Ten millilitres of
chromotropic reagent (disodium chromotropate
dihydrate, Kanto Chemical, Tokyo, Japan) was
mixed with 1 mL of this extraction within the test
tube. The test tube was then placed in boiling water
with constant shaking for 45 min, and a compound
of formaldehyde and chromotropic reagent was
generated. Results were measured at an excitation
wavelength of 560 nm. The values of residual
formaldehyde were calculated using the absorbance/
formaldehyde concentration curve. Three independent
experiments were performed.