Brettanomyces bruxellensis, a survivalist prepared for the wine apocalypse and other beverages
Introduction
Wine is the final product of multiple biotic and abiotic interactions that occur in a complex medium (i.e. grape juice). The former are facilitated by microorganisms of which some are beneficial and others detrimental to wine quality. Certain abiotic factors, which include temperature and the initial chemical composition, play a major role in the composition of the resulting wine. The aforementioned detrimental microorganisms are referred to as spoilage microorganisms. Amongst these, yeasts of the genus Brettanomyces, or their teleomorphs known as Dekkera, have been found to negatively alter the chemical composition of wine by producing by-products detrimental to the organoleptic properties of the final product (Loureiro and Malfeito-Ferreira, 2003). Of this genus, the species Brettanomyces bruxellensis is a spoilage microorganism of wine. B. bruxellensis however, not only occurs in wine but also in many other alcoholic beverages, where it is not considered as a spoilage yeast e.g. some speciality beers and cider (Davenport, 1976, Loureiro and Malfeito-Ferreira, 2006). These yeasts can persist through the harsh conditions that occur during the winemaking process, such as rising ethanol concentrations and increasing additions of sulphur dioxide; they have in recent years become a major oenological concern worldwide.
Generally, many other species of yeasts, that naturally occur on grapes in the vineyard, are present at the onset of alcoholic fermentation but are readily eliminated due to: (i) the increase in ethanol concentration, (ii) the release of toxic compounds (e.g. killer toxins, weak acids, phenolic compounds and sulphur dioxide) by the dominant yeast Saccharomyces cerevisiae, (iii) competition for space and nutrients and (iv) low oxygen conditions (Pretorius et al., 1999, Holm Hansen et al., 2001, Nissen and Arneborg, 2003, Pérez-Nevado et al., 2006). B. bruxellensis however, is commonly isolated in wine and is well suited to surviving on all surfaces in and around the winery: winery walls, presses, fermentation tanks as well as within the wood of barrels used for maturation (Fugelsang, 1997). These environments are opportune for the colonisation of B. bruxellensis while the must is fermenting but even more so when the wine is ageing in barrel. In addition, the formation of biofilms by B. bruxellensis makes disinfection challenging, as biofilms are relatively resistant to chemical cleaning agents and sanitisers (Oelofse et al., 2008). Generally, grape juice is an environment inhabited by many yeasts, including S. cerevisiae the dominant yeast during alcoholic fermentation as well as other non-Saccharomyces yeasts. Amongst these yeasts, B. bruxellensis can also persist in this medium (Renouf et al., 2006). The ability however, of B. bruxellensis to reproduce and grow in wine, as opposed to grape juice, may have resulted in its adaptation to surviving in low nutritional environments.
The aromatic profile of wines is negatively influenced by the yeast B. bruxellensis and tends to be characterised by mousy, medicinal, wet wool, burnt plastic or horse sweat smells/scents. These off-aromas are said to be a part of the “Brett” character in wine (Licker et al., 1999). This is potentially problematic for winemakers, as this can result in serious economic losses (Loureiro and Malfeito-Ferreira, 2003, Fugelsang, 1997). The contamination of wines by B. bruxellensis has increased in recent years due to winemaking techniques changing to favour the production of wines that contain more residual sugar and that may be unsulphited, unfiltered, aged on the lees or aged longer in barrels. All of these factors are considered favourable to B. bruxellensis growth (de Orduña, 2010, Alston et al., 2011).
Although there has been a lot of research on factors influencing the growth of B. bruxellensis, it is surprisingly still uncertain which nutrients (and the concentrations thereof) are required for this microorganism to proliferate in inhospitable environments such as wine. It can be assumed from its ability to proliferate in a nutrient-poor medium that unlike S. cerevisiae, B. bruxellensis displays low nutrient requirements. In this review, the many ecological niches of B. bruxellensis will be reviewed with a primary focus on the nutritional composition of the products or niches concerned, also taking into account the adaptations of this yeast allowing it to survive in these media with varying compositions. Formerly, the history and sources of isolation of B. bruxellensis will be briefly discussed as this organism has been isolated from many different industries and matrices and has been referred to by various names since its discovery. In addition, the more common niches, products and matrices that B. bruxellensis is isolated from, will be discussed with regard to the potential nutrients available to the yeast. The general carbon and nitrogen sources utilised by B. bruxellensis, and its ability to utilise the nutrient sources from the different matrices it is isolated from, will be thoroughly reviewed. Overall, this review aims to provide a systematic and argumentative summary of B. bruxellensis’ nutritional requirements in an attempt to characterise the biological adaptations connected to surviving in a harsh medium.
Section snippets
History of Brettanomyces
Brettanomyces was given its name due to its close connection to the British brewing industry, as it was first isolated in the secondary fermentation of English beers. Indeed, once the initial fermentation by Saccharomyces was completed, it was noted by Claussen, that a secondary slow fermentation occurred. Later, he identified the yeast dominating this secondary fermentation and named it Brettanomyces (Claussen, 1904). Derived from the Greek ’’Brettano’’ meaning British and ’’myces’’ meaning
The ecological niches associated with B. bruxellensis
As mentioned above, B. bruxellensis has been isolated from many ecological niches. These various products and environments display common features/characteristics. Indeed, they are characterised by high alcohol concentrations, with the exception of soft drinks, fruit juices and the initial onset of alcoholic fermentations, and tend to be harsh matrices for microorganisms to live in as they contain limited nutrients, including low residual sugar concentrations and nitrogen sources, depending on
Chemical characterisation of B. bruxellensis’ habitats and the role of B. bruxellensis therein
The major ecological niches and products that host B. bruxellensis will be discussed in detail below. A specific focus will be placed on the nutrient and chemical composition of these different media, a summary of which is outlined in Table 2, and the accommodation of these media for the growth of B. bruxellensis.
The nutritional requirements of B. bruxellensis
Like S. cerevisiae, B. bruxellensis is ethanol tolerant, facultatively anaerobic, Crabtree positive and petite positive, allowing the yeast to produce offspring without mitochondrial DNA (Hellborg and Piskur, 2009). It can therefore ferment preferentially in the presence of high glucose in oxygenated conditions (Kurtzman and Fell, 1998, Piškur et al., 2006). However, unlike S. cerevisiae, B. bruxellensis can appear in situations in which nutrients are scarce (as reviewed in the previous
Conclusion
B. bruxellensis has been isolated from many products and environmental niches. These environments can be divided into those that are poor and generally not conducive for yeast growth, for example bottled wine or beer, as nutrients are scarce (Gilliland, 1961, Shantha Kumara and Verachtert, 1991, Ibeas et al., 1996, Esteve-Zarzoso et al., 2001) and those that are rich in nutrients, such as bioethanol plants, fruit juices and cider (Morrissey et al., 2004, Souza-Liberal et al., 2007, Passoth
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