Capillary electrophoresis for tryptophan and tyrosine was conducted
to validate if the index is correlated with the relative tryptophan
or tyrosine contents. For CE separation, all soy sauces were
subjected to ethanol precipitation instead of the salting out process
since the residual high salt content hampered the electrophoretic
separation process. Tryptophan and tyrosine were separated and
detected by A280 at 5.85 and 6.65 min, respectively. Since glutamic
acid is the representative amino acids in soy sauce (Yokotsuka,
1961), the F445
300 signal of the FIA system was calibrated with glutamate
to calculate the total amino acids content as its glutamate
equivalent. The ratios of tryptophan content (obtained by CE)
against the total amino acid content (obtained by FIA as the glutamate
equivalent) were compared with the aforementioned index,
DF445/F445
300 (Fig. 5b), and the results show a high correlation
(R2 > 0.8595) between this two parameters. The samples with
higher DF445/F445
300 values conserved more tryptophan during the
manufacturing process, which implied that these soy sauces were
made by traditional fermentation but not acidic hydrolysis. Tyrosine
was reported to be also labile (>20%) in acidic hydrolysis
(Pickering & Newton, 1990), but the regression coefficient between
DF445/F445
300 and tyrosine/total amino acids was only 0.1551. The
correlation shown in Fig. 5b was chiefly due to the influence of
tryptophan contents, so DF445 was with sufficient selectivity in
this case.