RESEARCH ON 2-METHOXY-3-ISOBUTYLPYRAZINE IN GRAPES
Dominique ROUJOU DE BOUBEE
(School of Oenology, University of Bordeaux II)
Pyrazines (1,4-diazines) are nitrogen-containing heterocyclic compounds that are quite widely
distributed in nature in both the animal and plant kingdoms. The food industry is the area in
which these compounds have been the most extensively studied. They are considered to be
the heterocyclic compounds most widely represented in food aromas (Vernin and Vernin,
1982). They can be classified into three groups depending on their origins: those formed by
heat treatment, those formed by micro-organisms and those present in the natural state in
plants. Amongst the methoxypyrazines in the last category, the most important ones are 2-
methoxy-3-isopropylpyrazine (IPMP), 2-methoxy-3-sec-butylpyrazine (s-BMP) and 2-
methoxy-3-isobutylpyrazine (IBMP). IBMP was identified for the first time in green pepper
(Capsicum annuum var. grossum) by Buttery et al. (1969). Its detection threshold in water is
estimated to be 2 ng/L, and those authors consider it to be responsible for the characteristic
aroma of green peppers. It was subsequently identified in several raw vegetables such as chili
peppers (Capsicum frutescens), beans (Phaseolus vulgaris), broad beans (Vicia faba), lettuce
(Lactuca sativa), spinach (Spinacea oleracea), etc. (Murray and Whitfield, 1975). Those
authors note that one of the other methoxypyrazines is often dominant in some species. This is
the case for IPMP in asparagus (Asparagus officinalis), peas (Pisum sativum), cucumber
(Cucumis sativus), lettuce, potatoes (Solanum tuberosum) or even sow-thistle (Sonchus
oleraceus) and for s-BMP in beets and carrots (Daucus carota sativa).
In 1975, IBMP was identified for the first time in Cabernet Sauvignon grapes (Vitis
vinifera L. cv. Cabernet Sauvignon) by Bayonove and Cordonnier, who claimed that it was
responsible for the green pepper aroma that is characteristic of this variety. In 1982, Augustyn
et al. identified IBMP in Sauvignon blanc grapes (Vitis vinifera L. cv. Sauvignon blanc). Over
the last 10 to 15 years, its contribution to the vegetal and green pepper aromas of Cabernet
Sauvignon, Merlot and Sauvignon blanc wines has been demonstrated (Harris et al., 1987;
Maga, 1989; Allen et al., 1989; Allen et al., 1991; Allen et al., 1994; Kotseridis et al., 1998;
Roujou de Boubée et al., 2000). These authors also show that s-BMP is rarely detected in
wines, whereas IPMP is found at levels below its detection threshold in water (2 ng/L). As
such, 2-methoxy-3-isobutylpyrazine would seem to be the key compound involved in the
green pepper aroma of Cabernet Sauvignon, Sauvignon blanc and some Merlot wines. With
these varietals, the IBMP concentration significantly exceeds the detection threshold.
This compound is present in grapes, it has no known precursor and its concentration
decreases during ripening (Allen et al., 1989; Lacey et al., 1991) under the influence of light
(Heymann, 1986; Allen and Lacey, 1993; Hashizume and Samuta, 1999). Consequently, a
high concentration in grapes at harvest is associated with a lack of ripeness and has a
negative impact on wine aroma quality. Winemakers and oenologists commonly associate
such aroma characteristics in grapes with low anthocyanin content and with mediocre “tannin
It is therefore crucial to know what conditions influence IBMP concentration in grapes
and wine. In this paper, we present the results of our research on the methoxypyrazine aroma
of wines and on the conditions under which IBMP forms or breaks down in the grapevine.
1. The Methoxypyrazine Character of Wines and Its Evolution during Winemaking
1.1. The Organoleptic Impact of 2-Methoxy-3-Isobutylpyrazine in Wines and Its
Distribution in Wines of Different Varieties
The quantification method that we developed (Roujou de Boubée et al., 2000) is based on
that developed by Harris et al. in 1987 and then used by Allen et al. in 1994. By improving
the ease and rapidity of extract preparation, we have been able to perform analyses with good
repeatability on relatively large series of samples.
