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A Technical Guide for Wine Producers
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RECENT ARTICLES | WYNBOER HOME
Modulation of Sauvignon blanc aromas through yeast strain, nutrition and seasonal variation
Paul Bowyer1, Charlotte Gourraud2, Marie-Laure Murat3 and Tertius van der Westhuizen1
1 LAFFORT Australia, 5 Williams Circuit, Pooraka SA 5095, paul.bowyer@laffort.com
Introduction
Recently
we have focused much research on the impacts that yeast strain and
yeast nutritional status have on the production of both varietal and
non-varietal (i
e fermentation-derived) aroma production in Sauvignon Blanc (Van der
Westhuizen et
al.,
2008; Swiegers et
al.,
2008; Bowyer et
al.,
2008). Saccharomyces
cerevisiae
winemaking yeast is the key factor in enabling must to express its
aromatic potential (Murat et
al.,
2001, Swiegers et
al.,
2005; Dubourdieu et
al.,
2006). Some specific pathways are responsible for releasing aromatic
compounds from their odourless precursors in grapes. Volatile thiols
are a good example of this phenomenon. Indeed,
4-mercapto-4-methylpentan-2-one (4MMP) and 3-mercaptohaxan-1-ol (3MH)
are released from their odourless precursors by yeast during
alcoholic fermentation (AF), while mercaptohexyl acetate (3MHA) is
derived from 3MH.
Volatile
thiols are extremely odoriferous molecules, which give particular
wines significant fruity aromas, even at very low concentrations
(Dubourdieu et
al.,
2006). The volatile thiols (4MMP, 3MH, and 3MHA) were initially
identified in Sauvignon Blanc wines and are still mainly associated
with this grape variety, yet they also contribute to the aromas of
wines made from grape varieties such as Pinot Gris, Riesling, and
Gewürztraminer, Colombard, Chenin Blanc, Rolle, Petit Manseng,
and Gros Manseng (Tominaga et
al.,
2000). It is also responsible for the fruity aromas of rosé
wines made from Merlot, Cabernet, Syrah, and Grenache (Murat, 2001;
2005; Ferreira et
al.,
2002).
Winemakers
are well aware that poor management of yeast nutrition has a negative
impact on wine fermentation kinetics. One new concept of yeast
management consists of adding nutrients to Active Dry Yeasts (ADY)
during the rehydration phase (Dumeau et
al.,
2004; Van der Westhuizen, 2006), viz.
Dynastart®.
This new generation of yeast rehydration nutrients brings
considerable improvement to membrane structure and fluidity, thus
enhancing yeast viability. Dynastart®,
used yeast during rehydration, includes growth factors (vitamins and
minerals, but no nitrogen) and survival factors (sterols and fatty
acids).
While
its effect on fermentation kinetics is clearly recognized, there has
been very little scientific research into the impact of Dynastart®
on the release of volatile thiols. Initial results published by
Swiegers et
al.
(2008) revealed that it had a significant effect on both volatile
thiol release and fermentation ester production. Aside from chemical
measurement of aromatic chemicals, ultimately it is a combination of
winemaker and consumer preference that dictates the success of any
given wine. Moreover, the aromatic composition of wines is known to
evolve noticeably over time, with new characters being produced and
others decreasing in intensity (Ribéreau-Gayon et
al.,
2006), and to vary from season to season. The questions remained:
Are
winemakers able to perceive the differences in these wines based on
quantifiable aromatic differences, justifying the use of a
rehydration nutrient?
Is a winemaker’s
concept of “Sauvignon blanc aromatic typicity”
consistent with analytical data, and between regions and countries?
To
what extent does seasonal variation impact upon the aromatic
composition of Sauvignon blanc?
Materials
and methods
Winemaking
Fermentations
were conducted in Australia, in an independent research centre
(Provisor), using Sauvignon Blanc juice (2007, Adelaide Hills
provided by Yaldara Estate) with the following characteristics: Sugar
218 g/L, potential alcohol 12.9% v/v, pH 3.19, turbidity < 50 NTU.
