High-amylose wheat and maize starches have distinctly different granule organization and annealing behaviour: A key role for chain mobility

For regular starches, gelatinization occurs over a relatively narrow temperature range in excess water; however, high-amylose starches typically gelatinise over an extended temperature range by a process that is less understood. The present study compared granule organisation of high-amylose starches from wheat (HAWS) and maize (HAMS), and investigated the role of granule organization in determining the gelatinization heterogeneity and different responses to annealing. During gelatinization (starch/water = 1:2), the melting peaks of double helices and amylose-lipid complexes are merged in HAMS, whereas the peaks are separated for HAWS. It is hypothesized that the glucan chains in HAWS are relatively more mobile (i.e., fewer inter-chain interactions), compared to HAMS. Due to this mobility, hot water leaches amylose more readily from HAWS than from HAMS at similar levels of apparent amylose content. After two-step annealing, To (gelatinization onset temperature) of HAWS increased from 58 to 84 degrees C, while To of HAMS only increased from 70 to 82 degrees C. For HAWS, annealing apparently extended double helical lengths and made semi-crystalline lamellae more heterogeneous, but reduced double helix content. For HAMS with more organised granular structure, annealing has a relatively limited effect on lamellar layers and chain conformation. The study demonstrates that HAMS and HAWS have distinctly different granule organisation and behaviours during hydrothermal processing and that HAWS, with more mobile chains, has the potential to form thermostable molecular order. The molecular basis for these differences is proposed to be the greater proportion of very long chains/branches in HAWS than HAMS.