Impact of annealing on the susceptibility of wheat, potato and pea starches to hydrolysis with pancreatin

Native, and one- and two-step annealed wheat, pea and potato starches were subjected to hydrolysis with pancreatin (1.34 nKat/mg starch, 37 °C, pH 6.0). While annealing increases enzyme resistance for wheat, pea and potato starches in the first (rapid) phase of hydrolysis, it increases the extent of degradation in the second (slower) phase for wheat and pea starches. Annealed potato starches, however, are still more resistant than native potato starch in the second phase of hydrolysis. Environmental scanning electron microscopy shows that enzymic degradation of wheat starch granules does not proceed uniformly throughout the granule population. Pancreatin action does not affect differential scanning calorimetry (DSC) gelatinisation characteristics of all studied native and of annealed potato starches. Although enzymic hydrolysis has no great effect on the DSC gelatinisation behaviour of native starches, partial enzymic solubilisation of the granules enhances the effects of annealing. After 2 and 120 h of solubilisation, DSC thermograms of annealed wheat and pea starches show somewhat broader peaks with lower enthalpies than those of the corresponding unhydrolysed starches. 13C CP/MAS NMR data of extensively (46%) degraded and undegraded native wheat starch granules show no change in double helix content, whereas after 57% solubilisation of one-step annealed wheat starch, a decrease in the proportion of double helices is observed. The 13C CP/MAS NMR signal at 31 ppm increases by a factor 2.0 for 46% solubilised native wheat starch, and by a factor 2.3 for 57% solubilised annealed wheat starch, indicating resistance of amylose-lipid complexes to pancreatin hydrolysis. Dissociation enthalpies, however, are higher than can be predicted from a concentration of complexes. The enthalpy of dissociation of amylose-lipid complexes, after enzymic hydrolysis, increases more for annealed than for native wheat starch. All the above suggest that, during annealing, molecular changes occur that have an impact on pancreatin hydrolysis.