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S. KASAPIS and S. Sablani. Food Science and Nutrition, Sultan Qaboos University, P.O. Box 34, Al-Khod, Muscat, 123, Oman

Rheological studies have been employed to probe the thermal effect on the phase behaviour of starch but measurements have been confined to temperatures below 100°C. However, most of food-related applications of starch (e.g. UHT, bakery fillings, extrusion, etc.) are performed at higher temperatures and may lead to additional physicochemical transformations.

The present work focuses on changes in the viscoelastic properties of starch above 100°C with a view to discussing them in terms of underlying fundamentals.

Small deformation oscillatory studies were performed on wheat flour paste with a starch content of 75.4%. The moisture content of our dough was reduced from about 32% at 100°C to 6.5% at 130°C. Heating produced a sigmoidal profile of viscoelasticity with a disproportionate viscous element also seen in the glass transition of semiamorphous synthetic polymers and high sugar/polysaccharide mixtures during cooling. Within the temperature range of 100 to 130°C, mechanical spectra from 10e-1 to 10e2 rad/s were obtained at constant temperature intervals of three degrees. The time-temperature superposition principle was used to produce a smooth composite curve of twelve decades of frequency from a single set of shift factors for both the storage and loss modulus of the starch network (G' and G", respectively).

It is proposed that the loss of water with heating reduces the available free volume between neighbouring chain segments thus generating a high-density thermoplastic melt suspending granule fragments. The configurational rearrangements of the disordered chains contribute mainly to an energy dissipating process, as observed in the vitrification of cooled high solids systems. The equation of Williams, Landel and Ferry was modified with a 'moisture term' in order to describe the temperature function of viscoelasticity and predict the rheological glass transition temperature of starch (163°C).