Determination of the high-molecular weight proteins in wort and beer by precipitating them with magnesium sulfate
This method is suitable for wort and beer.
The high molecular weight proteins are precipitated by magnesium sulfate, and the nitrogen content in the sediment is determined, e.g., according to Kjeldahl.
In order to measure the portion of the high molecular weight protein fractions in wort and beer, precipitation with magnesium sulfate is recommended. Through gel chromatography it has been established that magnesium sulfate precipitates nitrogenous compounds possessing molecular weights of approximately 2600 Da and higher [2]. A strong correlation between the nitrogenous substances precipitated by magnesium sulfate and beer foam has also been observed [3, 4].
Determination of low-molecular weight nitrogenous compounds in wort and beer (< 2600 Da) by means of molybdate precipitation
This method is suitable for wort and beer.
Determination of low-molecular weight nitrogenous compounds in wort and beer (< 2600 Da) after precipitating the high and middle-molecular weight nitrogenous compounds with molybdate
The nitrogen content in the filtrate is determined using Kjeldahl or Dumas.
Precipitation with phosphomolybdic acid provides information regarding the medium molecular weight nitrogenous substances present in samples of wort and beer. Nitrogenous substances with a molecular weight greater than 2600 Da are precipitated [1]. However, a quantitative elimination of all nitrogenous substances of an equivalent molecular weight does not occur, which indicates specificity toward certain nitrogenous substances.
Nitrogenous substances of a medium molecular weight can be calculated as the difference between the nitrogenous substances precipitated with molybdic acid and those precipitated with magnesium sulfate [2, 3].
The reagent phosphomolybdic acid itself is not necessary for the analysis; a solution of sodium molybdate is sufficient. Through the addition of sulfuric acid, enough phosphoric acid is released from the phosphates present in wort and beer to form phosphomolybdic acid.
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.