Inter- and Intra-Laboratory Variation in Amino Acid Analysis of Food Proteins

Amino acid compositions of casein, egg white, beef, soy isolate, rapeseed concentrate, pea flour and wheat flour were studied by seven collaborating laboratories. Samples were hydrolyzed with 6N HCl, performic acid + 6N HCl, and (with one exception) 4.2N NaOH. Amino acids were then determined by ion-exchange chromatography using automatic analyzers. One laboratory, however, determined tryptophan by treatment with p-dimethylaminobenz-aldehyde. The colorimetric determination of tryptophan was comparable to the tryptophan values determined by ion-exchange chromatography. Interlaboratory variation of tryptophan (coefficients of variation, CV, up to 24%), cystine and methionine (CV up to 17%), was greater than that of most other amino acids (CV up to 10%). Intralaboratory variation for all amino acids was less than 5%.