Methoxylated galacturonic acids

The two most striking features of these time-courses are that, though the initial speed of the reaction increases markedly with the pH, the final level does not, as the reaction stops much earlier. This was not due to a drop of pH below the values suitable for P-elimination, as the pH change were slight (-0.2 at pH 11, no measurable change at pH 9). In order to check wether this plateau was due to disparition of suitable reaction sites, i.e. methoxylated galacturonic acid residues, we have investigated in detail the deesterification reaction. 

This reaction is also important in alkaline solutions, where ester bonds are mainly hydrolysed and this reaction is accompanied by depolymerisation of the polysaccharide chains (P-ehmination). The depolymerisation reaction occurs at the glycosidic bond of the non-reducing methoxylated galacturonic acid residue (Figures 4.22 and 4.23). Pectic acids are not depolymerised. 

The molecular weight (Mw), degree of esterification (DE), and content of anhydrous galacturonic acid (AG) of the citrus pectin are shown in Table 1. The molecular weight was estimated by intrinsic viscometry (calibrated against known pectin products), and the degree of methoxylation and the galacturonic acid content by titration. 

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