In oenology, the value of interest is the threshold beyond which the green pepper aroma
caused by IBMP becomes perceptible in wine. However, the determination of a detection
threshold in wine does not yield an indicative value, owing to the great variations in
composition from one wine to another and in descriptors used from one taster to another. As
such, we sought to establish not a threshold value for a given wine, but rather a representative
threshold for a given wine type, in our case a red Bordeaux-type wine. To do this, 50 red
wines from Bordeaux and the Loire Valley (Cabernet Sauvignon, Cabernet franc and Merlot
varieties from several different vintages for the Bordeaux wines; Cabernet franc from the
1991 and 1992 vintages for the Loire Valley) were evaluated by a panel of 10 persons at the
Bordeaux School of Oenology. The wines were noted on a scale of 0 to 5 for the intensity of
their green pepper aroma. The median of the scores for each wine was calculated and then
correlated with the IBMP content determined by gas chromatography coupled with mass
spectrometry (GC/MS). The relationship is linear between the median scores and the IBMP
contents (r_ = 0.739, p>0.01%) (Figure 1). Wines with no green pepper aroma (median=0)
contain 10 ng/L of IBMP on average. Those with only a slight green pepper aroma
(median=1) have an average content of 15 ng/L, whereas beyond this level, the perception of
the green pepper aroma is medium to strong.
Figure 1: Correlation between the median of tasting scores obtained with 50 red wines from
Bordeaux and the Loire Valley and the concentration of 2-methoxy-3-isobutylpyrazine as
determined by GC/MS.
We can therefore estimate that the detection threshold for the green pepper aroma, also known
as the methoxypyrazine aroma, is 15 ng/L of IBMP in red Bordeaux and Loire Valley wines.
In other words, this is the concentration at which the tasters identify a vegetal aroma in those
We then performed a statistical study on 96 red and white wines made from the Cabernet
Sauvignon, Cabernet franc, Merlot and Sauvignon blanc varieties in order to determine those
y = 0.1533x - 1.3106
R2 = 0.7389
0 5 10 15 20 25 30 35 40
Median of tasting scores
in which IBMP plays a role in the vegetal aroma. This study shows that IBMP is the main
contributor to the vegetal aroma in Cabernet Sauvignon, Cabernet franc and Sauvignon blanc
wines. However, this compound is perceptible in only a minority of Merlot wines.
Once the methoxypyrazine character had been defined, its threshold determined and the grape
varieties identified, we focused on how the IBMP content changes during the winemaking
1.2. Extractability of 2-Methoxy-3-Isobutylpyrazine during Winemaking.
Observations on the Evolution of IBMP Content in Wines during Bottle Ageing.
Whole clusters of manually harvested Sauvignon blanc grapes were placed in a pneumatic
press (Bücher, 70 hL). The juice samples obtained by simple crushing of the berries during
press filling and at the beginning of the press cycle (0.2 bar; 30 min) had the highest IBMP
contents (Table 1).
Table 1: IBMP concentration (ng/L) of Sauvignon blanc musts sampled at different press
levels during the 1998 harvest.
Free run 0.2 bar (30 min)
Free run 0.2 bar (60 min)
0.8 bar (90 min)
1.4 bar (120 min)
2 bar (140 min)
2 bar (180 min)
The musts extracted subsequently contain less IBMP, and as the volume of liquid is lower,
these fractions contribute less to the final IBMP content. The final IBMP concentration differs
little from that of the first free-run juice obtained during press filling. IBMP is easily extracted
from grape clusters during crushing and at the beginning of pressing, at least in the case of
pneumatic pressing of whole clusters.
Table 2 shows the effect of settling on the IBMP content of musts.
