Spherulitic self-assembly of debranched starch from aqueous solution and its effect on enzyme digestibility

• Well-organized, positive spherulites were formed in debranched normal rice starch.
• Negative spherulites were observed in debranched waxy rice starch.
• Extra-long chains stabilized the radial organization of spherulites.
• Short chains were preferentially organized in tangential direction of spherulites.
• Digestibility of spherulites depends on the morphological and surface features.

Debranched waxy rice starch (DWRS) and debranched normal rice starch (DNRS) were crystallized into spherulites from aqueous solutions. DNRS, which contained extra-long chains, yielded small sintered-like spherulites with rough surface, which exhibit positive birefringence and, therefore, had radial orientations. Large spherulites with smooth surfaces, negative birefringence and tangential orientation were formed with DWRS which essentially contained short linear chains. Both spherulites showed a B-type crystalline structure with an exceptionally high crystallinity and a similar melting behavior. However, DWRS spherulites had higher resistance to enzymatic hydrolysis than DNRS spherulites. The higher enzyme resistance of DWRS spherulites was attributed to their tangential organization and their smoother and denser surface as observed by light microscopy and scanning electron microscopy. The impact of the different structural features of the two types of spherulites is discussed in relation to their resistance to enzymatic hydrolysis.

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