Total amino acid (TAA) composition was analyzed according to
the method described by Spackman, Stein, and Moore (1958) with
modifications. Briefly, 1.0 g sample and 10 ml of 6 M HCl were
added into an empty tube, which was then sealed under vacuum
and incubated at 110 ± 1 C for 24 h. For determine the free amino
acid (FAA) composition, 2 ml of hydrolysates at different time
intervals of hydrolysis were mixed with equal amount (w/v) of
7% (v/v) 5-sulfosalicylic acid dehydrate respectively and incubated
for 30 min to remove protein. The supernatant was then collected
by centrifugation at 16,000g for 20 min and used to determine the
FAA composition. The amino acid compositions for the samples
were measured by L-8900 Amino Acids Automatic Analyzer (Hitachi
LTD, Japan).