Making sense of FAN (free amino nitrogen)
By Moritz Kallmeyer
Master Brewer of Drayman’s Brewery & Distillery, Silverton Pretoria, June 2004
For yeast cell multiplication the yeast requires a source of nitrogen to form amino acids and nucleotides so that proteins, nucleic acids and co-enzymes can be formed. Yeast can remove the amino group from any one amino acid and use it to form other amino acids, from there the term “free” amino nitrogen. It is thus not necessary to provide a balanced mixture of amino acids corresponding to the mixture present in the yeast’s proteins. Instead a certain total amount of FAN must be provided to supply the N atoms needed by the yeast to synthesize amino acids which in turn is used to form proteins. The higher the amount of FAN in the wort the more fusel oil (higher alcohols & esters) is produced. It is thus desirable to regulate FAN levels by taking into account changes in grist composition and seasonal variations of raw materials, to control yeast growth. To prevent fusel oil formation in high gravity brewing about 25% of the malt is replaced with a carbohydrate adjunct which does not provide the excess amino acids and so the yeast does not form such high concentrations of higher alcohols. The yeast needs about 100mg of FAN per liter in the case of worts made with adjuncts and 200mg/L for all malt worts, to successfully ferment the wort.
The vast bulk of wort amino acids are pre-formed in the malt during malting. In the Kurz/ Hoch mashing system which is more dilute, less than10% of the amino acids formed in wort arise during mashing which is good. Thick, all-malt mashes with extended low temperature stands would increase amino acid formation by as much as 50% because soluble protein is available for attack by peptidases enzyme. This could negatively influence yeast performance (rate of growth, rate of fermentation and spectrum of flavour compounds produced). Furthermore if excessive amounts of amino acids survive into the final beer they could foster the growth of spoilage organisms in unpasteurized products.