Phase and polymorphic transitions of starches at low and intermediate water contents

Abstract

The plasticizing and structuring roles of water are described for native and hydrolysed starch at different temperatures between the glass and melting transition temperatures. Water, which acts as a plasticizer of amorphous regions, is also present in the crystallites and improves the crystallinity, increasing the resolution of X-ray diffraction diagrams. Polymorphic conversions and crystallinity modifications may be induced by hydrothermal treatments at temperatures below the melting temperature of the most stable form among the recognized allomorphs, i.e, the B-type water-rich crystalline hydrate of tuber and amylose starches, the A-type lower hydrate of natural cereal starches and the complexed single helical forms obtained in less polar environments either hydrated (Vh) or anhydrous (Va). The conditions needed to obtain various solid state conversions are given for Vh to Va or Vh to B-type at ambient temperature and B to A upon heating. In all cases for given temperature and hydration conditions, the kinetics depend on the chain length involved. In contrast to the situation with short independent chains obtained by acid hydrolysis of starch, which can rearrange themselves easily, annealing and ‘heat moisture treatments’ never increase the crystallinity of native starches although their melting behavior is strongly modified. These different transitions are discussed in terms of glass transition and melting temperatures, composition and known structures of starchy substrates.