Duplicate fermentations were conducted in 500 L stainless steel tanks
at a constant temperature of 14.5 ºC +/- 0.5 (58°F), which
was maintained until the end of fermentation (to eliminate the
variance in aromatic production observed with temperature
fluctuation). Yeast was added at 20 g/hL (200 ppm). Fermentation
kinetics were monitored by measuring density. Development of the
yeast strains inoculated in both tanks was confirmed by genetic
analysis (i e DNA authentication), half-way through fermentation.
The volatile thiols and fermentation esters were assayed by the SARCO
laboratory (Bordeaux) after post-AF sulphating. All winemaking
parameters were identical in all samples, except for the yeast
strain(s) used:
Zymaflore
X5®,
a strain isolated from breeding, selected for its fermentation
performance, as well as its marked capacity to release varietal
volatile thiols and produce fermentation esters;
Zymaflore
X5®,
rehydrated with 30 g/hL (300 ppm) Dynastart®,
a specific yeast rehydration nutrient;
Zymaflore
VL3®,
a strain selected from the ‘terroir’ for its capacity to
reveal volatile thiol aromas;
Strain A, a commercial
strain widely used to produce Sauvignon Blanc wines;
Mix 1, inoculated with a
blend of two commercial strains used to produce Sauvignon Blanc
wines (50/50), rehydrated separately;
VL3/X5®,
inoculated with a blend of Zymaflore
X5®
and VL3®
yeast strains (50/50), rehydrated separately.
Bottling
Screwcap
closure; pH 3.26 ± 0.02; TA 6.7 ± 0.3 g/L tartaric acid
equivalents; FSO2
28 ± 2 ppm; TSO2
142 ± 20 ppm).
Tastings
Tastings
of all experimental wines were conducted in the following wine
regions: McLaren Vale, Barossa Valley, Clare Valley, Langhorne Creek,
and the Limestone Coast (Australia), Marlborough and Hawkes Bay (New
Zealand) and South Africa (winemakers from various regions). All
wines were masked. The
tasters were all winemakers who were directed to concentrate on wine
aromas, then instructed to: (1) Rank the wines in order of decreasing
Sauvignon Blanc typicity (ie 1 = most typical Sauvignon Blanc, 6 =
least typical Sauvignon Blanc); (2) Indicate their preferred wine
from a masked pair in terms of aromatic typicity and intensity,
corresponding to those made from Zymaflore X5 and Zymaflore X5
rehydrated with Dynastart®.
Preference data were compiled thusly: the rank of each wine was
totalled, hence a lower score indicates higher preference.
Results and discussion
Fermentation kinetics
The
individual yeast strains and the two yeast mixtures showed very
similar fermentation rates.
In this trial, fermentation did not seem to be affected by the
coexistence of two different strains. Zymaflore VL3 is known to be a
strain more suited to fermenting at higher temperatures (16 - 18°C),
hence its slower fermentation rate under these conditions. In
contrast, rehydration of the LAFFORT hybrid strain Zymaflore X5 with
Dynastart®
resulted in a much faster fermentation rate (figure 1), illustrating
the significant impact of the complex nutrient on the overall
viability of the yeast cells and general metabolic activity.
Chemical analyses do not show significant differences between the
experiments (data not shown).
Figure
1. Chart
indicating the fermentation kinetics displayed by the yeast strains
examined. Note the marked improvement in fermentation rate when
Dynastart®
was used, with a fermentation duration some 40% less under the
conditions of the trial.
Initial aromatic
measurements
Analysis
of the final wines’ aromatic composition confirmed previous
observations (data not shown) that Sauvignon blanc grapes from the
Adelaide Hills do not contain high levels of the 4MMP precursor.
Significantly, only Zymaflore X5 was able to express this character
in the final wines (figure 2).
Both Mix 1 and the Zymaflore X5/VL3 mixture showed very similar
results in terms of varietal thiol content (3MH, 3MHA in particular)
in the final wines. The expression of the varietal volatile thiols
in the dual strain fermentations was greater than some single
commercial strains, but did not exceed (within experimental error)
the expression level of Zymaflore X5. Significantly, rehydration
with Dynastart®
resulted in a doubling of the release of 4MMP and 3MHA, and a
generous increase in the release of 3MH.
