Distribution of methyl substituents over crystalline and amorphous domains in methylated starches

Granular potato starch and amylopectin potato starch were methylated in aqueous suspension with dimethyl sulfate to molar substitution (MS) up to 0.29. The percentage of amorphous starch compared with crystalline domains increased with increasing MS. Prolonged treatment of these methylated starches with hydrochloric acid below the swelling temperature resulted in the release of d-glucose and small d-glucose-oligomers from the amorphous domains. The granular structure was maintained during the acidic treatment, indicating that the crystalline lamellae were less affected by acid. The amorphous domains contained on average about twice as many substituents per glucose unit as the remaining crystalline network. The distributions of methyl substituents in trimers and tetramers, released from amorphous domains and prepared from crystalline fractions, were determined by FABMS and compared to the outcome of a statistically random distribution. Quantification of the degree of heterogeneity of the thus-obtained trimers and tetramers showed a much larger deviation from random substitution in the crystalline fractions compared with the amorphous ones. These results are in agreement with our previous study that describes substitution patterns in branched and linear regions of methylated starch granules.