The Oxford Companion to Beer definition of
free amino nitrogen (FAN)
is defined as the sum of the individual amino acids, ammonium ions, and small peptides (di- and tripeptides) in wort. FAN is an important general measure of these nutrients, which constitute the nitrogen that yeast can assimilate during brewery fermentation. FAN is a degradation product of protein in raw materials such as malted barley, wheat, barley, and sorghum. Even if attenuation of wort sugars proceeds normally, production of the same quality of beer is not always guaranteed, suggesting that wort sugar removal alone is not a good indicator of yeast performance. Some brewing scientists regard FAN as a better index for the prediction of healthy yeast growth, viability, vitality, fermentation efficiency, and hence beer quality and stability. Proper FAN levels will be determined by using suitable malts and the correct mashing process. The level of FAN is determined primarily by the extent to which the proteolytic enzymes are able to act. The FAN itself is produced by the enzyme carboxypeptidase, a very heat-resistant enzyme that is present in abundant quantities in most malts and is accordingly seldom in short supply. Its substrates are the peptides that are produced by the proteinases that catalyze the hydrolysis of the storage proteins within the grain. These enzymes primarily act during malting, and so it is the extent of protein modification in malt that primarily determines the amount of FAN that will be developed in mashing. Wort nitrogen is used by yeast to accomplish its metabolic activities, particularly the synthesis of new amino acids and, hence, proteins. The majority of the FAN is consumed within the initial 24–36 h of fermentation, after which yeast growth generally stops. However, differences in FAN uptake between lager and ale yeast strains have been identified. The concentration of wort FAN required by yeast under normal brewing conditions is directly proportional to yeast growth and affects beer maturation. There is also a correlation between initial FAN levels and the amount of ethanol produced.