Significance of Secondary Nitrogen Metabolites for Food Quality

Nitrogen is essential for all organisms and a major constituent of amino and nucleic acids, proteins, proteids, nucleotides, enzymes and secondary compounds. In its functions N cannot be replaced by any other mineral as for instance the complementary features known for potassium and sodium (Marschner 1986). N deficiency in plants results in reduced protein synthesis and a diminished biological value of the crop. Characteristic symptoms of severe N deficiency are a stiff appearance together with apical chlorosis of the older leaves, which may proceed towards necrosis of the plant tissue. Plants show a poor degree of tillering, reduced growth rate and finally diminished crop yield. The symptomatological threshold for macroscopic symptoms ranges from 1.1 to 4.8% N and critical N concentrations vary between 1.2 and 5.9% N depending on crop type, stage of development and crop yield (Reuter et al. 1997).

Secondary metabolites contribute significantly to food quality, either as nutritives, or antinutritives. From N-containing secondary metabolites the structure of about 12,000 are alkaloids, 600 non-protein amino acids, 100 amines, 100 cyanogenic glycosides and 100 glucosinolates is known (Wink 1999a). It is the objective of this section to provide basic information about different aspects of food quality with special view to secondary compounds.