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Volatile phenols from Brettanomyces
in barrelled red wines:
Are they stable? And where do
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Brettanomyces is a yeast that can impact the aroma of wines
through production of 4-ethylphenol (4-EP) and 4-ethylguaiacol
(4-EG). These compounds can give undesirable off-aromas at
concentrations above their odor thresholds (~500 and ~50μg/L,
respectively; μg/L=parts-per-billion), but can also have favorable
influences on wine bouquet at lower levels. Managing the growth
of this microorganism and its metabolites has become an important
enological research and application priority. The literature often
speaks of 4-EP and 4-EG as if they were stable, ‘terminal’
compounds that – once formed – do not undergo any future physicochemical transformations despite changes that the wine experiences
during aging. These processes may alter the total concentrations
of 4-EP and 4-EG over time, in contrast to a steady-state level that
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64 The Australian & New Zealand Grapegrower & Winemaker
persists in the barrel/bottle after a Brettanomyces infection. Here
we discuss the current state-of-the-art regarding the stability and
partitioning behavior of these compounds in wine. We hope that
our update will stimulate research, and highlight the complexity of
understanding the impacts Brettanomyces has on wines.
Enzymatic and abiotic degradation of 4-EP and 4-EG
Whether 4-EP and 4-EG can be enzymatically or abiotically
degraded in wine is one of the most poorly understood aspects of
Brettanomyces research. For example, Reeve et al. (1990) reported
that Pseudomonas putida JDI oxidized 4-EP to 1-(4-hydroxyphenyl)ethanol via the enzyme 4-ethylphenol methylenehydroxylase
(4EPMH). The pathway proceeds via by 4EPMH catalyzed
dehydrogenation to a quinone methide intermediate, hydration of
the side chain to yield 1-(4-hydroxyphenyl)-ethanol, followed by a
second 4EPMH catalysed dehydrogenation of the alcohol to give 4hydroxyacetophenone.
If p-cresol methylhydroxylase (PCMH), a similar enzyme to
4EPMH that also contains flavoprotein and cytochrome c subunits,
is used on 4-ethylphenol, both 4-hydroxyacetophenone and 4vinylphenol are produced (McIntire et al., 1999). Other enzymes
are also known to oxidatively transform 4-EP and related volatile
phenols (e.g., vanillyl-alcohol oxidase; van den Heuvel et al., 1998).
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Potential degradation products of 4-EP and 4-EG can also contribute
important aromas to wines. For example, 4-hydroxyacetophenone has
a sweet, flora odour, 4-vinylphenol has an almond shell aroma, and
4-vinylguaiacol and 4-acetovanillone (produced from 4-EG) have
clove/curry and vanilla odours, respectively (http://www.flavornet.
org/flavornet.html). These compounds could be contributors to the
complex Brett aroma that has been difficult to chemically describe.
Ketones, aldehydes, and vinyl compounds can also be integrated
into the tannin framework during aging (Fulcrand et al., 2006),
representing another ‘loss’ pathway for primary, secondary, and
tertiary Brett metabolites that further complicates how the Brett
aroma evolves over time.
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We lack survey data on what enzymes persist in wines during
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The Australian & New Zealand Grapegrower & Winemaker 65
4-ethylguaiacol concentration (µg/L)
4-ethylphenol concentration (µg/L)
Fig.1. Time series of 4-EP (left set of panels) and 4-EG (right set of panels) concentrations in Merlot (a), Cabernet Sauvignon (b), Syrah/Shiraz (c), and Pinot
Noir (d) wines during oak barrel aging.
and practitioners to better link known enzyme capabilities (often
determined in non-wine matrices) with their possible presence in
wines. Additionally, winemakers often add enzymes to their wines
for a variety of reasons. We need to understand whether any of
these commercial formulations can degrade 4-EP and 4-EG, or
(as is discussed below) whether they may release bound forms of
these two analytes. Along this line of reasoning, anecdotal reports
of Brettanomyces aromas developing after prolonged bottle storage
may not be due to the growth of this micro-organism per se. Rather,
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the changes in enzymatic activities brought about by chemical or
other microbial alterations in the wine during aging may release
‘stored’ 4-EP and 4-EG.