Table 2 : Effect of settling on the IBMP concentration (ng/L) of Sauvignon blanc musts
After settling (200 NTU)
The clarified musts (200 NTU) contain about half as much methoxypyrazine as the unsettled
musts. A part of the IBMP seems to interact with the grape solids and is thus eliminated by
must clarification. It has previously been observed that settling limits the grassy aroma of
white musts by lowering the contents of C6 aldehydes and alcohols (Dubourdieu et al., 1986)
and of methionol (Lavigne, 1996). The effect of settling on IBMP content in musts has never
been reported before.
Under real conditions, the IBMP contents of Cabernet Sauvignon 24 hours after
tanking-down and at the end of maceration are not very different (Figure 2). Most of the
IBMP in the free-run wine is therefore extracted into the aqueous phase prior to alcohol
Figure 2: Evolution of the IBMP concentration during winemaking with Cabernet Sauvignon
grapes in 1997 (CS 97) and 1998 (CS1 98 and CS2 98).
To monitor the kinetics of IBMP extraction as precisely as possible with Cabernet
Sauvignon grapes, a micro-batch of wine was made in the laboratory (7 kg of grapes in 15-
litre stainless steel drums).
Figure 3: Evolution of the IBMP concentration during micro-batch winemaking from
Cabernet Sauvignon grapes in 1999 (CS 99).
The extraction of IBMP from the grapes into the must is even quicker in this case
(Figure 3). Within 24 hours, before the alcohol fermentation even begins, all of the IBMP
found in the wine after racking has already been extracted from the grapes. The IBMP content
is not increased by the successive punching-down operations performed during fermentation
nor by post-fermentation maceration. Finally, the final concentration of the wine after racking
does not seem to be influenced much by the frequency of pump-overs or by the skin contact
time. However, as Kotséridis et al. (1999) observed, the IBMP content of the press wine can
be greater than that of the free-run wine (Table 3). As such, IBMP certainly participates in the
1 2 3 4 5 7 8 9 14 15 16 17 18 23
Days after tanking down
CS 97 CS1 98 CS2 98
increased vegetal character of many press wines. A certain fraction of the IBMP associated
with the solid parts of the grapes can therefore be extracted during the mechanical operations
Table 3: Evolution of the IBMP concentration (ng/L) in Cabernet Sauvignon press wines
(1998) during the press cycle. Batches 1 and 2.
Start of pressing (0.2 bar)
Pressing after 1 hr. (0.8 bar)
Pressing after 2 hrs. (2 bar)
Quite often, it can be observed that the thermovinification of red grapes (heating of the
grapes to between 60 and 80°C for a short period to promote extraction of phenolic
compounds and to destroy oxidases) leads to a decrease in the vegetal character of some
wines. Of the 5 examples presented (Table 4), thermovinification systematically decreases the
IBMP concentration to below the 15 ng/L threshold, and as such the vegetal character is no
longer perceptible in those wines. This decreases varies from 29 to 67%, depending on the
Table 4: Influence of thermovinification on the IBMP content (ng/L) of 5 Cabernet Sauvignon
We thus sought to provide an analytical interpretation for an empirical observation. In
the laboratory, a Cabernet Sauvignon must doped with IBMP was heated by means of a water
bath to 60°C in a flask connected to a rotary vacuum evaporator in order to recover the
volatilised fraction. It was shown that all of the IBMP that disappeared from the must was
found in the evaporated fraction, which shows that this compound is volatilised during
heating (boiling point of IBMP = 50°C). Thermovinification can be of interest in cases where
the grapes have not reached optimal ripeness for different reasons (difficult weather
conditions, unfavourable soil/exposure, excessive yield, etc.) or where the grapes are mouldy.
This technique can lead to highly coloured, supple, fruity (ester-type) and less vegetal wines.
The evolution of IBMP content for one Cabernet Sauvignon wine and one Sauvignon
blanc wine during bottle ageing was also monitored. After three years of ageing in a dark
cellar, no significant change was recorded. The chemical stability of IBMP explains this
result. Thus, one should not count on time to diminish this olfactory defect in the bottle.
