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

Abstract

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.