Brettanomyces Character in Wine
©Richard Gawel
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Introduction
The desirability or otherwise of the wine character known as "Brett" is one of
the most controversial issues of recent times. Arguments have been made for
Brett character being a complexing and a legitimate expression of natural,
uncomplicated winemaking, while others view it simply as an unattractive wine
fault that results from poor winery hygiene and sloppy winemaking.
Figure 1: Brettanomyces bruxellensis forming pseudomycelium
© 2004 High Power Ultrasonics Pty Ltd
The Aroma and Flavour of
Brett Character
But what is Brett character and how and why does it appear in some wines? The
wine character described as "Bretty" comes in various forms. It is the combined
result of the creation of a number of compounds by the yeast Brettanomyces
bruxellensis, and its close relative, Dekkera bruxulensis. The three
most important known aroma active compounds are 1) 4-ethyl phenol (4-ep), which
has been variously described as having the aromas of Band-aids®, antiseptic and
horse stable 2) 4-ethyl guaiacol (4-eg) which has a rather pleasant aroma of
smoked bacon, spice or cloves and 3) isovaleric acid which has an unpleasant
smell of sweaty animals, cheese and rancidity. Other characters associated with
Brett include wet dog, creosote, burnt beans, rotting vegetation, plastic and
(but not exclusively caused by Brett) mouse cage aroma and vinegar.
The Formation of Brett
Character in Wine
Brettanomyces has been isolated from the outside of grapes and from
winery equipment. However its, favoured winery haunt is the oak barrel as it
often provides for conditions that strongly favours its growth.
Certain conditions are known to favour the growth of Brettanomyces during
winemaking. If low free sulfur dioxide levels are coupled with high wine pH and
warm temperatures during barrel maturation, then issues may arise. If older oak
is used and the wine has a reasonable amount of dissolved oxygen, …. look out!
Furthermore it is thought that Brett can also multiply after bottling if the
wine contains residual fermentable sugars, a situation made more likely if the
wine was minimally filtered. Lets look at the why's of these factors.
Brettanomyces proliferates under warm cellaring conditions. Twenty
degrees C is an ideal temperature, with even small reductions in temperature
seriously hamper its growth. Sulfur dioxide is an anti-microbial agent that is
added by winemakers throughout the winemaking process. If it is added in
sufficient amounts, and the pH of the wine is reasonably high (SO2 is more
effective at higher acidity levels), then the growth of Brett will be retarded.
On the other hand, high alcohol levels and the existence of even small amounts
of fermentable sugars such as glucose suit the growth of Brett, as they are its
preferred source of energy for growth. Some recent research under laboratory
conditions suggest that Brett does not grow at alcohol levels above 13%.
However, this result is not consistent with the observation that many wines with
alcohols far in excess of this have gone bretty under winery conditions.
Filtering the wine before bottling can reduce the numbers of Brett cells, and
hence the incidence of Brett character that develops in the bottle. However,
there is anecdotal evidence that filtered wines that are sound at the time of
bottling can randomly become infected with Brettanomyces after a period
of time, probably as a result of the bottled wine containing residual sugar and
being stored in warm conditions.
It is widely acknowledged that the majority of wines with Brett character,
became that way during the period of barrel maturation, particularly if second
use (or older) oak barrels were used. Brett can colonise a barrel between fills,
and can begin to reproduce when the barrel is refilled with new wine. Figure 1
shows Brett extending pseudomycilium into the surface of an oak stave. Topping
up barrels with a wine which contains Brett cells, may also contribute to those
barrels 'going Bretty'. Shaving and re-toasting the inside of re-used barrels
significantly reduces the incidence of Brett growth. However, it is also worth
noting that the use of new barrels does not guarantee that Brett will not
appear. Recent work in California has shown that new barrels filled with
sterilised wine can still sustain populations of Brett high enough to produce
above threshold levels of 4-ep.
But why does oak maturation particularly favour Brett growth? Firstly, Brett is
a slow growing yeast that does not compete well against other micro-organisms.
During alcoholic fermentation the wine yeast Saccharomyces out easily
out-competes it. Two possible reasons are that it naturally grows slower than
Saccharomyces, and that it prefers aerobic conditions for growth. During primary
ferment, the wine is saturated with carbon dioxide which makes for a hostile
environment for Brettanomyces. On the other hand, barrel maturation is a
step in conventional winemaking that provides both the time and the lack of
competition needed for Brett to successfully grow to levels which results in
sensory modification to the wine. Wines stored in barrel are usually lower in
SO2 and are kept warmer than at any other time (other than during ferment of
course). This is necessary so as to encourage malolactic fermentation (MLF).
Lastly, the necessary processes of racking off lees and regularly topping up
barrels ensures that there are always reasonable levels of dissolved oxygen in
the wine. For all these reasons, it is thought that the time between the
completion of primary fermentation and the start of MLF this is the most likely
time that Brett multiplies and produces brettiness in wine.
Brettanomyces Character is Seen Primarily in Red Wine.
Why?
