Determination of amino acid composition of foods by photometric methods, part 1 : determination of tyrosine, tryptophan and phenylalanine

Abstract

Today to determine the amino acid composition of foods, the most commonly used technics are the ion exchange column chromatography (IEC) with post-column derivatization with ninhydrin and the high-performance liquid chromatography (HPLC) with pre-column derivatization. Both technics are excellent in their own right, either for solving special tasks or for determining all amino acids in a protein. However, the methods require special purpose instruments (IEC) or special equipment (HPLC), which are expensive and require highly qualified analysts to operate, which cannot be afforded by smaller laboratories and in-process inspection units. The methods we propose, after proper sample preparation, can be implemented with a UV-VIS spectrophotometer capable to measure the aromatic amino acids and the coloured amino acid derivatives in the range of 200-800 nm. In this paper, we give a report on the photometric methods developed for the determination of total amino acids with ninhydrin, determination of aromatic amino acids, measurement of the combined amount of tyrosine and tryptophan, separate determination of tyrosine, tryptophan and phenylalanine. We plan to quantify the most suitable of the mentioned methods and make them applicable to food testing laboratories. Since the indole group of tryptophan is virtually completely degraded during the 6 M hydrochloric acid hydrolysis used in the hydrolysis of proteins, this publication protein hydrolysis methods that are suitable for determining the tryptophan content of foods is described.