Screening Brett
Brettanomyces
Methodology for the Screening and Isolation of Brettanomyces in the Grape: plot-scale application
(1) Pascal Barbin, Pierre Strehaiano, Patricia Taillandier - (2) Jean-François Gilis
(1) Chemical Engineering, Bioprocesses and Microbial Systems Laboratory
(2) Oenodev
Introduction
Nowadays, yeast belonging to the Dekkera/Brettanomyces genus is a recognised problem agent in the wine industry (Loureiro et al. 2003, Henick-Kling 2003).
Responsible for marked, physico-chemical (animal, pharmaceutical, volatile acidity) changes in musts during development and in both cellar and bottled wines, this yeast
triggers a non-negligible decrease in product quality (Licker et al. 1999) and consequential financial losses. Hence, many studies and investigations are being carried
out in an attempt to pinpoint the development and behaviour of this undesirable yeast.
It is of paramount importance to trace the origin of the contaminant in order to understand this form of contamination, which is likely to strike any cellar in any
region of production. And, in this respect, the mystery has only been partly unveiled. Brettanomyces, or "Brett" as some wine connoisseurs know it, is considered by some
to be an almost intrinsic factor in cellars and wine and spirit storehouses. For others, however, it is associated with hygiene defects and benefits from poor cleaning
and maintenance conditions to proliferate and contaminate production units year in, year out (Chatonnet 2000). Nevertheless, a source directly linked to the raw material
- the grape - cannot be overlooked.
Brettanomyces on the grape - a topic for discussion
Numerous studies have been carried out since Pasteur's initial investigations of 1878 to list the natural microflora of the grape. Twenty genera and 70 species of
micro-organisms (yeasts, bacteria and fungi) have been detected and listed to date (Barnett et al. 1990), highlighting several thousand strains.
And although some general studies of grape microflora refer anecdotally to the presence of Brettanomyces on berries, few actually focus on a specific investigation
of the problematic yeast, which would officially confirm its presence in grapes on the one hand, and define its role in cellar contamination on the other hand.
Although Pasteur himself quoted Brettanomyces as part of the natural flora of the grape, this concept is still widely debated today. Some authors conclude that
these yeasts have never been detected during the grape (wine) harvest or in musts at the start of fermentation (Larue et al. 1991, Wright and Parle 1973). Others, on the
other hand, use molecular biology methods to demonstrate, without further ado, that Brettanomyces has, in actual fact, been found in the grape and can account
for between 3 and 15% of the indigenous, non- Saccharomyces flora depending on the year and geographical location in question, e.g. Chile (Ganga and Martinez 2003).
To date, only one study clearly highlights the presence of Brettanomyces in the vineyards of Graves, Libournais and Médoc at various stages of berry maturation
(Renouf and Lonvaud 2006).
Our work is carried out against this controversial backdrop and we wanted to check whether the presence of Brettanomyces could be detected on the grape and to
specify the factors conducive to its development. Our investigation focuses on the study of a plot of land in the Buzet region of France.
Need to develop a specific method
No specific procedure had been recommended for the detection and isolation of Brettanomyces directly from the raw material - the grape - until very recently. Despite the fact that tests and screening media have been developed over a period of several years for samples of contaminated musts or wines (Couto et al. 2003, Rodrigues et al. 2001), not one was described for the direct investigation using samples of grapes on the vine. Our study therefore initially comprised the proposal of a straightforward and cost-effective, detection and isolation protocol, basing our approach on simple, discriminatory, eco-physiological criteria.