Measurement
of the fermentation ester concentrations illustrated additional
differences between the wines (figure 3). Strain A can be seen as a
high isoamyl acetate (IA,
banana) producer which, in the absence of high levels of the volatile
thiols, was a notable feature of the aromatic profile of this wine.
The mixed-strain fermentations did not show significant differences
in terms of fermentation ester production compared with each other or
with Zymaflore X5, nor with commercial Strain A (except for the noted
higher IA production by Strain A). In this experiment only yeast
Mixture 1 allowed the production of 18% more hexyl acetate (HA, pear)
than the Zymaflore X5/VL3 mixture or Strain A. In order to obtain a
significant increase of ester compounds, rehydration of the yeast
with Dynastart®
gave the best overall results, with gains of between 25 to 45% under
the conditions of this trial.
Figure
2.
Results
of the different yeast strains on the release of the varietal
volatile thiols 4MMP (broom,
box tree)
and 3MH (grapefruit)
and on the conversion of 3MH to 3MHA (passion
fruit),
expressed using the aroma index [C]/PT (Concentration / Perception
Threshold) in June 2007. Note that 3MHA is plotted at 1/5th actual
intensity for reasons of visual clarity.
Figure
3.
Results
of the different yeast strains on the concentration of fermentation
esters expressed using the aroma index [C]/PT (Concentration /
Perception Threshold) in June 2007. (IA) Isoamyl acetate (banana);
(PEA) Phenyl ethyl acetate (tea);
(PE) Phenyl ethanol (rose);
(HA) Hexyl acetate (pear).
Note the difference in y-axis scales between varietal (figure 2) and
non-varietal aromas. Note also that HA is plotted at 1/2 actual
intensity for reasons of visual clarity.
Aromatic longevity
In
order to examine the impact of bottle ageing on the aromatic
composition of Sauvignon Blanc, the X5 + Dynastart®
wine from the 2007 trial was re-analysed for volatile thiol and
fermentation ester content in January 2008, after cellaring at 15 °C.
The data are presented in figure 4.
Figure
4.
The
complete aromatic profiles of the X5 + Dynastart®
wine at two time intervals 7 months apart (cellared at 15°C).
Note that 3MHA is plotted at 1/5th actual intensity for reasons of
visual clarity. Note also the greater significance of the varietal
volatile thiols to the aromatic profile of the wine compared with the
fermentation esters. Aroma intensity expressed as
Concentration/Perception Threshold, where a value of ≥1 indicates
contribution to wine aroma. 4MMP = broom / box tree; 3MH =
grapefruit); 3MHA = passion fruit; IA = banana; PEA = tea; PE = rose
petal; HA = pear.
Clearly
there is a difference in the relative stabilities of the three
varietal aroma marker chemicals over time. 4MMP and 3MHA are
particularly unstable, whilst 3MH is artificially bolstered by the
fact that 3MHA yields 3MH upon decomposition. Thus, within a 7-month
time span the boxwood/broom (4MMP) and passion
fruit (3MHA) aromas had dropped dramatically in both wines. In terms
of fermentation ester stability, phenyl ethanol (PE, rose-petal) and
isoamyl acetate (IA, banana) are relatively stable, whilst phenyl
ethanol acetate (PEA, tea) and hexyl acetate (HA, pear) showed the
most instability.
The
significance of these aromatic observations is clear: to produce
wines of maximal aromatic intensity Dynastart®
gives
significant advantage. If varietal typicity is a priority, the only
strain capable of generating all three Sauvignon Blanc varietal thiol
characters in this trial was Zymaflore X5. In order that these wines
retain the most aromatic intensity for the longest shelf-life,
initial aromatic intensity must be boosted as much as possible, hence
Dynastart®
should be used.
Winemaker tasting
preferences
Australia: South
Australia
The
results of tastings by winemakers from South Australia of the
experimental wines are presented in figure 5.
Figure
5.
Tasting
preferences for South Australian winemakers (n=73). Columns indicate
the sums of preferences, hence a lower score indicates higher
preference.
In
the regions of McLaren Vale, the Barossa Valley, the Clare Valley,
Langhorne Creek and the Limestone Coast wine typicity and preference
was split roughly into three groups. The most favoured wine was that
made with Zymaflore X5 + Dynastart®.
Strain A and Mix 1 were favoured next, whilst X5, VL3 and the X5/VL3
mixture were placed equally third.
