Vanillin assays in methanol were carried out
as recommended by Price et al. (1978), in a 30 OC water
bath with a reaction time of 20 min. The vanillin reagent
contained 4% concentrated HCl and 0.5% vanillin in
methanol. Absorbance was read in a Zeiss PMQ-I1 spectrophotometer,
at 500 nm, the wavelength of maximum absorbance, by using a 1-cm cell. Samples were dissolved
in methanol.
For assays in glacial acetic acid, the vanillin reagent
contained 4% concentrated HC1 and 0.5% vanillin in
glacial acetic acid. The absorbance was read at 510 nm
as described above. Samples were dissolved in glacial
acetic acid, except for tannins, which were dissolved in a
minimum volume of methanol and diluted in glacial acetic
acid. The reaction time was shortened to 5 min, and the
methanol content was kept as low as possible to minimize
the effect of methanol on the reaction (see below)