Stability of tryptophan during food processing and storage. 2. A comparison of methods used for the measurement of tryptophan losses in processed foods.

Tryptophan losses in stored milk powders and in different model systems representing the major reactions of food proteins during processing and storage were determined using four different chemical methods and in a rat assay. Similar tryptophan values were obtained by the three chemical methods which included high pressure liquid chromatography (HPLC) after sodium hydroxide hydrolysis, colorimetric reaction with p-dimethylamino-benzaldehyde (p-DAB) after barium hydroxide hydrolysis, and fluorescence of the Norharman derivative after NaOH hydrolysis. Tryptophan losses in the treated proteins as measured by the alkaline-hydrolysis methods were generally smaller than those determined by the rat assay. Good agreement however was obtained when the chemical value was multiplied by the true nitrogen digestibility. Determination of tryptophan by reaction with p-DAB after papain (EC 3.4.22.2) digestion gave lower values in the processed proteins than the other chemical methods or the rat assay. A method using alkaline-hydrolysis is recommended, preferably combined with HPLC-measurement of the liberated tryptophan.