Little is known about the abiotic transformations of 4-EP and 4EG in wines. Reduction-oxidation cycles remain active in aging red
wines through the interactions of polyphenols (e.g., anthocyanins,
tannins, etc.), small amounts of oxygen, and the presence of
trace metal concentrations (e.g., iron, copper, manganese, etc.)
(Oszmianski et al., 1996; Waterhouse and Laurie, 2006; Danilewicz,
2007). These cycles are thought to produce hydrogen peroxide, a
powerful oxidant that should be able to oxidise 4-EP and 4-EG to the
corresponding 4-hydroxyacetophenone and 4-acetovanillone under
In the January issue of this journal, we reported (Rayne and
Eggers, 2007) that an industry-sponsored survey of 4-EP and 4-EG
in wines from the Okanagan Valley (Canada) conducted in midsummer 2006 found a barrel of Pinot Noir with 3900μg/L 4-EP and
1300 μg/L 4-EG. Our continuing monitoring of this barrel showed
that these high levels from mid-July 2006 subsequently decreased to
<1 to5 μg/L for both analytes by early September 2006. The 4-EP
and 4-EG in a 4L sample of the wine remained at these lower levels
throughout the autumn and early winter of 2006 until the wine was
discarded. A preliminary analysis of the wine in late 2006 suggested
that 4-hydroxyacetophenone and 4-acetovanillone could account for
nearly 50% of the mass balance of 4-EP and 4-EG, respectively, that
had been degraded.
In addition, 13 barrels of wine at a nearby winery monitored
from June 2006 through January 2007 exhibited relatively consistent
time-series profiles of 4-EP and 4-EG (Fig. 1). Concentrations of the
two analytes were generally low in early summer (June), increased
substantially by mid/late summer and declined throughout the
autumn and early winter. In several cases, 4-EP and 4-EG levels
after the summer peak declined to concentrations at or below
what was in the wine before any infection. Reliable monitoring for
Brettanomyces using 4-EP and 4-EG as indicators thus requires the
timing of sampling to be taken into account. Samples collected on
either side of the ‘Brett peak’ may suggest no problems, giving a
What these results suggest is that we may need to devote research
efforts into the potential post-Brettanomyces degradation of 4-EP
and 4-EG in wines. Our observed autumnal declines in 4-EP and
4-EG, after the late summer production, may also be because of
slow partitioning into the oak wood and lees, as is discussed in the
Partitioning of 4-EP and 4-EG in the Barrel
Key towards a more comprehensive understanding of how 4-EP
and 4-EG behave in the barrel or bottle depends on their partitioning
behaviours. This area is not well enough understood for these two
important wine aroma compounds. At present, we can make the
following physico-chemical generalisations about 4-EP and 4-EG:
At typical wine pH values (3 to 4) the hydroxyl groups of 4-EP and
4-EG are not appreciably dissociated (pKa~10; Varhanickova et
al., 1995), which reduces the hydrophilic nature of the compounds
compared to their ionic (dissociated) states. Thus, 4-EP and 4-EG
show a strong preference towards association with organic matter,
whether it be in solution (e.g., colloidal and dissolved tannins) or
a surface (e.g., the inside of an oak barrel). This insight based on
partitioning factors in both polar protic (e.g., n-octanol) and nonpolar aprotic (e.g., hexane and cyclohexane) solvents, and onto
activated carbon (a surrogate for toasted oak), that are greater
than one (Hansch and Leo, 1979; Blum et al., 1994; Piraprez et
Typical 4-EP and 4-EG levels in wines (generally <1 mg/L)
are several orders of magnitude below their solubility limits in
model alcoholic solutions (~4000 to 7000 mg/L; Barrera-Garcia
et al., 2006). Thus, 4-EP will never precipitate as a pure solid
(crystallize) from a wine (in contrast to tartaric acid, which can
be precipitated by cold treatment) – it will always be either in the
dissolved phase or adsorbed to organic material.
