A comparison of some methods for the determination of methoxyl groups in commercial pectin preparations

The results of methoxyl groups determination in pectin preparations were compared. The methods of direct (I, II) and indirect (III, IV) methanol determination connected with enzymatic (II, IV) or alkaline (I, III) pectin demethylation were applied. Higher values were obtained using indirect methanol methods than in the case of a direct methanol determination. The difference between these groups of methods was statistically significant. The precision of all the methods was high. The degree of methylation (DM) was calculated based on methanol content; moreover in the methods III and IV it was also determined from the ratio of methylated carboxyl groups to a total of acidic groups. From the difference (%) between these two ways of calculating DM the purity of pectin can be determined. The method based on enzymatic demethylation and direct methanol analysis is recommended as a precise and selective one.