Modelling of physical ageing in starch using the TNM equation

Gelatinised wheat starch, freeze dried and equilibrated at different RH, was aged at different temperatures and for different times. The Tool-Narayanaswamy-Moynihan (TNM) model was used to describe the ageing for all samples under all conditions. Three TNM parameters: x, Δh* and A were determined experimentally using, respectively, the peak shift method (x) and the dependency of T'(f) (the limiting value of T(f)) on the cooling rate (Δh* and A). The non-linearity parameter x and the non exponential parameter β were also estimated by optimising a fit of the experimental normalised specific heat at different ageing times and temperatures to curves generated using the TNM model. The TNM model successfully described the normalised experimental data. It was found that the intermolecular forces were strong and the relaxation times depended more strongly on the glass structure than the glass temperature. The hydration level of the starch had a direct impact of the breadth of the relaxation time distribution. A dependency of the non-linearity parameter x on ageing temperature (peak shift method) was observed. This suggests that physical ageing is more complex than is described by TNM formalism.