The aroma of Sauvignon blanc or Muscat wines is determined by the grape ripening
conditions and also, in large part, by the winemaking conditions (Peyrot des Gachons, 2000;
Günata, 1984). For the Bordeaux varieties, we show here that obtaining fruity wines with no
vegetal character depends mostly on the grape ripening conditions. This means that to get a
low IBMP concentration in the wines, it is imperative to understand what happens prior to the
harvest. What is the metabolism for IBMP in the grapevine? In what parts of the grapevine is
this compound found? How does its concentration change during the reproductive cycle and
what factors affect it?
2. Evolution and Location of 2-Methoxy-3-Isobutylpyrazine in Various Grapevine
Organs during the Reproductive Cycle
2.1. From Fruit Set to Veraison
The determination of IBMP content in grape berries at an early stage (prior to veraison)
enables us to observe a synthesis phase preceding the start of veraison (Figure 4), as
previously reported by Hashizume and Samuta (1999).
Figure 4 : Evolution of IBMP content in Cabernet Sauvignon grapes from berry touch to
Normally, the breakdown of IBMP is rapid initially and then it slows down as harvest
approaches. However, in 1999, we observed in this vineyard block that the IBMP content
increased in the berries between 31 July and 12 August (mid-veraison). This phenomenon,
which had never been observed previously, is related to specific weather conditions. Between
27 July and 10 August, approximately 180 mm of rain fell on the block. The soil water
reserves were thus restored, and the grapevines, which had been close to the end of their
growth cycle, began to grow again. These vineyard observations do not enable us to draw any
definitive conclusions. However, it would seem that IBMP synthesis is related to the
vegetative growth of the grapevine. This could explain why vigorous vines that stop growing
relatively late in the season produce grapes that generally have high IBMP levels. This result
should, however, be compared with those we obtained in the following experiment set up by
researchers at INRA Bordeaux (Tandonnet et al., 1996). A block of Cabernet Sauvignon vines
was subjected to three soil-water statuses: normal (vintage conditions), irrigated (the vines
received 4.6 mm of water/day from late June to late August) and dry (the soil was covered
with a tarp from late June to harvest time). The grapes were vinified in small batches, and the
IBMP concentrations were determined in the wines in 1994, 1995 and 1996 (Figure 5). We
can note that irrigation leads to a significant increase in IBMP concentration in the wines
12-Jul 17-Jul 22-Jul 27-Jul 01-Aug 06-Aug 11-Aug 16-Aug 21-Aug
(+79% in 1994 and +39% in 1996 with respect to the “normal” wines). In 1996, tarping-over
at soil level led to a 57% decrease in the wine IBMP levels with respect to the “normal”
wines. Perhaps by inducing high vine vigour, irrigation leads to greater IBMP synthesis.
Figure 5 : Effect of different soil water statuses on IBMP concentration for Cabernet
Sauvignon wines in 1994, 1995 and 1996 (normal: vintage conditions; irrigated: 4.6 mm of
water per day from late June until veraison in late August; dry: tarping-over at soil level under
the vines from late June until ripeness).
Regardless of the cause, IBMP synthesis seems to occur between fruit set and two to
three weeks prior to the onset of veraison. At this stage, the stems contain a large proportion
of IBMP (Figure 6). Inside the berry, IBMP is found mainly in the skin (72%) and also in the
seeds (23.8%). The pulp contains very little IBMP (4.2%).
At this stage, IBMP is found in the berries, but we cannot determine if it is synthesised
in situ. IBMP has in fact been identified in Cabernet Sauvignon leaves at the time of grape
harvest (Hashizume et al., 1997). This finding suggests that IBMP could be synthesised in the
leaves. We have therefore examined whether IBMP is present in the leaves upon berry touch.
During this analysis, the clusters were grouped as a function of their insertion point on the
shoot. On the primary shoot, we distinguished between leaves in the basal area (the first three
to four leaves from the base), the leaves in the intermediate zone and the leaves in the apical
zone where growth has not finished.
