Applicable to products having contents, expressed in nitrogen, of 2 mg/kg or more. Oxidation of nitrogen(III) of a test portion to nitrogen(V) by potassium permanganate. Reaction of nitrogen(V) with 3,4-xylenol under specified conditions to form a nitrate derivative. Distillation of the nitrate derivative and absorption in a sodium hydroxide solution. Spectrophotometric measurement of the yellow coloured nitrophenol at a wavelength of about 435 nm. Has also been approved by the IUPAC.