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Responses to ethanol

Yeast responding to increased ethanol concentration demonstrates an increased unsaturation index, and fluidity, of their membranes (Beaven et al., 1982; Sajbidor and Grego, 1992; Lloyd et al., 1993; Odumeru et al., 1993; Alexandre et al., 1994a). This relationship is not absolute, and oleic acid (18:1) appears to be a key determinant ofethanol tolerance in S. cerevisiae, rather than unsaturation index, per se (You et al., 2003). S. cerevisiae cells also exhibit an increase in plasma membrane H+-ATPase activity in response to ethanol exposure (Rosa and Sa-Cor- reia, 1991, 1996; Alexandre et al., 1994b; Monteiro and Sa-Correia, 1997), counteracting the increased influx of protons across the plasma membrane of ethanol-exposed cells (Leao and Van Uden, 1984).

Fujita et al. (2006), found that homozygous diploid mutant strains of S. cerevisiae lacking genes involved in vacuolar H+-ATPase function were sensitive to ethanol, 1-propanol, and 1-pentanol (Fujita et al. 2006).

Mitigating for ethanol

Typically in brewing, strains are selected for high- gravity brewing on the basis of their capacity to ferment according to profile and tolerate higher ethanol concentrations. Dilution of yeast slurries post cropping is a standard protocol used to ensure that cells are not exposed during storage to higher than necessary ethanol levels. However, there have been some studies suggesting other factors that might mitigate for the consequences of ethanol.

Increases in monounsaturated fatty acids, and corresponding decreases in saturated fatty acids have also been observed in ale and lager yeast strains exposed to ethanol, either directly through supplementation or during fermentation (Odumeru et al., 1993). The membrane composition of brewing yeast is influenced by wort composition, and supplementation of high-gravity wort with ergosterol and oleic acid (in the form of Tween 80) has been shown to significantly improve fermentation rate (Casey and Ingledew, 1985) and ethanol productivity (Dragone et al., 2003). However, it is still unclear whether this improved fermentation performance is due to an increase in ethanol tolerance because of changes to lipid membrane composition or simply due to improvement of the nutritional status of the growth medium, as suggested by Casey and Ingledew (1985).

Magnesium ions have a role in maintaining membrane integrity, and reduce the proton, anion, and nucleotide permeability of membranes exposed to ethanol (Salgueiro et al., 1988; Petrov and Okorokov, 1990; Hu et al., 2003). Increasing the bioavailability of Mg prior to or during ethanol shock reduces the synthesis of the heat shock proteins (Birch and Walker, 2000) and increases the viability (Walker, 1998; Birch and Walker, 2000; Hu et al., 2003) and growth (Ciesarova et al., 1996) of cells. In addition, supplementation of fermentation media with magnesium has been shown to increase fermentation rate and ethanol productivity (Dombek and Ingram, 1986; Stewart et al., 1988; DAmore et al., 1990; D'Amore, 1992; Ciesarova et al., 1996; Walker and Maynard, 1997; Rees and

Stewart, 1999). Impaired fermentation rates in the absence of sufficient concentrations of Mg2+ may also be related to its role in regulating the activity of glycolytic enzymes such as pyruvate kinase (Morris et al., 1984).

Exposure of cells to ethanol stress (16%) induces the synthesis of trehalose (Sharma, 1997) and increased accumulation of trehalose has also been observed in ale and lager brewing yeast strains exposed to 10% ethanol (Odumeru et al., 1993). Alexandre et al. (2001) demonstrated that genes involved in trehalose synthesis in yeast are up- regulated in response to ethanol stress.

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