Figure 6: Location of IBMP in different components of Cabernet Sauvignon grape clusters
prior to veraison (4/08) in 1999.
Normal Irrigated Dry
Skins Pulp Seeds Stems
The leaves on the secondary shoots (or summer laterals) were also divided into groups.
Firstly, we show that the leaves contain IBMP at this stage (Figure 7). They are therefore
capable of synthesising this compound. Secondly, we can note that the basal leaves have a
very high IBMP content, i.e. much greater than in the other leaves or in the clusters.
Figure 7: IBMP content in clusters and leaves (l.) at different levels of Cabernet Sauvignon
Knowing that many products synthesised in the leaves are then transported to the berries, we
decided that it would be interesting to determine whether IBMP is transported from the leaves
to the clusters. To do this, we used fruit-bearing cuttings of Sauvignon blanc obtained in
accordance with the protocol described by Ollat et al. (1998a) and Gény et al. (1998). Twelve
fruit-bearing cuttings of Sauvignon blanc (4 cuttings _ 3 repetitions A, B and C), each with
one cluster, were selected between the small-pea stage and the beginning of veraison for their
homogeneity (physiological stage, cluster size and compactness). On each of the cuttings,
eight leaves were selected from all along the shoot. Each leaf was placed in a plastic container
and then treated with a solution of 2(2H3)methoxy-3-isobutylpyrazine (1 mg/L), which is the
deuteriated analogue of IBMP and is used as an internal standard for GC/MS. The deuteriated
analogue solution of IBMP was deposited on the leaf every morning and evening for three
days. On the fourth day, the clusters were harvested. For the leaves treated with the
deuteriated IBMP solution, the leaf blade was removed (to avoid any contamination risk).
Only the petiole was sampled. The leaves not treated with deuteriated IBMP (i.e. the young
leaves near the apex) as well as the apices were sampled and gathered together. Finally, the
clusters were picked.
The distribution of deuteriated IBMP in the shoot was then measured (Figure 8). Firstly,
we can note that the deuteriated IBMP is detected in the petioles, which shows that it
penetrated the leaf blade and that it was transported. The great quantity of deuteriated
methoxypyrazine found in the petioles for repetition C (80%) seems to indicate that the
compound deposited on the leaf had not yet fully migrated towards the other plant organs. For
the three repetitions, we can also observe a low level of redistribution to the growing parts of
the vine (i.e. young leaves and apex, which always contain less than 10%). This corroborates
the fact that at this stage, the cluster is the organ to which metabolites are preferentially
Figure 8: Distribution (in %) for three repetitions (A, B, C) of deuteriated IBMP treatment on
Sauvignon blanc shoots after deposition on the leaves and migration.
1 repetition = 4 cuttings.
Finally, we found deuteriated IBMP in the stems and then in the berries. We therefore
demonstrate that this compound is transported by the phloem from the leaves to the berries.
Knowing that leaves contain high IBMP levels during ripening and that this compound
can migrate from the leaves to the clusters, we can imagine that the leaves form IBMP
reserves that can supply the clusters. We can also postulate that the berries are also capable of
synthesising methoxypyrazine. Before veraison, synthesis seems to occur faster than
breakdown. Perhaps at this stage, the berries have not yet acquired the capacity to break down
IBMP (or only at very low levels). The IBMP content in the berries would thus be the result
of transport from the leaves and of synthesis in situ.
Unfortunately, no advances have been made in the field of IBMP biosynthesis in plants,
and even the origin of this compound remains unknown. Murray et al. (1970) put forth the
hypothesis of a condensation between glyoxal (which is also involved in the formation of
other methoxypyrazines, according to these authors) and leucine (after having accepted an
amide group). In line with this hypothesis, a study on the biosynthesis of 2-methoxy-3-
isobutylpyrazine was undertaken using cell cultures of Cabernet Sauvignon. For the first time,
we demonstrate that an undifferentiated callus of Cabernet Sauvignon is capable of
synthesising IBMP. Moreover, adding leucine, i.e. the supposed precursor of IBMP, to the
culture medium increases IBMP production by the cells. However, in one experiment, the
addition of stable isotopes (L-leucine-d10 and 15NH4Cl) did not lead to isotopic enrichment of
the IBMP produced by the cells.
