) result in lower odor thresholds compared to their ortho and para analogues, and odor descriptors differ for the various isomers and as a function of alkyl chain length [8] (e.g., 4‐methylguaiacol versus eugenol). In comparison, ortho halogenated compounds (Chapter 18) tend to have lower thresholds than their isomeric or methylated counterparts, with odor qualities again dependent on substitution pattern [9, 10].

12.4 Origins in wine and effects on volatile phenol profile

12.4.1 Oak storage

Several volatile phenols, including phenol, guaiacol, syringol, vanillin, cresols, and eugenol (Table 12.1), are extracted when storing wine in contact with toasted oak wood (e.g., oak barrels, chips, or staves). These compounds arise from thermal degradation of lignin and are generally positive contributors to wine aroma when present at appropriate concentrations. Analogously, heating other lignin‐containing foodstuffs (e.g. roasting coffee or peanuts, kilning of malt for beer and whiskey) will generate volatile phenols that contribute to the characteristic odors of these products.3 The extent of heating (i.e., amount of oak barrel toasting, Table 12.2) and time in contact with toasted oak influences the level of lignin degradation compounds in wine, as does wine alcohol content, barrel age, oak origin, and seasoning (Chapter 25) [19, 20]. Typically, dry red wines will be subjected to oak contact but few white wines receive such treatment, with Chardonnay, Semillon, and botrytised white wines being the main exceptions. Table 12.3 provides a guide to the concentrations that can be encountered for a range of volatile phenols in oaked wines.

Table 12.2 Volatile phenols extracted into model wine (12% v/v/ethanol, 5 g/L tartaric acid, adjusted to pH 3.5) after two weeks of maceration of French oak wood shavings obtained from light, medium, and high toasting of barrels. Data from Reference [21]

Compound Average μg/g (% RSD) of dry wood at toasting level
Light Medium High
Phenol 0.47 (102) 0.83 (17) 0.41 (101)
Guaiacol 0.45 (72) 1.26 (13) 0.40 (60)
4‐Methylguaiacol 0.80 (28) 1.72 (23) 0.74 (20)
Syringol 0.94 (74) 3.66 (10) 0.83 (61)
Eugenol 1.87 (84) 1.27 (58) 1.65 (75)
Vanillin 27.17 (18) 49.81 (47) 25.45 (12)
4‐Ethylphenol 0.30 (78) 0.29 (13) 0.26 (72)
4‐Ethylguaiacol 0.04 (55) 0.14 (23) 0.04 (38)
4‐Vinylguaiacol 0.13 (42) 0.17 (66) 0.12 (37)

Table 12.3 Concentrations of selected volatile phenols in a number of wine varieties aged in contact with toasted oak wood [14, 18, 22, 23]

Compound Concentration range (μg/L)
Chardonnaya Tempranillob Cabernet Sauvignonc Merlotd
Guaiacol 3–28 25–76 5–44 16–139
4‐Methylguaiacol 1–8 16–66 <1–16 5–32
Syringol –e – – 68–488
Eugenol 10–26 30–96 13–52 NDf–19g
Vanillin 198–388 347–854 9–369 77–236
4‐Ethylphenol ND–1 8–251 630–1850 –
4‐Vinylphenol 7–76 1406–2521 1–5 –
4‐Ethylguaiacol ND–5 5–105 22–306 –
4‐Vinylguaiacol 5–30 178–409 1–3 –

a Aged in medium toasted French and American oak barrels for 55 weeks.

b Aged in medium toasted Spanish, French, and American oak barrels for 52 weeks.

c Aged in medium toasted French and American oak barrels for 52 and 93 weeks.

d Aged in contact with light to heavy toasted French oak wood staves in stainless steel tanks for 52 weeks.

e –, not reported.

f ND, not detected.

g Eugenol + isoeugenol.

