Total vitamin C content was determined using the 2, 4-
dinitrophenylhydrazine method [21]. The 0.2 g dried sample was extracted
with 8 mL of 1% oxalic acid solution in ultrasonic bath for
30 min. The supernatant was filtered to 25 mL volumetric flask and
the volume was made up with 1% oxalic acid. 10 mL supernatant was
mixed with 0.2 g active carbon, fully shaken for 1 min and filtered
again. 5 mL supernatant was mixed with 5 mL of 2% thiourea solution.
Then, 4 mL mixed solution was added to 1.0 mL of 2% 2, 4-
dinitrophenylhydrazine solution, and was kept in 37 ± 0.5 °C water
bath for 3 h. After reaction, 5 mL of 85% sulfuric acid was gradually
added to the reaction solution. The absorbance of solution was measured
at 500 nm, and results were expressed as mg 100 g−1 DW.