From fruit set up through about two to three weeks before mid-veraison, IBMP is
synthesised and accumulates in the leaves and/or berries, perhaps from leucine. Thereafter,
the IBMP content drops up through harvest.
2.2. From Veraison to Ripeness
The shape of IBMP content curves during ripening does not change as a function of
whether this content is expressed as ng/L, ng/kg of fresh matter or pg/berry (Figure 9).
0% 20% 40% 60% 80% 100%
Apical zone Petiole Stems Berries
Figure 9: Evolution of IBMP content (expressed in ng/L, in ng/kg of fresh matter or in
pg/berry) in Cabernet Sauvignon grapes from berry touch till harvest (1999). Mid-veraison
occurred on 11 August.
As such, and in contrast with what occurs with tartaric acid, for example, the decrease in
IBMP content expressed in ng/L is independent of the dilution that occurs through the
increase in berry volume during this period. This compound is thus truly broken down during
ripening. Several studies have revealed a strong relationship between exposure of the cluster
to light and the decrease in IBMP concentration (Allen and Lacey, 1993; Noble et al., 1995;
Hashizume and Samuta, 1999). They thus confirm the first studies performed by Heymann
(1986) and then by Maga (1989), which showed that methoxypyrazines are broken down by
A photo-degradation study of IBMP in a solvent (methanol, 10% v/v) and in wine
(white and red) enabled us to confirm that it breaks down when it is exposed to normal
daytime sunlight. We then showed that the breakdown products (including 2-methoxy-3-
methylpyrazine, which has been identified as a reaction intermediate) are present in very low
quantities and do not seem to have an organoleptic impact. Until now, it was possible to
suppose that an IBMP breakdown product could be involved in the aroma of Cabernet
Sauvignon wines. While this variety has a strong green pepper aroma when unripe, this
vegetal character disappears when conditions permit it and only then can a great Cabernet
Sauvignon wine with fruity and toasty aromas be made. Our work seems to show that the
origin of these aromas is not directly related to IBMP breakdown.
12-Jul 22-Jul 01-Aug 11-Aug 21-Aug 31-Aug 10-Sep 20-Sep 30-Sep
ng/kg of FM
As we have seen, prior to veraison, IBMP begins to break down in the berries. However, the
distribution of this compound in the clusters remains the same throughout ripening (Figure
Figure 10: Distribution (in %) of IBMP in different components of Cabernet Sauvignon
clusters during ripening in 1999.
Regardless of the phenological stage, the pulp contains little IBMP and the stems contain a
lot. However, from veraison to harvest, the proportion of IBMP decreases in the stems and
increases in the skins. It also decreases slightly in the seeds during this period.
Upon harvest, IBMP is found mainly in the stems. As such, we can understand why the
green pepper character in a wine can be greatly influenced by destemming quality. Between
11 August and 23 September, the IBMP content increases most in the basal and intermediate
leaves (Figure 11). In fact, the IBMP concentration in the adult leaves evolves in the opposite
direction from that of the grapes. This result may appear paradoxical. Under identical
environmental conditions, IBMP accumulates in the leaves, whereas it is broken down in the
grapes during ripening. If IBMP is broken down by light, why is it that the basal leaves,
which receive as much if not more light than the clusters, display increasing levels of IBMP
during the ripening phase? Everything suggests that the metabolism of this compound is
different in the fruit and in the leaves.
0% 20% 40% 60% 80% 100%
Stems Skins Seeds Pulp
[IBMP] (ng/kg of FM)
Figure 11: IBMP in the clusters and leaves (l.) at different insertion levels on the shoot during
ripening of Cabernet Sauvignon (1999).
