ISO 6635:1984

The method consists in extracting a test portion with hot water, precipitating the protein by addition of solutions of potassium hexacyanoferrate(II) and zinc acetate, and filtrating the precipitate. Further, reduction of the nitrate to nitrite by metallic cadmium (only for determining the nitrate). Then addition of sulfanilamide chloride and N-(1-naphtyl)ethylenediamine and measurement of the red complex obtained at a wavelength of 538 nm.