Determination of the degree of methyl esterification of pectins in small samples by selective reduction of esterified galacturonic acid to galactose.

A procedure for the determination of the degree of methyl esterification of pectin in virtually any sample is described. Samples were dissolved or suspended in 1 M imidazole buffer, pH 7.0, cooled on ice, and reduced with sodium borohydride. Quantitative reduction of samples was accomplished after 1 h using at least 20 mg sodium borohydride/mg sample. The degree of methyl esterification was determined by either the increase in galactose content as determined by GLC of the sugar or by the change in galacturonic acid content by colorimetric uronic acid analyses. Sample requirements were at least as low as 100 micrograms per determination by GLC or 2 to 3 mg per determination by colorimetric uronic acid analysis compared to 5 mg or more per determination for other published procedures. The degrees of methyl esterification determined by the methods described have compared very favorably with those determined by established methods.