The traditional view of flavour modification during lagering is examined in the light of present knowledge of yeast metabolism. The long, cold secondary fermentation improves beer flavour by adsorption of nonvolatile compounds on to yeast cell surfaces, by reactions between phenolic compounds and peptides and by a slow clarification and stripping of volatiles through release of carbon dioxide. In addition to the elimination of undesirable flavours such as diacetyl and green beer flavour, however, there is generally recognized an improvement in palate fullness and mouthfeel that can be shown to be connected with yeast behaviour during lagering. Such behaviour is likely to be associated with breakdown of nondividing cells rather than with biosynthetic activities. analyses of yeast and beer during a traditional 6 week lagering period showed yeast viability and fermentative power declined rapidly after the first three weeks and after six weeks there were increases in the pH, alpha amino N and invertase activity of the beer. The amino acid content of the beer was increased by 12% while in the bottoms, containing the yeast sediment, the increase was 11 fold. A similar liberation of nucleotide material and of organic phosphate (mainly hexose and pentose phosphates) and inorganic phosphate was observed. Total and individual amino acid levels were compared in the beer and in the intracellular yeast pool. Findings indicate that the major metabolic sequences involved in maturation are breakdown of cell constituents and accumulation of the degradation products in endogenous pools, with a gradual loss in selectivity of the plasma membrane and release of amino acids, peptides, nucleotides, phosphates and other materials. Accumulation and secretion were enhanced by increasing temperature.
Keywords: amino acid beer flavour nitrogenous compound