Screening brettanomyces
Methodology for the Screening and Isolation of Brettanomyces
in the Grape: plot-scale application
Results
Brettanomyces on the grape: presence confirmed
Results of initial studies conducted by Oenodev :
Our study was initiated, supported and conducted in collaboration with the Oenodev Company, which, for several years now, has led a huge, international campaign to conduct both local and laboratory investigations to highlight the problem of Brettanomyces from the vine to the bottle.
As far back as 2001, the Oenodev R&D team joined forces with the Chemical Engineering laboratory to carry out a huge, international campaign to conduct both local and laboratory investigations into the problem of Brettanomyces from the vine to the bottle. Samples were taken from the vine before harvesting in France and South America. These samples were targeted on bunches presenting with damaged seeds.
This groundbreaking work laid the foundations of this study, implemented the first grape screening protocols and yielded the initial results.
| Location | Number of samples | % Positive results | Direct screening |
|---|---|---|---|
| Argentina | 27 | 11 | no |
| Uruguay | 44 | 7 | yes |
| France | 49 | 18 | no |
This study demonstrated that Brettanomyces is found on grapes although the frequency and concentration of contamination vary according to the geographical region. On average, Brettanomyces yeasts were detected in 12.5% of samples of damaged bunches. The following vines were affected: Merlot, Cabernet Sauvignon, Cabernet Franc, Tannat, Fer Servadou, Ancelota, Zynfandel and Malbec. Since no samples were taken from any other vine, it may be concluded that all of these varieties are affected with no specific vine characteristics being highlighted. These encouraging results have triggered an in-depth analysis of the subject through close collaboration between the Chemical Engineering Laboratory, the Buzet cellar and Oenodev in an attempt to optimise protocols and specify conditions for the development of Brettanomyces and its subsequent distribution in vines.
General evaluation of studies conducted at Buzet
During the 2004 and 2005 campaigns, a screening procedure was carried out on the Brettanomyces population inhabiting test plots of Buzet origin (within the framework of a thesis on Brettanomyces, supported jointly by the Cave Coopérative des Vignerons de Buzet and the Oenodev Company) using the strategy outlined. Once again, the presence of Brettanomyces in grapes was confirmed in the three major vines of this label of origin, namely Merlot, Cabernet Franc and Cabernet Sauvignon. Fifty-one of the 215 samples of bunches of grapes on the vine tested positive for Brettanomyces (i.e. 24%, a non-negligible proportion).
Application: Population distribution over a given plot
Presentation of the study and specific features of the plot in question :
In 2005, a study was carried out in a 3-hectare plot known as X, growing only the Cabernet Franc vine of the Buzet vineyard, to determine whether or not any yeast was present in the grapes and, if so, to establish whether or not this population was evenly distributed over the plot at the time of harvesting.
This plot was targeted because it had been linked with isolated batches of Brettanomyces-contaminated wines during earlier winemaking campaigns.
The 75 rows, ranging from 5 vines in the shortest case to 240 vines in the longest were pointing in the South-West / North-East direction.
Plot X is characterised by a rather raised relief (in the shape of a "hillock"), as illustrated by the photograph proposed in Figure 5. Furthermore, this plot is specific in that it has 3 entirely different borders: a country road, a copse and another vine-growing plot of land (Figure 6).
A combination of the relief, direction and presence of the copse marks the start in this plot of a shaded area, which is exposed to less sunlight compared to the rest of the plot (Figure 7: the photograph was taken at 11.00 on 29 September 2005). This area is thus characterised by morning humidity, which remains significant until later in the day (11.00 even 12.00 at the end of September) unlike the remainder of the plot, which is rapidly dried by the sun. In the morning, a difference in temperature of up to 8°C is also apparent between the shaded area and the exposed area, which is always bright.
Figure 5: Plot X: the raised relief of the plot is emphasised here.
(Bosquet : Copse - Parcelle X : Plot X - Autre parcelle : Other Plot)
Figure 6.: Plot X: the three borders and general topography.
(Autre parcelle : Other Plot - Bosquet : Copse - Parcelle X : Plot X - Route : Road)
Figure 7: Plot X: An area of morning shade (Zone d'ombre : Shaded area)
Sampling :
The plot was marked out in squares in order to collect samples from all of the vines and rows as follows: a sampling point every 30-40 vines, one row in 5 (apart from the first 15 short rows - on the copse side - where the network was more dense in order to cover the zone properly).
In total: 110 sampling points were screened for Brettanomyces in accordance with the procedure described. The samples were collected the day before harvesting.
Screening results :
After processing, the samples were screened for the presence of Brettanomyces in this plot at the rate of 12 positive points out of a total of 110 (i.e. ~11%).
Figure 8 illustrates the distribution of positive Brettanomyces points in this plot of land.
Figure 8: Plot X: presence of Brettanomyces (red dots: positive test)
The relatively tightly packed area of predilection is clearly visible. We, therefore conclude a heterogeneous population profile for Brettanomyces in the case of this particular plot, which suggests that factors that are favourable (or, to the contrary, unfavourable) to the growth of this yeast are present within the plot.
In an attempt to understand this phenomenon, it is interesting to compare this result to other observations made on the plot. Figures 7 and 8 can clearly be superimposed, highlighting a potential link between the presence of the shaded area and the presence of Brettanomyces in grapes.
We can thus postulate the presence of conditions favourable to the implantation and/or development of this yeast on grape berries based on the presence of shade and its more or less indirect consequences such as a humid, fresher, morning micro-climate.
Case of specific berries
Although this is an extremely well-worked and very well-maintained plot, we noticed damaged bunches of grapes during the sampling procedure. The vines affected were routinely found at the copse border (among the last 4-5 vines in the first 20 rows, approximately). As the photograph shows (Figure 9), these bunches of grapes appear to have been damaged by birds (the presence of which is certainly enhanced by the adjoining copse).
A series of samples was collected to compare healthy and damaged bunches of grapes for a given vine (10 point paired comparison overall).
The results confirm the presence of Brettanomyces in all of the damaged bunches of grapes collected with less evidence of Brettanomyces contamination in the healthy bunches (40% tested positive). Deterioration of the berries is thus exacerbated in the presence of yeast (scenario more conducive to microbial colonisation and the intervention of insects, especially fruit flies, etc.).
Figure 9: Plot X: berries damaged on the vine in the region adjacent to the copse.
Comment :
A preliminary study was carried out as far back as 2004 on the same plot of land, with a less compact sampling network (only 70 samples on the plot. Albeit less specific, the presence profile detected also highlighted an area of predilection for the yeast. This region is roughly equivalent to that specified in 2005, which would suggest a certain recurrence in terms of this phenomenon and, therefore, some recollection of yeast in this area of the plot.
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(1) Pascal Barbin, Pierre Strehaiano, Patricia Taillandier - (2) Jean-François Gilis
(1) Chemical Engineering, Bioprocesses and Microbial Systems Laboratory
(2) Oenodev
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