Sources of error in microbiological determinations of amino acids on acid hydrolysates. I. Effect of humin on amino acid values.

The history of amino acid assay has been marked by numerous controversies on the problem of destruction of amino acids by acid hydrolysis of the protein or food. It has been generally conceded that when such hydrolyses are carried out in the presence of certain carbohydrates amino acids are destroyed. Recently Baldwin, Lowry, and Thiessen (1) reported that autoclaving casein with dextrose resulted in a rapid destruction of amino acids, particularly lysine and arginine. Futrell et al. (2) found that unsatisfactory recoveries were obtained for tryptophan, lysine, histidine, and methionine when food composites* were analyzed. These findings led Mertz et al. (3) to use factors to calculate the correct amino acid content of their composites from amino acid values obtained on acid hydrolysates. While running assays for essential amino acids on acid hydrolysates (4) of raw and cooked foods in this laboratory, we made several observations which may account for apparent losses of amino acids. These were (1) the effect of large and small concentrations of pyridoxamine in the basal medium on the results obtained from hydrolysates containing the humin formed during acid hydrolysis, and (2) apparent loss of amino acids when hydrolysates are allowed to stand under toluene. The present paper deals with the effect of the humin, or some substances adsorbed by the humin, on the growth of Leuconostoc mesenteroides and Streptococcus faecalis.