12.4.2 Fermentation – release from grape‐derived glycosides

Compounds formed by lignin degradation in toasted oak are also naturally present in wood smoke. Volatile phenols can therefore give rise to undesirable aroma and flavor characters (medicinal, smoky, ashy‐aftertaste [24]) in wine when grapes become contaminated due to bushfire (wildfire) smoke in the vicinity of vineyards [25]. This smoke taint is of most concern in winegrape growing regions in proximity to areas where bushfires or forest fires are common, such as parts of Australia, North America, and South Africa. Originally, the focus was on the volatile compounds themselves, and guaiacol and 4‐methylguaiacol were used as indicators of contamination by smoke, but these compounds can be found at higher concentrations in oaked wines [17] (e.g., compare Tables 12.3 and 12.4). Further research discovered that other volatile phenols from smoke were also involved (Table 12.4), and that volatile phenols could be glycosylated in grape leaves or berries. These non‐volatile glycosides could be stored in grapes [26] and hydrolyzed by acid or enzymes during fermentation or storage, subsequently releasing the volatile compounds into wine [27] (Chapter ). These glycosides can also be released in‐mouth due to oral microflora (and therefore perceived retronasally) [17, 28], and the phenomenon of glycosylation of exogenous volatiles by grape berries and release during fermentation and storage (or enzymatically) has since been extended to studies involving the deliberate application of oak extracts to grapevines in the vineyard [29–31].

Table 12.4 Average concentration (and standard deviation, SD) of volatile phenols in unoaked wines elaborated from grapes affected by bushfire smoke. Data from Reference [17]

Compound Average concentration (SD) (μg/L)
Control wines
(N = 3)a		 Pinot Noir
(N = 9)		 Cabernet Sauvignon
(N = 2)		 Shiraz
(N = 3)
Guaiacol 5.0 (1.7) 18.2 (14.7) 23.5 (10.6) 32.7 (4.9)
4‐Methylguaiacol 2.3 (3.2) 3.8 (2.7) 5.0 (0) 4.7 (4.0)
4‐Vinylguaiacol NDb 4.9 (4.6) 2.0 (2.8) 5.7 (3.1)
Syringol 9.7 (5.5) 16.9 (6.0) 19.5 (4.9) 19.3 (4.0)
4‐Methylsyringol 2.0 (2.0) 5.1 (2.3) 7.5 (3.5) 6.0 (5.2)
4‐Allylsyringol 8.3 (9.1) 10.7 (5.6) 4.0 (2.8) 8.7 (7.4)
Phenol 2.7 (3.1) 24.3 (15.9) 34.5 (7.8) 28.7 (13.2)
o‐Cresol 2.7 (3.1) 10.1 (6.4) 7.5 (2.1) 5.0 (1.7)
m‐Cresol 2.0 (1.7) 7.1 (2.9) 7.5 (0.7) 2.3 (0.6)
p‐Cresol 0.7 (1.2) 5.0 (0.9) 3.5 (0.7) 2.0 (1.0)

a N, number of wines; the control wines were Pinot Noir (N = 2) and Cabernet Sauvignon (N = 1).

b ND, not detected.

12.4.3 Fermentation – metabolism of hydroxycinnamic acids

Volatile phenols can also arise from yeast activity acting on grape‐derived components, presenting a significant challenge to winemakers. Metabolism of HCAs in wine (Chapter 13) gives rise to “Brett” off‐flavor compounds such as 4‐ethylphenol (4‐EP) and 4‐ethylguaiacol (4‐EG, Table 12.1) from their 4‐vinyl analogs [32], which contrasts with the small amounts formed during lignin pyrolysis. The route from precursor acids involves enzymatic decarboxylation (by Saccharomyces or Brettanomyces) of HCAs to give 4‐vinylphenol (4‐VP) and 4‐vinylguaiacol (4‐VG, Table 12.1) from p‐coumaric and ferulic acids, respectively, and subsequent reduction of the vinyl group by Brettanomyces to form 4‐EP/4‐EG (Chapter 23.3). Although dependent on grape variety and wine style, the relative concentrations of Brett spoilage compounds tend to reflect the concentrations of the precursor acids (Chapter ). Typically, this leads to an approximately 8:1 ratio of 4‐EP to 4‐EG [33]. These compounds demonstrate additive effects and are major contributors to “Brett” aromas, although predicting sensory effects can be complicated by the presence of other volatile phenols, or due to masking, for example, by isovaleric and isobutyric acids [34]. Table 12.3 shows representative concentrations of Brett‐related volatile phenols in different wines, with the higher levels of ethyl analogs in particular being an indication of Brettanomyces activity during barrel aging. Wines are particularly susceptible to Brett spoilage during this time (and also while undergoing MLF) due to the low levels of SO2, slow ingress of O2 and potential for poor sanitation, especially of older barrels.

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Notes