Granule size and rheological behavior of heated tapioca starch dispersions

Rheological behavior of starch dispersions is strongly influenced by the granules. From granule size data of 2.6% tapioca starch dispersions (STDs) isothermally heated in the range 61°C¯74°C obtained by laser diffraction, the maximum average granule size was found to be 3.7×the raw starch granule mean diameter. Compared to corn and cowpea starches, heated tapioca starch granules had a higher initial maximum diameter that was less dependent on the heating temperature, and the ratio of the heated to unheated starch granule diameter ratio contributed to a higher increase of notional volume fractions (cQ) of the tapioca STDs. Shear-thickening flow behavior was observed in STDs heated at 61°C for 1, 5 and 15 min, and shear-thinning in all other dispersions. The Casson and Quemada models described well the apparent viscosity versus shear rate data of the STDs at different notional volume fractions. The power law consistency index increased exponentially with granule size reflecting increase in dispersion viscosity. The relative viscosity vs cQ data of the STDs followed Quemadaʹs structural model. A modified Cox¯Merz rule described the relationship between complex viscosity and apparent viscosity of a 4% dispersion.