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Beer fermentations are a highly selective niche

Yeasts used in the beer industry provide an excellent model to investigate the effects of artificial selection. For example, they can be recovered and reused after the fermentation process (unlike, e.g. bread yeasts) and are employed continuously throughout the year (unlike, for example, wine yeasts, where the fermentation scheme is tightly linked to the grape harvest season). Therefore, many current beer yeast strains can be considered to be the result of a centuries-long evolution experiment, performed by brewers in a highly selective niche, the brewing environment. It is therefore not surprising that multiple genetic adaptations to the beer-making process have been described.

First and foremost, beer yeasts show a remarkable efficiency in utilization of maltose, the prime carbon source in beer wort. Through extensive duplication and subsequent functional divergence of subtelomeric genes involved in maltose metabolism, present-day brewer's yeast is capable of hydrolysing a variety of sugars much more efficiently than its ancestors (Brown et al., 2010; Charron and Michels, 1988; Dunn and Sherlock, 2008; Gallone et al., 2016; Gonsalves et al., 2016; Pougach et al., 2014; Voordeckers et al., 2012b).

Table 6.2 Examples of (suspected) domestication traits of S. cerevisiae. Domestication traits are characteristics that have diverged between the domesticated strains and their wild ancestors

Trait

Industry

Responsible gene(s)

Reference

Stress tolerance

Copper tolerance

Wine

CUP1

Fay et al. (2004), Liti et al. (2009), Warringer et al. (2011)

Molasses toxin tolerance

Beer,

distillery

RTM1

Borneman et al. (2011), Ness and Aigle (1995)

Sulfite tolerance

Wine

SSU1

Perez-Ortfn et al. (2002)

Nutrient utilization

Fructose utilization

Wine

FSY1, HXT3

Galeote et al. (2010), Novo et al. (2009)

Malto(trio)se utilization

Beer

AGT1, MAL

Brown et al. (2010), Gallone et al. (2016), Gongalvez et al. (2016), Charron and Michels (1988), Dunn and Sherlock (2008), Stambuk et al. (2009), Steensels et al. (2014), Voordeckers et al. (2012b)

Xylose utilization

Wine

XDH1

Wenger et al. (2010)

Sensory quality

General wine aroma

Wine

Unknown

Hyma et al. (2011)

Acetate ester production

Fermented

beverages

ND

Steensels et al. (2014)

Phenolic off-flavour (POF) production

Beer

PAD1, FDC1

Dunn and Sherlock (2008), Gallone et al. (2016), Gongalves et al. (2016), Mukai et al. (2010, 2014)

Other

Flocculation/flor-

formation

Sherry, beer

FLO genes

Fidalgo et al. (2006), Christiaens et al. (2012)

Lag phase

Wine,

bakery

ARO8, ADE5,7, VBA3

Carmona-Gutierrez et al. (2013)

Mesophilic behaviour

Lager beer

ND

Dunn and Sherlock (2008)

Vitamin biosynthesis

Biofuel

SNO, SNZ

Stambuk et al. (2009)

Another trait in which beer yeasts often excel is flocculation, and more specifically the timing of the onset of flocculation. Flocculation is the ability of cells to stick to each other and form aggregates that rapidly sediment to the bottom, or rise to the top, of the fermentation medium (see Chapter 1). This is an important trait in the beer industry, since it provides an easy and cheap way to separate the yeast cells from the finished beverage (Verstrepen et al., 2003). However, early flocculation leads to inefficient or even stuck fermentation processes. Therefore, many brewer's yeasts have been selected to flocculate at the exact moment when all fermentable sugars have been converted into carbon dioxide and ethanol. Moreover, some reports suggest that brewers have fine-tuned the flocculation behaviour of their yeast strain by selecting specific layers of yeast sediment for re-inoculation of a subsequent fermentation batch (Powell et al., 2004). The genetic basis of this phenotype, and the remarkable speed at which yeasts are able to switch their flocculation behaviour, has been studied intensively (Christiaens et al., 2012; Verstrepen et al., 2003). It was shown that flocculation behaviour is controlled by the FLO genes, encoding flocculins, and that the instability of the tandem repeats present in these genes enables relatively rapid expansions and contractions in the gene size, thereby allowing for the fast isolation of spontaneous mutants with altered flocculation characteristics (Verstrepen et al., 2005). Lastly, recent evidence also pinpoints 4-vinylguaiacol (4-VG) production (or rather the absence thereof) as an important feature of brewing yeasts. Indeed, genetic analysis shows that disruptive genetic mutations in the causative genes (PAD1 and FDC1) were heavily selected for in brewing yeasts, while this was never encountered in wild strains (Gallone et al., 2016; Gonsalves et al., 2016). Interestingly, this strong adaptation to the beer environment came with a cost. General stress resistance (temperature tolerance, ethanol tolerance, salt tolerance), which is vital for survival in nature, is often impaired in brewing yeast. This loss of ‘survival skills' is typical for domesticated organisms (imagine releasing a Chihuahua in the wilderness), and is one of the clearest signs of human interference with the organism's evolution.

In conclusion, S. cerevisiae combines several natural features that allow it to thrive in industrial fermentation processes. Moreover, these features were further enhanced or specialized during domestication.

 
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