Naegeli amylodextrin and its relationship to starch granule structure. II. Role of water in crystallization of B‐starch

Abstract Although native B‐type starch loses its sharp X‐ray pattern on strong drying, the wet Nägeli amylodextrin prepared from such starch gives a sharp B‐pattern, that is only slightly reduced in sharpness by complete drying. Nägeli amylodextrin dried admixed with a noncrystalline “filler” (e.g., starch sirup) gives an X‐ray pattern essentially identical to that of the wet amylodextrin, except that the 16‐Å (No. 1) ring is greatly reduced in sharpness and intensity. Failure of the B‐structure to collapse during drying indicates that water is not intercalated between turns of a helix or otherwise required to maintain the geometry and packing arrangement of the starch molecular chains. Rather, for native starch granules or amylose fibers, water relieves intercrystallite strains and consequent crystallite distortion induced by strong drying. For Nägeli amylodextrin, the small molecules of starch sirup can penetrate the interstices between crystallites and prevent strain build‐up during drying. For native starch granules, the interstitial regions are gel‐like in character and less permeable to starch sirup molecules. A model for B‐starch is proposed, that employs intertwined (double) helices.