The following nitrogenous substances are found in wort and beer:
Amino acids, peptides, proteins, nucleic acids and their derivates, ammonium salts, nitrates, heterocyclic compounds as well as aliphatic and aromatic amines. In practical applications, amino acids, peptides, proteins and ammonium salts are of particular interest. The amino acids and ammonium salts are the most important sources of nitrogen for the yeast. They shape to a large extent which fermentation by-products are created through yeast metabolism and at which concentrations. The medium and high molecular weight proteinaceous compounds (peptides, proteins) are especially significant for the colloidal stability (shelf life), mouthfeel and for foam stability. For this reason, it is necessary to measure not only the total amount of protein but also the relative amounts of each of the proteinaceous compounds in wort and beer.
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.