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The Australian & New Zealand Grapegrower & Winemaker 67
In terms of actual experimental data with wines, Chatonnet et al.
(1997) have stated that the presence of tannins in red wine had no
influence on the synthesis of 4-EP, although their data does support a
reduction in 4-EP levels when 1g/L of either catechin or procyanidins
was added to the solution during culturing of Brettanomyces in 10
mg/L p-coumaric acid. These results may support the hypothesis
that tannins are capable of complexing 4-EP and 4-EG in red wine,
leading to a significant portion of the analyte being in an associated
form (perhaps >25% if the values of Chatonnet et al. (1997) are
Both 4-EP and 4-EG can non-covalently interact with the
flavylium nucleus – part of a process termed copigmentation. Dufour
and Sauvaitre (2000) have determined association constants (Ka)
between anthocyanins and a variety of aroma substances in wine,
including 4-EG (Ka=6.8 M-1 in a model wine solution). For vanillin
(Ka=52 M-1), complexation rates of up to 7% total analyte were
calculated, with a general negative relationship between ethanol
content and the percent bound. With an approximately 8-fold lower
association constant for 4-EG compared to vanillin, one would expect
a substantially lower associated fraction (say, ~1-2% in wine). These
values are lower than can potentially be inferred from the empirical
observations of Chatonnet et al. (1997), but substantial inter-annual,
varietal, and enological effects could exist. Thus, a greater dataset on
the complexation of aroma compounds with tannins is needed before
we can better understand the role of inter-molecular interactions in
wines and the effects on the wine bouquet.
Barrera-Garcia et al. (2006) has recently reported detailed data
on the sorption of volatile phenols at the oak wood/wine interface.
After 30 days of contact, wood-model wine partition coefficients
(using Quercus robur (pedunculata) French oak) of 19.4 at 10°C and
27.3 at 25°C for 4-EP, and 13.5 at 10°C and 16.4 at 25°C for 4-EG
were observed. The sorption process for the two analytes was found
to be biphasic, with (1) a fast initial sorption onto the active sites
of the wood surface within the first day, and (2) a slower diffusion
process between the second and the eighth days where migration
of the analytes into the wood matrix can be observed. Sorption
plateaus of about 1000 mg of analyte per kg of wood were found,
well above the amounts of 4-EP and 4-EG that would be present in
a barrel, suggesting that for new wood, saturation of binding sites in
the barrel wood is not a limiting factor.
These results raise questions regarding how the age of a barrel,
and the cleaning procedures used, can impact potential 4-EP and 4EG concentrations in the barreled wine. Assuming the same barrel
is used for sequential batches of wine, and that each batch of wine
has equal quantities of 4-EP and 4-EG at the start of barrel aging,
nutrient status, and Brettanomyces levels, and each batch produces
an equivalent total mass of these analytes within the wine, then we
might find subsequent batches to contain higher levels of 4-EP and
4-EG in the wine. This is because the active sites on the oak surface
may be still occupied with 4-EP and 4-EG, or other compounds
having a high oak wood sorptive potential, from previous batches
An unknown is the ability of a ‘between-batch’ barrel cleaning
protocols to effectively regenerate active sites for 4-EP and 4-EG
sorption on a barrel’s internal surface. Shaving and re-toasting may
regenerate the active sites, perhaps producing a ‘fresh’ oak surface
with a sorptive potential near that of a new barrel. Further work is
required, determining oak wood-wine partition coefficients for 4EP and 4-EG under sequential exposures of the same wood to the
same starting solutions – and with industry standard cleaning or
re-toasting procedures between determinations. At this point, the
current literature strongly suggests that a significant portion of the
total 4-EP and 4-EG ‘loads’ in a barrel may be sorbed to the inside
surface. The implications of this line of investigation for barrel
management in commercial wineries dealing with Brettanomyces
The amount and type of lees in the barrel, which can be
controlling by racking, will also influence the quantity of 4-EP and
