Fact Sheet WINEMAKING Controlling Brettanomyces during winemaking Introduction microbiological spoilage, volatile acidity (VA) and general wine instability problems. Brettanomyces is a yeast commonly found in wineries, which has the potential to cause The growth of Brettanomyces in wine is significant spoilage in wines through the affected by a range of factors, some of which production of volatile phenol compounds. are interlinked. This means that controlling These compounds, in particular 4-ethylphenol Brett requires a multi-faceted approach. If just (4EP), 4-ethylguaiacol (4EG) and 4- one factor is addressed in isolation, it is ethylcatechol (4EC), are associated with unlikely to be successful. However, if action is undesirable sensory characters such as ‘Band- taken on all or most of the factors discussed Aid’, ‘medicinal’, ‘horsy’, and ‘barnyard’, which the risk of Brett spoilage should be greatly are collectively often known as ‘Brett’ reduced. Key factors in a Brett control character. Because Brettanomyces can be strategy are outlined below. found across all Australian wine regions, and can cause such negative sensory effects, it is General sanitation sensible for all wineries to have a control Cleaning and sanitation in the winery are strategy in place, even if Brett spoilage extremely important in controlling a range of problems have not been experienced in the microbial spoilage problems, by helping to past. Steps taken to control Brett are also prevent the build-up of unwanted yeast, likely to have additional positive bacteria or moulds. During vintage, care consequences in avoiding other Updated March 2020 Fact Sheet WINEMAKING should be taken to ensure that crushers, rather than assuming that primary presses and must lines are cleaned and fermentation is complete. sanitised regularly (at least daily), so that populations of unwanted microorganisms can Sulfur dioxide (SO2) be kept to a minimum. Keeping processing SO2 is a very important wine additive, both in equipment clean will also help prevent the preventing microbial spoilage and in accumulation of organic material, which can minimising wine oxidation and promoting harbour microorganisms. wine longevity. Simply adding more SO2 is not Tanks and barrels should also be cleaned necessarily the best way to control Brett, regularly to prevent microbial cross- although in some cases this is appropriate. contamination when wines are transferred Rather, it is better to use SO2 in a way that is within the winery. Additionally, the microbial most effective. A more detailed discussion of status of any wines or barrels entering the SO2 use in winemaking can be found in winery, and the 4EP/4EG concentrations of Robinson and Godden (2003); however, there the wines, including those intended for are some simple principles that can be topping, should be ascertained. applied to minimise the risk of Brett spoilage. Residual sugar • To dramatically reduce the probability of microbiological problems, add some SO2 at Brettanomyces growth is strongly favoured by the crusher. Note that this may eliminate the presence of residual sugar in wine. all yeast and bacteria, which might result in Optimising the success of primary the need to inoculate for malolactic fermentation is therefore an important part fermentation (MLF). of a Brett prevention strategy. The simplest ways to minimise residual sugar in red wine • When adding SO2 to wine, remember that are to: only about 35 to 40% is yielded as free SO2 (the component that has antimicrobial • have the strongest yeast starter culture activity) – so add enough to make a possible by following supplier difference. One large addition is much recommendations – especially with high more effective than several small additions. sugar musts • Don’t forget about the relationship • aerate the fermentation when it is most between free SO2 and pH. The higher the active – conduct at least one aerative pH of wine, the more SO2 is needed to racking, or rack and return achieve the same antimicrobial effect. • avoid temperature shock of yeast when • Wine is particularly vulnerable to pressing – aim to keep wines within two microbiological spoilage during MLF, so it is degrees of fermenter temperature for at a good idea to make a big SO2 addition as least twelve hours during and after soon as MLF is completed – and to do pressing. everything possible to help MLF go through quickly. It is also important to check residual sugar levels in red wines using an enzymatic assay, • SO2 is less effective when added to wines with high turbidity. This doesn’t mean that Updated March 2020 Fact Sheet WINEMAKING SO2 shouldn’t be added to hazy wines, but microwaves and even blasting with particles if it is, more will be needed to have the of dry ice. same effect. It also means that working to maintain low turbidity throughout wine To remediate known contaminated barrels maturation will reap benefits. the AWRI recommends hot water as the most effective and practical sanitation method; that • The ratio of free to total SO2 is a useful is, filling barrels with hot water of at least winemaking tool and is worth monitoring. 