The influence of hydrochloric acid concentration and measurement method on the determination of amino acid levels in soya bean products

Accurate determination of amino acid levels in soya products facilitates optimum diet formulation and amino acid supplementation. A study was carried out to investigate the effect of hydrochloric acid (HCl) concentration and method of amino acid measurement on the measurement of amino acid levels. Six different soya bean products were evaluated. Hydrolysis was carried out with five different acid concentrations (1, 3, 6, 9 and 12 M HCl). Amino acids were analysed by both ion-exchange chromatography with ninhydrin detection and pre-column derivatisation with phenyl isothiocyanate. Both acid concentration and measurement method affected (P<0.05) measurements of most amino acids. Standard hydrolysis conditions (hydrolysis in 6 M HCl at 110°C for 24 h) rarely provided the maximal amino acid values, and thus, correction factors were calculated to standardise amino acid levels by dividing the maximum value by the value obtained with 6 M HCl. Therefore, when determining amino acid concentrations, the patterns of release and degradation of amino acids, as well as correction factors, should be considered to obtain the most accurate values.