4-EG in solution through sorptive removal processes. Chassagne et
al. (2005) examined the removal of these two compounds by wineproducing Saccharomyces biomass from a model hydroalcoholic
solution and a wine (both containing 1000μg/L 4-EP and 500μg/L
4-EG). Under the experimental conditions, the analytes were
primarily removed from solution by surface binding on the yeast
cells, giving partition coefficients ranging between 19 and 910
for 4-EP depending on yeast type. Pinot Noir yeast lees had the
highest partition coefficient (Kwy=910) for 4-EP, while active dried
yeast (Kwy=19), synthetic medium fermented yeast (Kwy=33), and
Chardonnay yeast (Kwy=45) lees had lower values. Similarly, for
4-EG, the lowest partition coefficients were observed for active
dried yeast (Kwy=27), synthetic medium fermented yeast (Kwy=27),
and Chardonnay yeast (Kwy=51) lees, with Pinot Noir yeast lees
having the highest value (Kwy=87). The high affinity of Pinot
Noir yeast lees for 4-EP warrants further investigation, possibly as
an intentional sorbent for Brett contaminated wines. Chassagne et
al. (2005) also found higher ethanol levels in the wine increased
the analyte solubilities, and thereby reduced the driving force for
sorptive partitioning onto the lees.
Additional studies on the quantitative relationship between
ethanol levels and sorptive scavenging of 4-EP and 4-EG in wines
would be valuable to help the industry better understand what its
options are in this regard. As well, confirmative studies are needed
to show where the majority of 4-EP and 4-EG mass is residing in
a barrel. Studies that quantify 4-EP and 4-EG levels in various
quadrants of the free wine column, in the lees, and as a function of
depth in the oak wood across a range of varieties, oak types, barrel
ages, and other factors would be particularly valuable to extend the
current theoretical framework.
Free and Bound Forms of 4-EP and 4-EG in Wines – A New
The relative concentrations of free and bound forms of 4-EP
and 4-EG, and how these forms potentially ‘interchange’ in a wine
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during barrel and bottle aging, has not been addressed. In general,
the free forms of wine aroma compounds are sensory active, while
the bound forms are not (Abbott et al., 1991; Clarke and Bakker,
2004). At this point in the research evolution of Brettanomyces and
4-EP/4-EG, we can only speculate on the potential importance of
The flavanoids – which are classified into two major groups,
anthocyanins and anthoxanthins – are phenolic compounds that
occur primarily as glycosides in plants (Markham, 1989). Red
wines are a particularly rich source of the flavanoids. The majority
of even the simple flavonols (a subset of the anthoxanthins) such
as quercitin and myricetin are conjugated in ‘bound’ sugar forms
in the wine at up to >95% of the total compound present, although
typical free flavonol content is about 20% to 50% of total levels
(McDonald et al., 1998). However, the flavonoid glycosides are
formed as secondary metabolites in the grapes (Harborne, 1994);
and thus, it can questioned whether sugar conjugation is relevant
for Brettanomyces metabolites that appear to be primarily formed
in the barrel.
But this is poorly understood, and we need to determine whether
there may be substantial quantities of glycosidically (or other) bound
4-EP and 4-EG entering the wine via the grapes, and what the fate
of these bound forms is during fermentation and aging. Could this
source – with the bound forms liberated by slow acid hydrolysis
at wine pH values or enzymatic hydrolysis due to the complex
metabolic changes that occur in the wine during barrel and bottle
aging – be a potentially significant cause of otherwise attributed
Brett contamination? Could Brettanomyces growth be inducing
enzymatic changes in the wine that releases the 4-EP or 4-EG
glycosides, in addition to any direct production of these analytes as
Intracellular b-glucosidase activity by Brettanomyces has been
reported (Mansfield et al., 2002), and this action could potentially
help liberate any glycosidically bound forms of 4-EP and 4-EG in
wines. Other microbes (e.g., lactic acid bacteria such as Oenococcus
oeni) present in the wine could also be sources of free 4-EP and
4-EG liberating enzymes through their extracellular glycosidases.
