Brettanomyces: Yesterday's Myths and Today's Realities
This saga is divided into 6 articles
Read the introduction , Ecology and Origin of Brettanomyces in wines and What are the factors and practices that influence the development of Brettanomyces in wines?
Brettanomyces is a yeast that has been known for a long time but has recently attracted a lot of attention. The latest fashionable subject, controversies, myths, miracle solutions are now legion and contribute to enormously complicating a serious problem but nonetheless not as complex as some would have us believe... The abundance of information of varying quality on this subject has rather detrimental to the management and solution of the Brettanomyces problem in red wines. The main objective of this article is to take stock of the key elements likely to significantly influence the development of microorganisms in wines and to identify practical and effective tools for controlling their development.
Brettanomyces control is first done in the vineyard
This may seem strange, but the agronomic factor greatly influences the risk of annoying contamination. Not because the source of inoculation is on the grape, but vine and the compositional balance of the grape significantly influence the susceptibility of the wine to the development of Brettanomyces.
Indeed, the imbalance in the mineral supply of the vine, and in particular the excess of potassium, inevitably leads to an increase in the pH of the musts and then the wines. In some cases, if the acidity is not corrected early, it will be impossible to use sulfiting to inhibit the development of Brettanomyces. On the other hand, a lack of nitrogen fertilization, for example, excessive competition (poorly controlled grass cover) or excessive water stress (territory with low water reserves or poorly managed irrigation), risks producing grapes that are difficult to ferment. Without supplementation in the cellar, these grapes will experience slow or incomplete fermentation, ideal situations to promote the development of Brettanomyces in the short or medium term. There can therefore be “terroirs”, or rather agronomic situations, which naturally favor the development of this germ. It is the responsibility of the oenologist and especially the winemaker to correct or prevent these situations of imbalance to avoid undesirable developments in the wine a posteriori.
Use of sulfiting
Sulfur dioxide is the only antiseptic authorized in oenology but Brettanomyes/Dekkera is relatively resistant to this antiseptic. The content of active molecular sulfur dioxide, the only one which has antiseptic properties, depends mainly on the content of free sulfur dioxide and the pH (-0.2 pH unit = + 50% active SO2), more incidentally on the temperature (+1°C = +7% active SO2) and the ethanol content of the wine (+1% vol. = +5% active SO2). A concentration of active molecular sulfur dioxide of 0.5 mg/l is just sufficient to inhibit multiplication without completely preventing it. From 0.7-0.8 mg/l, the content becomes lethal if the temperature is high enough. Thus, if a content of 25 mg/l is sufficient to control the development of Brettanomyces at pH 3.60, at least 35 mg/l will be required at pH 3.75 and almost 60 mg/l from pH 3, 90… We can therefore clearly see that the use of sulphiting is strongly limited by the actual acidity of the wine; this parameter therefore represents a key element of the “natural resistance” of wine to contamination by this type of germ. The sulfiting of wines aged in barrels must be even more careful because the stability of sulfur dioxide in these conditions is much more uncertain. Breeding bungs aside for a prolonged period, the absence of topping up on the pretext that more watertight silicone bungs are used, the reduction in doses of sulfur dioxide or the absence of regular racking to limit the work of the cellar are all factors which have caused the explosion of contamination in cellars in recent years. Finally, we must always consider the sulfiting of wine as a preventive tool and not a curative one!
The drying of barrels after their cleaning by burning sulfur is a practice often abandoned due to its inconvenience and even recently called into question legally. However, the action of gaseous sulfur dioxide is particularly interesting to ensure the disinfection of the surface and the first millimeters of the barrel staves9. The practice of drying has demonstrated its effectiveness in preventing contamination by Brettanomyces when no other means of disinfection (steam in particular) is available in the cellar. Its replacement by alternative techniques such as rinsing with ozonated water cannot produce exactly the same result due to the essentially superficial action of the active ingredient. Among alternative physical treatments, no solution has the simplicity, productivity, effectiveness and cost of basic wicking... It is therefore to be hoped that this practice can continue for an unlimited time.
Brettanomyces polyploidy and the effectiveness of sulfiting
Brettanomyces exists through different species, but it is the species bruxellensis (ex intermedius) which seems largely dominant in oenology. The metabolic activities involved in wine spoilage are strongly dependent on the nature of the strain. Among these strains, the number of chromosomes varies between 4 and 9, but the ploidy factor (number of copies of these chromosomes) strongly influences the capacity for development in wine and especially the resistance of Brettanomyces to sulphites16. Triploid strains prove to be the most adapted, and therefore the most difficult to combat by conventional means and in particular sulfiting. These triploid strains have an adaptive advantage over others and therefore spread more easily in cellars. With the development of modern tools for genetic typing of strains, the determination of the ploidy of the populations present usefully complements the diagnostic arsenal to effectively adjust the means of response17 when a problem is detected.
Hygiene and disinfection of wine equipment
Cellar equipment and wine containers represent the main reservoir of Brettanomyces in the cellar. It is from its tanks that the contamination, limited to a batch of wine initially, can spread, depending on the movement of wines and the use of the equipment, to the entire volume. Disinfection of equipment is therefore a crucial part of controlling these germs, but any disinfection will only be effective after sufficient cleaning. The situation is more complicated with wood which has a much larger developed surface than stainless steel or epoxy resin. Additionally, this material is delicate and cleaning and disinfection procedures should not degrade it. Hot water and steam are the most effective and least expensive means of ensuring prophylactic or curative hygiene against microbiological infections in barrels. A specific publication has also been devoted to this subject and I refer readers to its consultation to obtain all the necessary information18.
16 CURTIN C. et al. 2012 Genotype-dependent sulphite tolerance of Australian Dekkera (Brettanomyces) bruxellensis wine isolates. Letters in Applied Microbiology 55, 56-61.
17 ALBERTIN W. et al. 2014 Development of microsatellite markers for the rapid and reliable genotyping of Brettanomyces bruxellensis at strain level. Food Microbiology 42, 188-195.
18 CHATONNET P. 2010 Cleaning and disinfection applied to wooden wine containers intended for winemaking and aging: part 2/3: Necessities, principles and methods of disinfection of wood in contact with wine Revue des Œnologues, No. 137, 38-43