Thua nao, a rich source of free-amino acids, is a fermented soybean, usually used as seasoning or flavouring
enhancer in northern Thailand. Free-amino acids (FAA) of unfermented/cooked soybeans, thua nao,
fermented by pure Bacillus subtilis TN51 (TNB51), and a naturally fermented product (TNMX), were investigated
by pre-column derivatisation with 9-fluorenylmethyl chloroformate, followed by reversed-phase
HPLC. Total FAA and essential amino acids were found at significantly higher concentrations in TNB51
thua nao than in TNMX thua nao (naturally fermented). Both fermented thua nao had much higher concentrations
of FAA than had their unfermented counterparts. With respect to taste-enhancing FAA, typical
bitter attributes of thua nao came mainly from hydrophobic and basic FAA, whereas an umami
attribute came predominantly from acidic FAA.