Quantification of methoxyl groups
Methoxyl group content was measured by the GC quantification method [24]. Briefly, 20 mg of milled and dried samples were dispersed in 5 mL 57% hydriodic acid at 130 °C for 30 min with frequent agitation. Following, 10 mL of n-pentane was added with a known concentration of an internal standard, ethyl iodide. After dissolution of the generated methyl iodide in the pentane solvent, the liquid was analyzed in a gas chromatograph instrument (Hewlett Packard HP 5890, Agilent Technologies) equipped with a flame ionization detector, using with DB-WAX column (40 m × 0.18 mm i.d. × 0.3 μm; Agilent Technologies Inc., CA, USA). Temperature for the injector and detector was set to 110 and 150 °C, respectively. Oven temperature was isothermal at 80 °C, and total run time was 7 min.