Branch-structure difference in starches of A- and B-type X-ray patterns revealed by their Naegeli dextrins

Abstract

Naegeli dextrins and debranched Naegeli dextrins were prepared from native starches that display A- and B-type X-ray diffraction patterns. In comparison with their debranched counterparts, Naegeli dextrins prepared from the A-type starches consistently possessed substantially more singly branched molecules than those prepared from the B-type starches. The results indicated that the A-type starches had branch points scattered in both amorphous and crystalline regions. The branch linkages present in the crystalline region might be protected during the exhaustive acid hydrolysis. The B-type starch had most branch points clustered in the amorphous region, making them more susceptible to the acid hydrolysis. These models are consistent with the previously reported amylopectin structures that the A-type starch has more short A-chains (dp 6–12) than the B-type starch. The short A-chain is likely attached to a B-chain with the branch linkage located in the crystalline region. The branch linkages present in the crystalline region and the short double helices derived from the short A-chains provide the ‘weak points’, which are more susceptible to enzymatic hydrolysis and to generate pinholes and pits to the A-type starches. Banana starch, a C-type starch resistant to enzymatic hydrolysis, produced Naegeli dextrins with substantially less singly branched chains than the A-type starches. Naegeli dextrins prepared from starches that display A-, B-, and C-type X-ray patterns have different structures. The structures of the Naegeli dextrins indicate that the A-type starch amylopectin has a scattered branch structure and the B-type has a clustered branch structure.