The objectives of this study were to investigate the adaptation mechanisms of Brettanomyces bruxellensis to wine and to sulphur dioxide (SO2). In particular, the nutritional requirements of B. bruxellensis were investigated in the presence/absence of oxygen, together with its nitrogen uptake in the presence/absence of SO2. Furthermore, its broader response to SO2 was investigated using a variety of approaches, from microscopy to transcriptomics (the expression of genes at any given time).
A synthetic wine medium was developed to perform the experiments of this study. Various environmental conditions were applied (e.g. presence/absence of oxygen, presence/absence of SO2) and a variety of analyses were performed including microbiological analyses (population dynamics and fermentation kinetics), chemical analyses (carbon and nitrogen sources), microscopic analyses using optical and electron microscopy, as well as transcriptomic analysis (which genes are switched on during which conditions).
The results of this study revealed the variety of adaptation mechanisms of B. bruxellensis to wine (i.e. the nutrients found in wine and SO2). It is a slow grower, because it has a long adaptation period. The data showed that SO2 has a major impact of the cell morphology, cell surface and metabolism, but that B. bruxellensis is able to slowly “repair” the damages and recover. Its main mechanism of detoxification is the efflux of SO2 via a sulphite pump. Moreover, B. bruxellensis has adapted to use a range of carbon and nitrogen sources amongst those left at the end of fermentation, including malic acid, ethanol and proline (an amino acid that cannot be utilised by S. cerevisiae under fermentative conditions). The results are fascinating because they reveal how B. bruxellensis has adapted to a harsh medium, containing nutrients mostly unassimilable by other yeasts (especially under anaerobic conditions) and SO2.
Practical recommendations for winemakers as a result of this study:
- Protection of wine with SO2 helps to control Brett growth but is not a guarantee that spoilage will not occur due to some strains’ ability to repair the SO2 damage over time. The use of other preservatives/means of protection is advised in addition to SO2.
- Such other means of protection include: making sure the wine does not contain access nutrients at the end of primary and malolactic fermentations (MLF); conducting MLF as soon as possible after primary fermentation to remove all malic acid; limiting wine’s contact with oxygen to slow B. bruxellensis’ utilisation of certain nutrients; and by limiting yeast cell autolysis at the end of fermentation to prevent the release of their stored nutrients into the wine.
Researcher: Prof. Benoit Divol