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pH downshift

Publications concerning fermentation stresses frequently omit pH. This is unfortunate, since wort pH during a lager or ale fermentation is typically reduced from 5.5 to approximately 4.1 (Coote and Kirsop, 1976; Rowe et al., 1994). The practice of using acidified worts changes the extent of the downshift considerably. Any decrease in fermentation pH occurs as a result of production of carbonic acid (CO2), secretion of organic acids, and the consumption of buffering compounds (basic amino acids and primary phosphates) in the wort (Coote and Kirsop, 1976). Coote and Kirsop (1976) found a similar buffering capacity in wort and beer and calculated that the removal of buffering materials and release of organic acids are insufficient to account for the magnitude of the pH drop typically observed during fermentation. They concluded that the secretion of H+ by yeast accounts for the discrepancy. The protons that are released potentially cause a decrease in the intracellular pH (pHi) (Ullah et al., 2012), thereby decreasing the activities of metabolic functions. In addition, changes in pHi affect signal transduction (Dechant et al., 2010), protein interaction (Young et al., 2010), and cell division rate (Orij et al., 2012). As a result of their charged state, the anions released also accumulate inside the cell where they may increase the internal turgor pressure and exert growth-inhibitory interactions, depending on the nature of the anion (Ullah et al., 2012). The plasma membrane ATPase is, therefore, important in maintaining the intracellular pH, allowing the cell to adapt to acidic conditions. However, the energy demand for the activity of this enzyme may result in a depletion of the cell's energy reserves in the form of ATP. Very little is known about the impact of lactic acid on brewing yeast during lambic beer fermentations or the production of Berliner Weiss beers; however, it can be hypothesized that during these fermentations the pHi is significantly impacted.

The final pH of the fermentation is dependent upon the buffering capacity of the wort, initial wort pH, and the extent of yeast growth (Heggart et al., 1999; Narziss et al., 1983). Although it has been suggested that brewing yeasts readily tolerate the 1.5 to 2.0 unit down-shift in pH that occurs during fermentation (Boulton and Quain, 2001), recent evidence suggests that brewing lager yeast strains demonstrate some sensitivity to changes in this parameter, manifesting in modified specific growth rates and reduced replicative lifespans (Maskell, 2003).

pH can have a sizeable impact on the production of flavour components by yeast. A lowering of pitching wort pH from pH 5.75 to 5.46 can result in a reduction in dimethyl sulfide production of up to 50% (Anness and Bamforth, 1982). Furthermore, the rate of conversion of a-acetolactate to the detrimental butterscotch flavour diacetyl was increased 4-fold when wort pH was lowered from pH 5.5 to 4.0 (Haukeli and Lie, 1978).

The downshift in pH during a typical lager fermentation also results in a change in gene expression. In laboratory studies, S. cerevisiae experiencing a shift from pH 5.5 to 3.5 in minimal media demonstrated an increase in the expression of 36 genes (Kapteyn et al., 2001) involved in the cell wall, carbohydrate metabolism, redox metabolism, and stress.

 
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