The impact of wort nitrogen limitation on yeast fermentation performance and diacetyl. Discussion

Wort free amino nitrogen (FAN) quantitates amino acids and peptides in wort and, as such, is an important measure of those yeast nutrients in brewery fermentations. To determine the wort FAN concentration at which fermentation performance is affected, lab-scale fermentations were run using worts that were adjusted to different FAN levels. It was found, under the fermentation conditions used, that fermentation performance was negatively affected in worts containing 14.4 mg/100 ml FAN or less. Assimilable nitrogen was found to be the critical nutrient since amino acid supplementation reversed the effects of fermentation performance while supplementation with minerals and lipids had only minor effects. Wort FAN concentration also affected diacetyl production, which declined as wort FAN concentrations decreased down to 14.4 mg/100 ml; below this concentration, diacetyl production increased coincident with valine depletion. We propose that residual beer valine could be used to optimize wort FAN concentration for minimum diacetyl production without affecting fermentation performance.