The concentrations of several organic acids have been estimated quantitatively in commercial and experimental worts and beers. All of the worts contained acetate, pyruvate, lactate, pyroglutamate, malate and citrate. During fermentations, yeast excreted pyruvate, lactate, succinate, usually 2-oxoglutarate and malate, and sometimes 2-hydroxyglutarate and citrate. The concentration of pyroglutamate in wort was unaffected during fermentations with brewing strains of yeast, and acetate concentrations oscillated during fermentation. Pyruvate was re-assimilated by yeast towards the end of some fermentations. The total concentration of organic acids in beer was higher than that in the wort from which it was fermented. The strain of yeast, wort compositioa, wort pH value and availability of oxygen to the yeast and wort prior to fermentation all affected the organic acid concentrations in beers, and the reasons for these effects are discussed. Pyruvate, 2-oxoglutarate and 2-ydroxyglutarate concentrations were found to be dependent on the extent of yeast growth, whereas malate, lactate and succinate were dependent on the degree of sugar utilisation during fermentation; acetate excretion demonstrated a negative correlation with yeast growth. It is concluded that organic acids may be excreted by yeast during fermentation (a), to control the intracellular pH value, (b) as a means of removing from the cell excess carbon resulting from amino acid metabolism, and (c), to control the intracellular balance. Exeretion of organic acids into the fermenting wort, and assimilation of basic amino acids from it were found to be significant in causing the pH value to decrease during fermentations, and solution of carbon dioxide and absorption of primary phosphates contributed to a small extent. The initial buffering capacity of wort was important in determining the pH value of beer, although the buffering capacity measured in the relevant range (pH 4 to 5) changed little during fermentations, and therefore did not contribute to the observed pH fall during fermentation. It is concluded that small peptides and, to a lesser extent, citrate, are responsible for buffering worts and beers within the pH range 4 to 5, these compounds neither being assimilated nor excreted to any great extent by yeast during fermentation.
|Date of Award||1977|