|M.Sc Student||Shamai Keren|
|Subject||Polymorphism of Resistant Starch|
|Department||Department of Biotechnology||Supervisors||Professor Havazelet Bianco-Peled|
|Dr. Eyal Shimoni|
Part of the dietary starch, called Resistant Starch (RS), is resistant to enzymatic digestion in the human intestine. The microbial flora in the colon ferments a fraction of this RS, termed RSIII. RSIII has health benefits such as pro-biotic effects, improving lipid and cholesterol metabolism, and reducing the risk of colon cancer. Potential uses of RSIII as a food additive require development of methods for producing it in a controlled manner. One possible rout to produce RSIII is heat-induced gelatinization of native starch followed by recrystallization. Previous studies have shown that the amount of RSIII produced and its enzymatic resistant is effected by the starch composition, heat treatment and recrystallization conditions. Yet, very little is known on the colloid structure of resistant starch, and the effect of the different crystal polymorph structures (i.e. molecular-level structure, as determined using x-ray diffraction) on RS III prebiotic behavior is not clear.
This research is part of a project that explores the relation between the structural properties and prebiotic behavior of RSIII. We developed a procedure by which different polymorphs of RSIII can be produced from the same source, and applied this procedure to three different types on native starch. Our results revealed that for all types of starch studied, the polymorph structure was determined by the crystallization temperature, but wasn't affected by the chemical composition or the crystalline structures of the native starch. Retrogradation at low temperature leads to formation of polymorph B, with some degree of order. Small-angle x-ray scattering (SAXS) experiments have shown that the crystallinity is much lower than in the granular form but the lamellas are arranged in long-range periodicity. Retrogradation at high temperature, however, leads to formation of a mixture of polymorphs A and V. No defined structure can be seen, the degree of crystallinity is very low and the system is better described as dilute particulate system. DSC measurements showed no differences in the thermal stability between the polymorphs.