Low molecular weight nitrogenous substances, especially amino acids in the wort, have an influence on the course of fermentation and the formation of fermentation by-products. For a beer’s aroma profile, the concentration and composition of the amino acids are therefore of considerable significance. Due to their reactivity with reducing sugars (Maillard reaction), this is most apparent during kilning in the malthouse as well as mashing and boiling in the brewhouse. The reaction products influence the redox potential, the color and the aroma of a beer.
Determination of the FAN content in wort and beer
This method is suitable for wort and beer.
Determination of FAN content by photometrically measuring the color intensity of Ruhemann's purple from the ninhydrin reaction.
The ninhydrin reaction is the best known of the color reactions in amino acid chemistry [1]. Ninhydrin is an oxidant and brings about the oxidative decarboxylation of amino acids, producing CO2, NH3 and the formation of an aldehyde. The aldehyde produced in this reaction possesses one less carbon atom than the original amino acid that served as the reactant. Reduced ninhydrin then reacts with non-reduced ninhydrin and the NH3 that was liberated to generate a blue pigment (all amino acids except for proline) or in the case of proline, a yellow pigment. Fructose also takes part in the color reaction as a reducing compound. Potassium iodide present in the solution used for dilution preserves the ninhydrin in an oxidized state and thus inhibits undesirable secondary reactions. The solution to be analyzed is heated together with ninhydrin at a pH of 6.7 and the resulting color is measured at 570 nm [3].
This method quantifies amino acids, ammonia and also the terminal α-amino groups of the peptides and proteins. Proline is also measured in part at the wavelength employed in this method.
The method is non-specific for α-amino nitrogen, because γ-amino butyric acid found in wort also generates a color reaction in the presence of ninhydrin.