I recently had a fellow homebrewer graciously review my process and an interesting topic was mentioned that I have begun researching. I have not concerned myself with FAN thinking I don’t need to know how to build a watch to tell time. Brewing can be as easy as soak some grain, boil it with some hops, throw in some yeast and out pops beer. …but the more I dig the more I find it’s not that simple. This brewing stuff can get deep with the quickness. I thought I’d run it up the flag pole and see how others view FAN.
It is well understood that excess FAN correlates to beer staling and is negatively correlated to beer stability. Beer stability refers to the capacity of beer to maintain its original quality, flavor, and chemical profile as long as possible. (Jesse Bussard, Craft Malting)
Free amino nitrogen (FAN) is a term used to describe the amount of nitrogen-containing compounds found in wort that may be metabolized by yeast during the fermentation process. FAN includes amino acids usually found in wort along with ammonia and small peptides. (Hill & Stewart, 2019)
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. (Graham G. Stewart, The Oxford Companion to Beer)
There are many mechanisms that result in the staling of beer, including oxidation mechanisms (e.g. lipids, free radicals, higher alcohols, and iso-alpha-acids) and aldol condensation resulting in the formation of a cardboard-like or papery off-flavor. However, the majority of beer staling results from amino acids transforming into chemical compounds called “Strecker aldehydes” (SA). (Yin, X. S. Malt. American Society of Brewing Chemists, 2021). That’s where the excess FAN plays a role.
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. (Graham G. Stewart, The Oxford Companion to Beer)
The comment I received was based on not using O2 to aerate my wort. By pitching the correct amount of yeast into properly aerated wort, FAN can be consumed and create a more stable beer. Sounds easy enough.