If the intention is to determine the concentration of a single amino acid in the hydrolysate or in its free form in fermentation solutions, juices or extracts, chromatographic separation can be omitted altogether if a reagent or enzymatic reaction is used that is specific for this particular amino acid. Marko-Varga et al. (1993) reported on enzymatic detection systems specific for amino acids. Pohlmann et al. (1990) have developed an enzymatic flow-injection method which is specific for lysine and allows up to 30 assays per hour. Using a similar flow-injection analysis, Lavagnini et al. (1993) determined lysine concentrations in feed hydrolysates and Chen et al. (1996) analysed supplemented lysine in fish food. Vrbova et al. (1992) described an enzyme electrode specific for l-lysine which enabled its determination in wheat extracts. A fluorimetrie assay of lysine in feed hydrolysate following reaction with l-lysine oxidase was described by Hikuma et al. (1991) which showed a high degree of concordance with IEC for compound feeds, fish meal and soy. Ninhydrin-ferric reagent was used by Hsieh et al. (1995) for the determination of lysine in culture broth. Colorimetric assays of tryptophan have been mentioned above.
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