Desktop version

Home arrow Education

  • Increase font
  • Decrease font


<<   CONTENTS   >>

Brettanomyces (Dekkera)

Of the non-Saccharomyces beer-spoiling yeasts, Brettanomyces spp. have arguably the greatest potential impact on beer and have been the subject of detailed scientific investigation. This is partly because they are desirable for the production of certain beer types, including lambic, guez, and some saison-style products, but also because they are considered to be amongst the most dangerous spoilage microbes in alcoholic beverages, particularly in wine production (Schifferdecker et al., 2014). Brettanomyces cells are typically sausage- or bullet-shaped, but overall morphology can be variable. They are able to produce pseudohyphae and can sporulate; as mentioned previously (see section ‘Taxonomy of beer-spoiling yeast species'), the sexual forms of Brettanomyces were historically classified as belonging to the species Dekkera. Reclassification of many yeasts within the genus has resulted in five species: B. bruxellensis, B. anomalus,

B. custerianus, B. naardenensis, and B. nanus. All of these have been isolated from beers, but arguably the first two are the most frequently reported. In the wild, Brettanomyces yeasts are associated with trees and the surfaces of fruit; it is thought that Brettanomyces strains may be introduced to breweries by insect vectors such as fruit flies (Christiaens et al., 2014).

Yeasts belonging to this species are often described as ‘survivalists' since they are ethanol tolerant and resistant to low pH which means that they are well adapted to withstand the fermentation process (Steensels et al., 2012). As well as being recognized contaminants of beer and wine, they can also cause issues in the manufacture of

Table 11.3 Characteristics of typical aerobic (non-fermentative) beer-spoiling yeasts

Genus

Common species

Characteristics

Beer spoilage potential

Additional information

Brettanomyces

(Dekkera)

B. anomalus (D. anomola)

B. bruxellensis (D. bruxellensis) B. lambicus

Elongated cell structure can form short chains Fermentative yeast in presence of oxygen due to the Custers effect

Cannot ferment sucrose and limited fermentation with maltose

Produces acetic acid and 4-ethyl phenol (horse blanket/barnyard character)

Sometimes forms a pellicle

Causes off-flavours especially if present in bottle-conditioned beers Can be found in unpasteurized draught beer

Ascomycete Brettanomyces is the anamorphic form Dekkera is the teleomorph, producing spores Sometimes used as the primary yeast or as a secondary culture in lambic style and ‘Brett’ beers, or in some traditional UK cask- fermented ales

Candida

C. boidinii C. stellata C. tropicalis C. vini

Spherical-shaped

cells

Fermentation of glucose and sometimes maltose Grows poorly under anaerobic conditions

Fermentation, turbidity, and off-flavours Can form films (pellicles) Spoilage often limited to aerobic production stages

Ascomycete Some teleomorphs of Candida are Pichia species Other species may be associated with the human microbiome

Debaryomyces

D. hansenii

Small spherical cells Weak or no fermentation

Turbidity and yeasty off- flavours

Sometimes forms a pellicle or deposit Spoilage often limited to aerobic production stages

Ascomycete

Osmotolerant yeast found as a contaminant throughout the food industry

Lindnera

(Williopsis)

L. saturnus

Spherical-shaped

cells

Weak or no fermentation

Produces strong estery flavours

Often associated with killer activity

Ascomycete

Has been studied as a means of producing banana (isoamyl acetate) flavouring for the food industry

Pichia

P anomola

Typically ovoid/

Turbidity and yeasty off-

Ascomycete

(Hansenula)

P fermentans P.

membranifaciens

ellipsoidal or rodshaped cells Prefers aerobic conditions Fermentation weak or absent

flavours

Can cause elevated ester production Sometimes forms a pellicle or deposit

Hansenula is an obsolete synonym of Pichia Pichia are teleomorphs and produce spores.

Some Candida species are anamorphs of Pichia species

Rhodotorula

R. glutinis R. mucilaginosa

Typically ovoid/ ellipsoidal May produce pseudohyphae Fermentation absent

Can assimilate sugars, reducing fermentation efficiency

Survives in pitching yeast but typically does not spoil beer

Basidiomycete Can act as a nitrate reducer, potentially contributing to apparent total N-nitroso compounds (ATNC) in beer

soft beverages and dairy products. Although Bret- tanomyces yeasts are facultative anaerobes, due to the Custers effect (see section, ‘The Custers effect', below) they readily convert sugar into ethanol and acetic acid in the presence of oxygen. Furthermore, they are able to degrade and ferment complex sugars (dextrins) that are not readily utilized by the culture yeast. Consequently, they pose a significant threat to unpasteurized beers in which oxygen ingress may occur and spoilage ofbeer is characterized by the formation of particularly high concentrations of acetic

acid. In addition, these yeasts exhibit an enhanced capacity for biomass generation, causing turbidity, while in the case of extreme infections a surface film may also be visible. Brettanomyces are also responsible for the production of volatile phenolic compounds (see section, ‘Production of phenolic compounds', below) such as 4-ethylphenol (often described as barnyard, horsey or medicinal) and 4-ethylguaiacol (bacon, spice, cloves, smoky), and volatile fatty acids such as isovaleric acid (sweaty saddle, cheese, rancid). As can be imagined, many of these flavours are undesirable in the vast maj ority of beers produced worldwide. For a comprehensive review of positive characteristics associated with the use of Brettanomyces strains for beverage production, the reader is directed to Steensels et al., 2015; see also Chapters 4, 6 and 7.

 
<<   CONTENTS   >>

Related topics