One final matter concerning Brett is rarely mentioned. It occurs almost
exclusively in red wines. Why is this so? Red wines have a much higher level of
tannin like substances called coumaric and ferulic acid than do white wines as
they are extracted from the skins of grapes during red wine fermentation. The
wine yeast Saccharomyces and some lactic acid bacteria such as
Lactobacillis have enzymes which degrade these acids to weakly smelling
intermediates called 4-vinyl phenol and 4-vinyl guaiacol (Step 1 of Figure 2).
These compounds are then enzymatically degraded over a period of months by
Brettanomyces to the strong smelling 4-ethyl phenol and 4-ethyl guaiacol
respectively (Step 2 of Figure 2). Incidentally Brettanomyces is the only
major micro-organism in wine that has the ability to transform 4-vinyl-phenol
into the potent band-aid® smelling, 4-ethyl phenol. Hence 4-ethyl phenol is
rightly considered to be the "trademark" aroma of Brettanomyces growth in
wine. Where you find 4-ethyl phenol you will invariably find Brett, and vice
versa.
Surveys of Australian wines have shown that detectable levels of 4-ethyl phenol
is more likely to be seen in darker coloured wines, with Shiraz and Cabernet
wines than wines made from either Pinot noir and Grenache. The reason for this
is unclear, but may involve the coumarates which are a form of coloured
anthocyanins found in red wines.
Figure 2: Pathway to the formation of 4-ethyl phenol and 4-ethyl guaiacol in
wine
The Prevalence of Brett Character
Has Brett character become apparently more prevalent in recent years? Some
commentators believe that we have simply become more aware of it and that it has
always been around. I am sure that there is some truth in this. Upon personal
reflection, I feel that classic Hunter Shiraz with its 'sweaty saddle' aroma and
flavour is a very likely case in point. However, in my opinion, the
overpowering, fruit destroying, antiseptic like aromas and flavours that are now
occasionally encountered in wines sourced from every winemaking region of
Australia is a relatively new phenomenon. The trend in this country today is to
produce red wines picked from riper grapes. In addition to maximizing flavour
development in some varieties, this also results in wines that are on average
higher in pH and alcohol. Furthermore, residual sweetness is being retained in
some commercial red wines in an attempt to fill out the palate and to give it
greater apparent fruitiness. These trends together with the use of minimal SO2
and filtration, has enhanced the conditions under which Brett is retained and
thrives.
The Desirability or Otherwise of Brett Character in Wine
But is the action of Brett desirable? In my humble opinion, the answer depends
on degree. As well as producing a band-aid aroma, Brett can create an array of
'interesting' smells that can excite those that are inclined to be excited by
them. Furthermore, the ratio of the rather unattractive 4-ethyl phenol to the
rather pleasant smelling 4-ethyl guaiacol varies substantially from wine to
wine, with reports varying from 3:1 to over 40:1. In the latter case, it is
highly likely that the wine would smell like the inside of a band-aid box, while
in the former, the aroma would in all likelihood be far more spicy and savoury
like. The reason for these differences between wines are not completely
understood but are likely to be either due to differing ratios between wines
coumaric and ferulic acids (the respective precursors of 4-ep and 4-eg), or to
different strains of Brettanomyces being more effective in producing one
compound relative to the other. Very recent research with five different strains
of Brettanomyces has not lent much support to the latter possibility. Under
laboratory conditions the different strains produced roughly equal proportions
of 4-ep to 4-eg in the same red wine. But the search for strains of Brett which
may be low 4-ep producers will no doubt continue.
In some wine growing regions such as Bordeaux, the Rhone and, dare I say it, the
Hunter Valley, it is now acknowledged that some wine producers have developed
'house styles' over time that have actually been defined by some form of Brett
character. Many of these producers, or the media, or both, have naively
attributed these unusual and sometimes complexing characters to being 'an
expression of the soil'. However, overwhelming scientific evidence in the form
of elevated 4-ethyl phenol levels in their wines have forced them to admit to
the less romantic notions of the microbiological origin of these characters.
This is not to say that they necessarily will, or indeed should, do anything
different in the future, as many Bretty house styles have become widely accepted
and in some cases revered by the wine tasting public. But in the cases where a
wine smells more of a hospital ward than it does wine, surely the wine-maker
should begin to reflect on what wine drinkers seriously value. That is, real
fruit and real complexity. Unfortunately some winemakers (possibly in an attempt
to save their career), have attributed the accidental making of overtly Bretty
wines as a serious attempt at making something different and complex. Wine
diversity is a wonderful thing and should be encouraged in the face of continued
'internationalisation' of wines. But as Pascal Chattonet once argued.
Brettyiness has nothing to do with a wines 'typicity' as claimed by some French
wine producers. His counterclaim is that wines that are overly Bretty do indeed
smell and taste much the same, so overt Brettyness mitigates against 'typicity'
and diversity. I'm in Pascal's camp. Real 'typicity' and 'expression' indeed
come from the fruit. A message that I hope is not lost on the winemaking
fraternity.
This work was
presented at the Australian Society of Wine Education National Convention.
Hunter Valley, Australia. 4th-6th of June 2004.
http://www.aswe.org.au/