Investigations into the softening of normal and waxy starches during heating at moderate water content

The objective of this study was to evaluate the ability of a single compression-relaxation test to qualify the softening behaviour of four different starch samples – normal wheat, waxy wheat, normal rice and waxy rice – during heating and within a range of water content (WC) close to that of starch in bread dough during baking (moderate WC). Temperatures were selected according to the gelatinization behaviour of the different starches. Compression-relaxation tests at 10% strain were performed to identify the step-responses of the materials. The fast compression minimised the relaxation effect during the test, enabling Young’s modulus and dynamic viscosity to be determined independently. Young’s modulus values ranged from 130 to 1,700 kPa, while dynamic viscosity values varied from 7×10 6 to 7×10 8 Pa·s across all samples analysed. Microscopic observations and DSC confirmed that starch granules swelled and gelatinized to different extents depending on local liquid water availability. Waxy starches softened more markedly, with Young’s modulus approaching that of gluten at 80°C. These findings demonstrate the effectiveness of compression-relaxation tests in assessing starch softening and provide a basis for modelling stress distribution around granules during gas cell expansion. • Compression-relaxation at 10% strain can be used to evaluate the softening behavior of starch granules. • Fast compression and long relaxation enable Young's modulus and dynamic viscosity to be determined independently. • Starch softening was explored at water contents of 40-50% (wb) and temperatures of 20-80 °C. • Waxy starches were confirmed to soften more than non-waxy starches, reaching a Young's modulus similar to that of gluten at 80°C. • Swelling and gelatinization varied with local water availability around starch granules.