These results confirm the advantages of leaf removal from the grapevine to decrease the
vegetal/green pepper aromas of grapes. By exposing the grapes to greater sunlight, leaf
removal increases IBMP breakdown, and it might also decrease the IBMP content in the
berries by removing organs that could be a source of IBMP supply for the clusters.
2.3. Effect of Vineyard Practices and Conditions on IBMP Content in Grapes
First, we measured the cumulative effect of all summer pruning and thinning work on
the IBMP content of Cabernet Sauvignon and Merlot grapes. This work includes debudding
(between budbreak and bloom), removal of summer laterals in the cluster zones on the east
side of the vine row (at the end of fruit set), leaf thinning in the fruiting zone on the east side
(at berry touch) and cluster thinning to limit the yield to around 50 hL/ha (at the start of
Table 5: Influence of summer pruning and thinning (SPT) work on the composition of Merlot
(M) and Cabernet-Sauvignon (CS) wines, 1997.
(g/L H2SO4 )
TPI: Total Polyphenolic Index
Summer pruning and thinning has a direct effect on decreasing the IBMP content of
Cabernet Sauvignon and Merlot grapes, from veraison till harvest. The photolabile nature of
IBMP explains this result. Grapes from the control group and from the thinned group were
vinified on a large scale (311-hL tanks) and then analysed at the start of ageing (Table 5). The
differences are more marked for Cabernet Sauvignon. The wine made from the “test” vines
(i.e. the thinned vines) has more alcohol, a higher phenolic content (+40% anthocyanins) and
its IBMP content is much lower than that of the wine made from the “control” vines (-39%).
The “control” wine is clearly marked by the green pepper aroma of IBMP, whereas IBMP is
not perceptible in the “test” wine.
It is widely acknowledged that the grapevine reacts differently depending on the date on
which summer vine work is performed. An experiment was conducted in 1998 on Cabernet
Sauvignon and Sauvignon blanc vines in order to determine how the timing of summer vine
work (summer lateral removal and leaf thinning in the cluster zone on the east side) affects
the IBMP content of grapes at harvest. This experiment led to the following conclusions. It is
generally important to perform lateral removal and leaf thinning early, i.e. between fruit set
and berry touch. If this is done, the grapes have a higher sugar content at harvest, they are
smaller in size and they contain less IBMP (Table 6). Late leaf thinning, while it does increase
the sugar content in the grapes, does not lead to a sufficiently great decrease in IBMP content
(this content is 65% greater than for a vine on which the leaf thinning and lateral removal
were done early).
Table 6: Influence of the timing of summer pruning and thinning (difference with respect to a
control group) on the composition of Cabernet Sauvignon grapes upon harvest – 1998.
Summer lateral removal
Lateral removal at fruit set
Lateral removal and leaf
& leaf thinning at fruit set
& leaf thinning after veraison
thinning after veraison
There are no significant differences in grape composition at harvest if this work is performed
at fruit set or when the berries are the size of small peas. In both cases, the work improves
ripening. From a practical point of view, this offers greater flexibility in scheduling summer
vine work. There is a 15-20 day period in which one can perform this work without affecting
final grape quality. Once this period is over, the risk becomes greater of harvesting grapes
with marked green pepper aromas. These results were obtained in the case of low-trained
vines with high planting density (8,550 vines/ha) and also with high-trained vines with low
density (3,300 vines/ha). Analogous results were found with Sauvignon blanc.
However, these results obtained in 1998 were not confirmed in 1999, when the IBMP
contents in the grapes at harvest were close to zero, regardless of when the summer vine work
was performed. The 1999 vintage was an atypical year, with heavy rainfall during veraison,
which lengthened the veraison period and led to heterogeneity in berry ripeness. Thereafter,
the weather was variable, with storms being intermixed with sunny, hot periods. These
weather conditions led to unusual vine function. We can observe that IBMP breakdown was
slower in 1999 than in 1998. The main difference between these two vintages lies in the
IBMP content at mid-veraison: it was three times greater in 1998 for the same vineyard block.