Furthermore, since poor correlations have been noted between
Brettanomyces colony forming units and 4-EP levels, Fugelsang
and Zoecklein (2003) speculated that this organoleptic metabolite
may be released upon yeast cell death and autolysis. Alternatively,
perhaps the cell death and autolysis also liberates enzymatic activity,
which then converts bound 4-EP and 4-EG to the sensory active free
forms in the bulk wine?
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68 The Australian & New Zealand Grapegrower & Winemaker
Post-Brettanomyces transformations of 4-EP and 4-EG in model
and real wine solutions, by abiotic and enzymatic routes, require
more attention. Because of subsequent changes in wine chemistry
during barrel and bottle aging, bound/sorbed forms of 4-EP and 4EG may also be released to aromatically active free forms, or vice
versa. Little is known in this regard, and at present, we cannot say
whether there will be viticultural and enological differences that
complicate any generalisations.
To complicate matters, recent analytical work by Pollnitz et
al. (2004) has shown us that the manner in which we extract and
analyze our aroma compounds from wine determines what we see.
In other words, the analytical method may degrade precursors into
target analytes, or free the bound forms. As these authors note,
“[g]enerally little or no consideration is given to the possibility that
extraction methods could increase the risk of artifact formation
during analyses.” In some cases, artifacts led to the target free
analytes (guaiacol, 4-methylguaiacol, cis-oak lactone, and vanillin)
being overestimated by more than an order of magnitude. If these
related analytes can be freed during analysis from their bound forms,
it implies we need to look at our analytical methods for 4-EP and 4EG in more detail, and to also consider ways of treating our samples
(e.g., acid/base hydrolysis, exposure to -glucosidase, etc.) to get a
better handle on what the total concentrations of these compounds
The discussions above may help us explain why some Brettanomyces
infected wines produce high, and apparently permanent, levels of
these compounds, whereas other infected wines apparently end up
with insignificant levels. Through targeted research, we may better
understand the complexity of the Brett aroma, and be able to control
it by directing the wine aging conditions to achieve the desired end
Fuselgang, K.C. and Zoecklin, B.W. (2003) Population dynamics and effects of
Brettanomyces bruxellensis strains on Pinot noir (Vitis vinifera L.) wines. American
Journal of Enology and Viticulture, 54, 294-300.
McDonald, M.S., Hughes, M., Burns, J., Lean, M.E.J., Matthews, D., and Crozier,
A. (1998) Survey of the free and conjugated myricetin and quercetin content
of red wines of different geographical origins. Journal of Agricultural and Food
Chemistry, 46, 368-375.
Thanks to the British Columbia Wine Grape Council (BCWGC),
the Investment Agriculture Foundation of British Columbia (IAFBC),
and the Western Diversification Program (WDP) for funding of
this research, and the support of wineries who generously donated
expertise and samples.
Abbott, N.A., Coombe, B.G. and Williams, P.J. (1991) The contribution of
hydrolyzed flavor precursors to quality differences in Shiraz juice and wines: An
investigation by sensory descriptive analysis. American Journal of Enology and
Viticulture, 42, 167-174
Barrera-Garcia, V.D., Gougeon, R.D., Voilley, A. and Chassagne, D. (2006)
Sorption behavior of volatile phenols at the oak wood/wine interface in a model
system. Journal of Agricultural and Food Chemistry, 54, 3982-3989.
Blum, D.J.W., Suffet, I.H. and Duguet, J.P. (1994) Quantitative structure-activity
relationship using molecular connectivity for the activated carbon adsorption of
organic chemicals in water. Water Research, 28, 687-699.
Hansch, C. and Leo, A.J. (1979) Substituent constants for correlation analysis in
chemistry and biology. Wiley: New York, USA.