Correlating
these data with the quantified aromatic wine profiles (figures 2 and
3) indicates important differences between the preferred wines. The
X5 + Dynastart®
wine
is the highest in varietal aromas of 4MMP, 3MH and 3MHA (the latter
by a margin of some 100 %). Mix 1, aromatically, does not appear to
be especially distinctive except that it lacks the varietal thiol
4MMP. The aromatic profile of Strain A, interestingly, differs from
X5 + Dynastart®
and
Mix 1 in several respects: it produced no 4MMP, contains the lowest
concentration of 3MH and has the second lowest concentration of 3MHA.
Conversely, Strain A produced the highest level by some margin of
the non-varietal ester isoamyl acetate (IA, banana), a character
which was a clearly distinguishing feature for this wine.
Examining
these results leads to a simple conclusion: winemakers preferred
these wines for different aromatic characteristics: Those favouring
the wine made with Zymaflore X5 + Dynastart®
clearly
show preference for the more complex varietal Sauvignon blanc
characters 4MMP, 3MH and 3MHA, whilst those favouring the wine made
from Strain A show preference for the non-varietal, uni-dimensional
character isoamyl acetate (banana aroma).
New Zealand:
Marlborough and Hawke’s Bay
The results of tastings
by winemakers from New Zealand (Marlborough and Hawkes Bay) are
presented in figure 6.
Figure
6.
A
comparison of the tasting preferences for winemakers in New Zealand
(n=59). Columns indicate the sums of preferences, hence a lower
score indicates higher preference.
New
Zealand winemakers, in a country considered by many to typify the
production of “new world” Sauvignon Blanc, indicated that
the highest Sauvignon typicity was displayed by the wine made from X5
+ Dynastart®.
Following this was a group of 4 wines: X5, Strain A, Mix 1 and
X5/VL3. Third preference was given to the wine made with VL3.
These
results correlate well with measured aromatic Sauvignon typicity,
excepting the preference for Mix 1, which contains no 4MMP, similar
levels of 3MH and 3MHA as the wines made from X5 and the mixture of
X5/VL3, and levels of fermentation esters no higher than the other
wines in the trial. Given that the winemakers were instructed to
focus on Sauvignon Blanc aromatic typicity, Mix 1’s high rank
in this region is without obvious aromatic
analytical correlation.
South Africa
Sauvignon
Blanc typicity data for South Africa (figure 7) do not reflect those
of any Australian wine region, nor those of New Zealand. The South
African winemakers involved indicated the greatest Sauvignon typicity
to be found in the wines made from Mix 1 and VL3. This indicates a
good level of aromatic analysis consistency, given that the
concentrations of all measured aromatic components of the wines made
from VL3 and Mix 1 are very similar, excepting only 3MHA and hexyl
acetate (HA, pear). Least typicity was indicated for the wines made
from X5 and Strain A, with X5 + Dynastart®
and
the mixture of X5/VL3 in between. Clearly the South African
winemakers involved did not strongly equate Sauvignon Blanc aromatic
typicity directly with the measured highest levels of the varietal
aroma compounds 4MMP, 3MH and 3MHA. Since the sample size of each of
the South African regions was very small, further studies of this
type are required to gain an idea of the preferred Sauvignon Blanc
characteristics.
Figure
7. The
tasting preferences for winemakers in South Africa (n=11). Columns
indicate the sums of preferences, hence a lower score indicates
higher preference.
Combined preference
When
all regions are combined the overall responses are indicated in
figure 8. Highest Sauvignon blanc typicity overall was found in the
wine made using Zymaflore X5 rehydrated with Dynastart®.
Roughly equal second preference was given to the wines made from
Strain A and Mix 1, with approximately 3rd equal preference given to
those made using Zymaflore X5 and the mixture of Zymaflore X5/VL3.
Figure
8.
The
combined preferences of all winemakers (n=143) surveyed, from
Australia, New Zealand and South Africa. Columns indicate the sums
of preferences, hence a lower score indicates higher preference.
