Effect of heating temperature on particle size distribution in hard and soft wheat flour

The particle sizes of soft and hard wheat (Triticum aestivum L.) flours at isothermal temperatures were determined by laser diffraction analysis. Flour samples were suspended in water at temperatures ranging from 30 to 80°C, for 20-60 min. All flour particles exhibited trimodal size distributions, with the particles of the first mode <10 μm, the second mode from 10 to 40 μm, and the third mode from 41 to 300 μm. Control experiments with isolated starch and gluten indicated that the first and second modes were mainly associated with starch granules, while the third mode was related to gluten and particle clusters. Soft wheat flours showed higher volume fractions in the first and second modes, indicating more dissociated starch granules. Particle size distributions of the wheat flours were temperature dependent. At 60°C, significant changes in particle size and distribution of flours were observed. This result was attributed to swelling of starch granules in response to elevated temperature. As temperature increased, the peak particle size increased. Hard wheat flours had a higher volume fraction of particles ranging above 120 μm at elevated temperature. Our results suggest that the difference in size of single particles and starch-protein aggregates, when comparing soft and hard wheat flour, is due to the strength of starch-protein interactions.