Monitoring the crystallization of amylose-lipid complexes during maize starch melting by synchrotron x-ray diffraction

Most starch granules exhibit a natural crystallinity, with different diffraction patterns according to their botanical origin: A-type from cereals and B-type from tubers. The V polymorph results essentially from the complexing of amylose with compounds such as iodine, alcohols, ol lipids. The intensity and nature of phase transitions (annealing, melting, polymorphic transitions, recrystallization, etc.) induced by hydrothermal treatments in crystalline structures are related to temperature and water content. Despite its small concentration, the lipid phase present mainly in cereal starches has a large influence on starch properties, particularly in complexing amylose. The formation of Vh crystalline structures was observed by synchrotron x-ray diffraction in native maize starch healed at intermediate and high moisture contents (between 19 and 80%). For the first time, the crystallization of amylose-lipid complexes was evidenced in situ by x-ray diffraction without any preliminary cooling, at heating rates corresponding to the usual conditions for differential scanning calorimetry experiments. For higher water contents, the crystallization of Vh complexes clearly occurred at 110-115 degrees C. For intermediate water contents, mixed A + Vh (or B + Vh for high amylose starch) diffraction diagrams were recorded. Two mechanisms can be involved in amylose complexing: the first relating to crystallization of the amylose and lipid released during starch gelatinization, and the second to crystalline packing of separate complexed amylose chains (amorphous complexes) present in native cereal starches. (C) 1999 John Wiley & Sons, Inc.