The
relatively low ranking of Zymaflore VL3 is interesting given the
strong commercial position that Zymaflore VL3 commands globally. The
marginally elevated volatile acidity (VA) in this wine compared with
the others (data not shown) possibly adversely affected the
preference ranking in this trial. The higher VA production of
Zymaflore VL3 in this trial was a direct result of the extended low
temperature of the fermentation, which is not recommended for this
strain. Although VL3 produces comparable levels of 3MH and 3MHA to
other strains, and excels in the production of the floral PE aroma,
it did
not produce high levels of the fermentation esters under the
conditions of this trial.
Perception
of the impact of Dynastart®
At
each tasting the participants were asked to indicate the wine showing
the greatest aromatic intensity and Sauvignon blanc typicity between
two masked wines, corresponding to those made from Zymaflore X5 and
Zymaflore X5 rehydrated with Dynastart®.
When all regions were combined, 4 out of 5 winemakers indicated that
the wine made using Dynastart®
was
superior in aromatic and varietal terms.
In
order to elucidate the exact manner in which Dynastart®
is
able to provide enhanced aromatic expression in the yeast, LAFFORT
has sponsored a doctoral research candidate at the Australian Wine
Research Institute.
Seasonal comparison
In
order to facilitate a direct seasonal aromatic comparison, the same
vineyard and winemaking techniques (except ferment temperatures of
14.5 °C in 2007 and 16.0 °C in 2008) were employed in a trial
in 2008. Aromatic data from vintages 2007 and 2008 for the wine made
using Zymaflore X5 + Dynastart®
are illustrated in figure 9.
Figure
9. A
comparison of the aromatic profiles of wines made from the same
vineyard in 2007 and 2008 using X5 + Dynastart®.
Note that 3MH and 3MHA are plotted at 1/10th and 1/45th actual
intensity for reasons of visual clarity. Aroma intensity expressed
as Concentration/Perception Threshold, where a value of ≥1
indicates contribution to wine aroma. 4MMP = broom / box tree; 3MH =
grapefruit); 3MHA = passion fruit; IA = banana; PEA = tea; PE = rose
petal; HA = pear.
Interestingly, 4MMP
levels were almost identical for both years. Massive differences
were observed, however, for 3MH and 3MHA: in the 2008 wine 3MH
increased 890 % over 2007, whilst 3MHA increased 750 % over 2007.
Increases in aroma intensity were also observed for IA (250 %) and PE
(rose, 680 %), whilst decreases were observed for PEA (tea, 60 %) and
HA (pear, 60 %).
Clearly
seasonal impact on wine aromas is not to be underestimated when
conducting yeast and/or nutrient trials
(Lee, et
al.,
2008). Winemakers must therefore exercise caution when undertaking
trials of this type to ensure that observed effects are correctly
correlated with genuine cause, and not merely the result of an
exceptional growing season. To illustrate the importance of this
point, the sums of the aroma intensities analysed for the 2007 and
2008 wines were 88 and 572 respectively, meaning that for the aromas
analysed the 2008 wine was found to be 6.5 times more aromatically
intense overall than the 2007 wine.
Acknowledgements
The authors would like to
thank Chris Day of Provisor (Adelaide, Australia) for his
involvement. We would also like to thank James Evers and Peter Ruchs
from Yaldara Wine Estate (Australia) for their support in this and
ongoing projects and for providing the Sauvignon Blanc juice.
Dr
Paul Bowyer is the Technical Manager for LAFFORT in the Australasian
region. He can be contacted by email,
paul.bowyer@laffort.com,
www.laffort.com.
References
Bowyer,
P., Gourraud, C., Van
der Westhuizen, T. & Murat, M.L. (2008). Yeast strain and
nutritional modulation of aroma intensity, longevity and winemaker
preference in Sauvignon blanc. The
Australian and New Zealand Grapegrower and Winemaker,
Annual Technical Issue, 47 - 58.
Dubourdieu,
D., Tominaga, T., Masneuf, I., Peyrot Des Gachons, C. & Murat,
M.L. (2006). The role of yeast in grape flavour development during
fermentation: the example of Sauvignon Blanc. Am.
J. Enol. Vitic.
57, 81 - 88.
Dumeau,
F., Mansour, C. & Masneuf-Pomarède, I. (2004). Le point
sur les activateurs de fermentation. Revue
des œnologues, No
113, p. 21 - 23.
