Loss of crystalline and molecular order during starch gelatinisation: origin of the enthalpic transition

The disruption of molecular oders which occur during the gelatinisation of starch granules has been studied by isolating dried samples from maize, waxy maize, wheat, potatoe, and tapioca starches after defined thermal pre-treatments. Residual molecular and crystalline order was quantified by 13C-c.p.-m.a.s.-n.m.r. spectroscopy and powder X-ray diffraction, respectively, and the results compared with residual gelatinisation enthalpy determined by d.s.c. For native starches, molecular (double-helical) order was significantly greater than crystalline order. Molecular and crystalline order were both found to correlated with the residual enthalpy of gelatinisation following thermal pre-treatment, indicating that both levels of structure are disrupted concurrently during gelatinisation. From the data obtained, predicted enthalpy values for the disruption of fully ordered crystalline analogues of the starches studied were calculated, and compared with values for essentially fully ordered and crystalline model material. This comparison suggests that the enthalpy of gelatinisation primarily reflects the loss of molecular (doube-helical) order.