Free amino acid concentration was measured by ninhydrin colorimetric
method [22]. The 0.5 g dried sample was extracted with 8 mL
of 10% acetic acid solution in ultrasonic bath for 30 min. Extracts were
centrifuged at 10 000g for 10 min. 2 mL of the supernatant was mixed
with 3 mL of ninhydrin and 0.1mL of ascorbic acid. The mixture solution
was heated in a water bath (100 °C) for 15min, and quickly cooled and
shaken after water bath. When the solution turned to royal purple, the
absorbance of solution was recorded at 570 nmusing a spectrophotometer,
and results were expressed as absorbance μg g−1 DW.