Harborne, J.B. (1994) The flavonoids: Advances in research since 1986.
Chapman and Hall: London, UK.
Mansfield, A.K., Zoecklein, B.W. and Whiton, R.S. (2002) Quantification of
glycosidase activity in selected strains of Brettanomyces bruxellensis and
Oenococcus oeni. American Journal of Enology and Viticulture, 53, 303-307.
Markham, K.R. (1989) Flavones, flavonols and their glucosides. Methods in Plant
Biochemistry, 1, 197-235.
McIntire, W.S., Everhart, E.T., Craig, J.C. and Kuusk, V. (1999) A new procedure
for deconvolution of inter-/intramolecular intrinsic primary and -secondary
deuterium isotope effects from enzyme steady-state kinetic data. Journal of the
American Chemical Society, 121, 5865-5880.
Oszmianski, J., Cheynier, V. and Moutounet, M. (1996) Iron-catalyzed oxidation
of (+)-catechin in model systems. Journal of Agricultural and Food Chemistry,
Piraprez, G, Herent, M.F. and Collin, S (1998) Determination of the lipophilicity of
aroma compounds by RP-HPLC. Flavour and Fragrance Journal, 13, 400-408.
Pollnitz, A.P., Pardon, K.H., Sykes, M. and Sefton, M.A. (2004) The effects of
sample preparation and gas chromatograph injection techniques on the accuracy
of measuring guaiacol, 4-methylguaiacol and other volatile oak compounds in
oak extracts by stable isotope dilution analyses. Journal of Agricultural and Food
Chemistry, 52, 3244-3252.
Chassagne, D., Guilloux-Benatier, M., Alexandre, H. and Voilley, A. (2005)
Sorption of wine volatile phenols by yeast lees. Food Chemistry, 91, 39-44.
Rayne, S. and Eggers, N.J. (2007) 4-Ethylphenol and 4-ethylguaiacol in wines
from the Okanagan Valley, and the Brettanomyces/Dekkera connection. The
Australian and New Zealand Grapegrower and Winemaker, 516, 52-59.
Chatonnet, P., Viala, C. and Dubourdieu, D. (1997) Influence of polyphenolic
components of red wines on the microbial synthesis of volatile phenols. American
Journal of Enology and Viticulture, 48, 443-448.
Reeve, C.D., Carver, M.A. and Hopper, D.J. (1990) Stereochemical aspects
of the oxidation of 4-ethylphenol by the bacterial enzyme 4-ethylphenol
methylenehydroxylase. Biochemical Journal, 269, 815-819.
Clarke, R.J. and Bakker, J. (2004) Wine flavour chemistry. Blackwell Publishing:
van den Heuvel, R.H.H., Fraaije, M.W., Laane, C. and van Berkel, W.J.H.
(1998) Regio- and stereospecific conversion of 4-alkylphenols by the covalent
flavoprotein vanillyl-alcohol oxidase. Journal of Bacteriology, 180, 5646-5651.
Danilewicz, J.C. (2007) Interaction of sulfur dioxide, polyphenols, and oxygen in a
wine-model system: Central role of iron and copper. American Journal of Enology
and Viticulture, 58, 53-60.
Dufour, C. and Sauvaitre, I. (2000) Interactions between anthocyanins and aroma
substances in a model system: Effect on the flavor of grape-derived beverages.
Journal of Agricultural and Food Chemistry, 48, 1784-1788.
Fulcrand, H., Duenas, M., Salas, E. and Cheynier, V. (2006) Phenolic reactions
during winemaking and aging. American Journal of Enology and Viticulture, 57,
Varhanickova, D, Shiu, W.Y. and Mackay, D (1995) Aqueous solubilities of
alkylphenols and methoxyphenols at 25°C. Journal of Chemical and Engineering
Data, 40, 448-451
Waterhouse, A.L. and Laurie, V.F. (2006) Oxidation of wine phenolics: A critical
evaluation and hypotheses. American Journal of Enology and Viticulture, 57,
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