Microstructure and mechanical parameters of five types of starch

Abstract

2Research was conducted to recognize interrelations between morphology and mechanical properties of starch. Microscopic examination, determination of particle size distribution, as well as direct shear and uniaxial compression testing were performed on five types of starch extracted out of: potato, wheat, corn, tapioca and amaranth. Regarding geometrical features (morphology) of granules, three distinct groups of materials were distinguished. Classification of materials based on the results of mechanical testing was found in a close agreement with classification based on morphology. Potato and wheat starches that had relatively large granules (d(0.5) of 41.5 and 20.2 µm) and bimodal particle size distribution showed stress-strain curves with fluctuations, particularly high in the case of potato starch. Tapioca and corn starches had smaller granules of similar sizes (d(0.5) of 15.6 and 13.8 µm, respectively). The uniaxial compression stress-strain curves of the two materials were also very similar, as well as their angles of internal friction. Granules of amaranth starch with d(0.5) of 3.0 µm were several times smaller than those of corn and tapioca starches (parameter d(0.5) is the size in microns at which 50% of the sample is smaller and 50% is larger). As a result, amaranth starch was characterised by relatively weak compressibility and flowability, the lowest of all the materials tested.