In addition to ethanol, yeast metabolic processes occurring during fermentation result in the formation of a number of intense, aromatic and volatile substances. These fermentation by-products primarily consist of alcohols and esters. Higher alcohols are formed from amino acids, which are converted to their corresponding α-keto acids by means of transamination reactions, through decarboxylation and reduction to alcohols. Esters are intracellular products of yeast metabolism. Esters are formed in enzymatically catalyzed reactions involving alcohol-acetyl-transferase from acetyl-CoA and the resultant alcohols produced during fermentation. These presence of these compounds can provide information about fermentation parameters. The quality of the wort (FAN profile, zinc concentration), pitching technology (aeration, number of yeast cells pitched per ml and yeast strain) as well as the temperatures at which fermentation is conducted greatly impact the subsequent concentration of fermentation by-products in the beer.
The determination of volatile fermentation by-products by means of steam distillation detects compounds that serve as indicators of technological parameters during wort boiling, pitching and aeration, fermentation and the physiological state of the yeast.
The method is suitable for beers of all original wort ranges and alcohol contents.
Volatile compounds in beer are concentrated through distillation and the distillate is quantitatively determined by direct injection into a gas chromatograph. The linearity of the detector and the determination of the concentrations of analytes in the sample are achieved by using multiple concentration levels within the relevant range and through evaluation of the relative area under the peaks.
This method is suitable for beers exhibiting all ranges of original gravity and alcohol concentrations.