Evaluation of the impact of annealing on gelatinisation at intermediate water content of wheat and potato starches: A differential scanning calorimetry and small angle X-ray scattering study
The DSC (differential scanning calorimetry) thermograms of wheat and potato starches at 50% (w/w) water content are characterised by two gelatinisation endotherms. Two separate phenomena coinciding with the two DSC endotherms can be distinguished in the SAXS patterns of 50% (w/w) suspensions of wheat and potato starches during heating from 25 to 95 °C at 2 °C/min: an increase in peak integral in the temperature domain of the first (G) endotherm and a marked decrease in peak integral in the temperature domain of the second (M1) endotherm. One- and two-step annealing affect only the G endotherm, leading to a shift to a higher temperature of up to 8 °C, sharpening of the peak and an increase in enthalpy, while the completion temperature of the M1 endotherm remains unchanged. Static SAXS measurements indicate that the repeat distances of crystalline and amorphous lamellae in wheat (105 Å) and potato (99 Å) starch granules are unaffected by annealing. One- and two-step annealing intensify the SAXS peaks. The most striking difference between the SAXS gelatinisation profiles of native and annealed starches is that there is no increase in peak integral at the onset of gelatinisation of annealed starches. The effects following annealing are interpreted as a decreased water absorption during gelatinisation. Annealing leads to a retardation of the initial swelling and cooperative melting of the granules, without altering the stability of the most perfect crystallites