In other words, for vintages like 1998, it is important to intervene early so that the IBMP in
the grapes breaks down as quickly as possible. However, in 1999, early leaf thinning
accelerated IBMP breakdown at the beginning, but at harvest no difference was observed
between the different lots (“test” and “control”), owing to the low levels of IBMP present
from the start.
This proves that it is not so much the weather conditions during the ripening period that
matter but rather those just prior to that period. It is probable that an early diagnosis (i.e. in
late July) of the grape IBMP content would help in determining whether or not lateral
removal or leaf thinning are needed in a vineyard block. While 1999 did not confirm the
observations made in 1998, lateral removal and leaf thinning are major vineyard techniques
enabling production of high-quality wine grapes. These practices are not widely used (since
they cannot be mechanised and are thus costly), but they can be advantageous if performed
early (at fruit set, the laterals are small and can be removed easily and quickly). Such
practices facilitate further summer vine work by eliminating some of the vegetation in the
fruiting zone; they improve the healthiness of the crop and lead to better aeration around the
clusters; they remove some of the organs requiring influx of nutrients and thus they promote
better redistribution of photosynthesis products for improved grape ripeness.
The application of an original protocol enabled us to define a upper limit for the IBMP
concentration beyond which the methoxypyrazine character is perceptible in red Bordeaux-
type wines. This level is on average 15 ng/L. We have been able to determine the contribution
of IBMP to the vegetal aroma of Bordeaux varieties. It plays a major role for Cabernet
Sauvignon, Cabernet franc and Sauvignon blanc and a minor one for Merlot.
We have also shown that the IBMP content of wine depends mainly on that of the
corresponding grapes and that it is only marginally affected by winemaking techniques. In the
case of traditional winemaking, IBMP is highly extractable, independently of pressing
conditions for white wines and of maceration time and the number of pump-overs for red
wines. Only the settling of Sauvignon blanc musts and a careful selection of press wines for
Cabernet Sauvignon can be used to limit the IBMP content of wines. In the case of
thermovinification, heating the grapes leads to a significant drop in IBMP concentration,
owing to volatilisation. We have observed no change in IBMP concentration in wines during
IBMP concentration increases in grapes from fruit set until about two to three weeks before
mid-veraison. This phenomenon seems to be influenced by the grapevine water status before
veraison. The use of Cabernet Sauvignon cell cultures has not enabled us to explain the
mechanisms of IBMP biosynthesis. Nevertheless, we have shown that the addition of leucine,
which is the supposed precursor of IBMP, leads to increased IBMP production. During this
period, IBMP is synthesised in the leaves, which is where it is mainly located. It is also in the
stems, skins and seeds (in order of decreasing importance). The pulp contains almost no
IBMP. We have revealed for the first time that IBMP is transported from the leaves to the
clusters during this stage.
The maximum IBMP content in the grape is reached before veraison. After this, the
compound begins to break down in the berries. The order of its distribution in the leaves and
clusters remains the same throughout the ripening period. This breakdown is the result of
IBMP’s sensitivity to light. However, none of the photo-degradation products, including 2-
methoxy-3-methylpyrazine, which we have identified here, appear to have any organoleptic
impact. Paradoxically, while it is broken down in the grapes, IBMP continues to accumulate
in the leaves. The metabolism of methoxypyrazine does not seem to be the same in leaves as
The IBMP content of grapes at harvest can be controlled by summer vine work. Operations
such as debudding, summer lateral removal, leaf thinning and cluster thinning lead to a large
decrease in IBMP content in Merlot and Cabernet Sauvignon grapes during ripening. It is
generally important to perform lateral removal and leaf thinning early, i.e. between fruit set
and berry touch. After that, the risks are greater of harvesting grapes with marked green
pepper aromas. The comparison of the results obtained in 1998 and 1999 enables us to
postulate that the key to the methoxypyrazine character of ripe grapes is related to the weather
conditions that prevail at an early stage and which lead to a certain initial IBMP content in the
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