Ferreira,
V., Ortín, N., Escudero, A., López, R. & Cacho, J.
(2002). Chemical Characterization of the Aroma of Grenache Rosé
Wines: Aroma Extract Dilution Analysis, Quantitative Determination
and Sensory Reconstitution Studies. J.
Agric. Food Chem.,
50
(14), 4048 - 4054.
Lee,
S.A., Rick, F.E., Clark, H., Dobson, J., Thomson, M., Reeves, M. &
Gardner (2008). Grape juice is the major influence on volatile thiol
aromas in Sauvignon Blanc. The
Australian and New Zealand Grapegrower and Winemaker,
Annual Technical Issue, 78 - 86.
Murat,
M.L. (2001). Recherches sur la vinification des vins rosés et
clairets de Bordeaux. Diplôme
d’Etudes et de Recherches de l’Université de
Bordeaux II Victor Segalen.
Murat,
ML. (2005). Acquisitions récentes sur l’arôme des
vins rosés, Partie I: Caractérisation de l’arôme,
Etude du potentiel aromatique des raisins et des moûts. Revue
des œnologues,
No 117.
Murat,
M.L., Masneuf, I., Darriet, P., Lavigne, V., Tominaga, T. &
Dubourdieu, D. (2001). Effect of the Saccharomyces
cerevisiae
yeast strains on the liberation of volatile thiols in Sauvignon Blanc
wine. Am.
J. Enol. Vitic.
52, 2: 136 - 139.
Ribéreau-Gayon,
P., Glories, Y., Maujean, A. & Dubourdieu, D. (2006). Handbook
of Enology Volume 2 (second edition), Wiley: Chichester, 60.
Swiegers,
J.H., Bartowksy, E.J., Henschke, P.A. & Pretorius, I.S. (2005).
Yeast and bacterial modulation of wine aroma and flavour. Australian
Journal of Grape and Wine Research,
11: 127 - 138.
Swiegers,
J.H., Ugliano, M., Van der Westhuizen, T. & Bowyer, P. (2008).
Impact of yeast rehydration nutrient on the aroma of Sauvignon Blanc
wine. The
Australian and New Zealand Grapegrower and Winemaker,
January,
66 - 69.
Tominaga,
T., Baltenweck-Guyot, R., Peyrot des Gachons, C. & Dubourdieu, D.
(2000). Contribution of Volatile Thiols to the Aromas of White Wines
made from Several Vitis
vinifera Grape
Varieties. Am.
J. Enol. Vitic.
Vol. 51, No 2, 178 - 181.
Van
der Westhuizen, T (2006). An overview of the requirements for
ensuring optimal yeast performance and a new development in yeast
preparation technology. The
Australian and New Zealand Grapegrower and Winemaker,
Annual
Technical Issue,
55 - 63.
Van
der Westhuizen, T., Bowyer, P. & Gourraud, C. (2008). Impact of
yeast rehydration nutrients and yeast strain choice on the aroma of
Sauvignon Blanc wine. The
Australian and New Zealand Grapegrower and Winemaker,
March issue, 48 - 52.
Summary
There are no absolute
answers when it comes to winemaker interpretation of varietal
characteristics and preference. Nevertheless, some clear conclusions
can be drawn from this investigation:
A winemaker’s
interpretation of what constitutes “Sauvignon blanc varietal
aromatic typicity” does not necessarily correlate with the
known varietal Sauvignon blanc aromatic compounds.
Winemakers in the warmer
regions tend to favour yeast strains producing fermentation esters
in Sauvignon Blanc, whilst those in the cooler regions tend to
favour strains expressing varietal aromas.
Decreases in some wine
aromatic characters can be expected over time. To counter this,
strains with high production of these compounds should be used.
The
use of Dynastart®
can
dramatically increase aromatic intensity in terms of both varietal
and non-varietal aromas, particularly for the varietal volatile
thiol 3MHA (passion fruit), leading to significantly higher wine
preference.
The
only yeast strain that was able to express all 3 varietal characters
in Sauvignon Blanc (4MMP, 3MH and 3MHA) in this trial was Zymaflore
X5.
Seasonal variations can
have a major impact upon wine aromatic intensity and composition. |
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