The primary purpose of brewery fermentation is the synthesis of alcohol from fermentable sugars and the production of particular flavour-active compounds by Saccharomyces yeast. Under normal fermentation conditions, final ethanol concentrations fall in the range of 3-6%, though under high-gravity fermentation this concentration may be greater than 10% (Briggs et al., 2004). The effects of ethanol toxicity on yeast physiology are diverse, though cellular membranes appear to be the main sites of ethanol damage. Specific effects include growth inhibition, reduced cell size (Canetta et al., 2006), reduced viability particularly for respiratory- deficient mutants of brewing strains (Gibson et al., 2009; Cheung et al., 2012), reduced respiration and glucose uptake (Pascual et al., 1988), reduced fermentation (Fernandes et al., 1997), enzyme inactivation, lipid modification, loss of proton motive force across the plasma membrane (Petrov and Okorokov, 1990; Mizoguchi and Hara, 1997), increased membrane permeability (Marza et al., 2002), lowering of cytoplasmic pH, and the induction of respiratory-deficient mutants (Jimenez et al. 1988; Ibeas and Jimenez, 1997; Chi and Arneborg, 1999).
Beers produced through high-gravity fermentation have also been found to have a greater flavour stability and consistency than those produced using normal gravity brewing (McCaig et al., 1992). This improved fermentation efficiency may, however, be at the expense of yeast physiological state and hence performance in subsequent fermentations.