Experiments are described in which the effects on yeast performance of subnormal free amino nitrogen (FAN) levels in the wort being fermented were investigated. It was found that the performance of the yeast strain used, under the conditions prevailing in the authors' laboratory fermenters, began to be impaired when the wort FAN content at the time of pitching was 14.4 mg/100 ml or less. Because changes in the FAN level in the wort can also affect the levels of other yeast nutrients, some of the low FAN worts were supplemented with minerals and lipids, but this had very little effect on the yeast, whereas the addition of amino acids restored a normal level of fermentative performance, showing that assimilable nitrogen is the critical nutrient. It was also found that the rate of diacetyl formation decreased with falling FAN until the minimum FAN level compatible with normal yeast performance was reached, after which further reductions in the wort FAN content led to increased diacetyl production. This is attributed to the fact that at high FAN levels, brewers' yeast synthesizes valine (forming the diacetyl precursor alpha acetolactate as a by-product) while consuming other amino acids, whereas at low FAN levels, valine is initially consumed, but its depletion leads to renewed synthesis. The authors therefore propose that the valine content of beer could be used as an indication of the correctness of the FAN level in the wort from which it was made, with a lack of valine indicating insufficient FAN.
Keywords : amino acid beer brewers' yeast diacetyl fermentation free amino nitrogen performance precursor synthesis wort