70°C and leaving it in the barrels for at least A decrease in this ratio indicates that free 30 minutes, or 85°C water for at least 15 SO2 is being lost or bound to other wine minutes (Coulter et al. 2003), ideally until the components, and if this occurs, the reasons outside of the barrel is hot to touch. The hot should be investigated. water may be reused to sanitise other barrels, although caution should be exercised with • During even the most careful transfer of a pumping hot water, due to the possible wine, 5 mg /L of free SO2 can be lost. negative effects on pump stators and hoses. For these reasons, it may be better to transfer pH the water using a syphon and gravity. Brettanomyces growth is favoured by high pH; however, this is predominantly due to the Barrel topping relationship between pH and SO2 The wine used for topping barrels can be a effectiveness. At the end of MLF, wines are potential source of Brettanomyces and other usually at their highest pH and lowest SO2 microbiological contamination. It is commonly concentration, which makes this a critical time poorly stored – often on ullage and without for potential Brettanomyces growth and wine adequate SO2. Simple steps taken to ensure spoilage. It is recommended that winemakers that topping wine is stored carefully, at low clarify red wines and make a single large SO2 temperature and without ullage, and is addition as soon as possible after MLF, rather maintained at a suitable level of SO2 to than a series of smaller additions. prevent microbial growth, could prevent widespread wine contamination. Storing Barrel sanitation topping wine at elevated SO2 concentrations While barrel sanitation should be an can be beneficial, because when used for important component of any Brett control topping it helps control the growth of film- strategy, it is crucial to remember that barrel forming microorganisms in barrels. sanitation alone will not solve a Brett problem. Additionally, any effort put into New barrels vs old barrels barrel sanitation will be wasted if the barrels While it might be commonly assumed that are re-contaminated with wine containing a older barrels pose greater risk, due to the high population of Brettanomyces yeast. possibility of Brettanomyces yeast having become established in the old wood, it should A wide range of barrel sanitation methods are be remembered that all other things being used in wineries around the world, including: equal, wine stored in new barrels will lose SO2 cold and hot water rinses, filling with SO2 faster than wine stored in older barrels. This solutions, steam cleaning, ozone, ultrasonics, factor should be taken into account for wine going into new barrels, or recently cleaned Updated March 2020 Fact Sheet WINEMAKING barrels, to ensure that SO2 concentrations are of volatiles has risen above the sensory maintained at high enough levels to inhibit threshold, and therefore have an advantage microbial growth. over after-the-fact detection by sensory evaluation. Filtration and clarification Silicon dioxide and activated carbon have Wines with high turbidity are generally at a been shown to have some ability to remove higher risk of microbial spoilage (including Brettanomyces-derived volatiles from wine Brett) than those with lower turbidity. This is when used as fining agents, although their due, at least in part, to the influence of high effectiveness and the dose required may vary turbidity on SO2 effectiveness. It is important, between wines. Fining trials are therefore when confronted with a hazy wine, to recommended. determine what components make up the haze, rather than simply to assume that the By far the most effective method of removing haze is benign in nature. If a haze is found to Brettanomyces-derived volatiles is reverse contain viable microorganisms, then filtration osmosis (RO). Mobile RO services are available of the wine before bottling is highly in most Australian wine production regions. It recommended. While some winemakers seem has been reported that up to 7% of 4- hesitant to filter red wines, it is the AWRI’s ethylphenol can be removed with each pass position that a well performed filtration of the of a wine through a RO machine, but care appropriate grade is a much better option should be exercised because positive volatiles than taking the risk of post-bottling microbial might also be removed, particularly with spoilage. repeated passes. It is therefore Other treatments and monitoring recommended that wine is repeatedly sensorially assessed during RO treatment, options and that samples are taken periodically for While the prevention of Brettanomyces comparative purposes as the treatment infection by combining the strategies progresses. discussed here has been shown to be highly In most cases, fining or RO will not reduce the effective and is strongly recommended, some concentration of Brettanomyces-derived other treatments for Brett are also available.