Granule size and rheological behavior of heated tapioca starch dispersions

Rheological behavior of starch dispersions is strongly influenced by the gr anules. From granule size data of 2.6% tapioca starch dispersions (STDs) is othermally heated in the range 61 degrees C-74 degrees C obtained by laser diffraction, the maximum average granule size was found to be 3.7 x the raw starch granule mean diameter. Compared to corn and cowpea starches, heated tapioca starch granules had a higher initial maximum diameter that was les s dependent on the heating temperature, and the ratio of the heated to unhe ated starch granule diameter ratio contributed to a higher increase of noti onal volume fractions (ce) of the tapioca STDs. Shear-thickening flow behav ior was observed in STDs heated at 61 degrees C for 1, 5 and 15 min, and sh ear-thinning in all other dispersions. The Casson and Quemada models descri bed well the apparent viscosity versus shear rate data of the STDs at diffe rent notional volume fractions. The power law consistency index increased e xponentially with granule size reflecting increase in dispersion viscosity. The relative viscosity vs ce data of the STDs followed Quemada's structura l model. A modified Cox-Merz rule described the relationship between comple x viscosity and apparent viscosity of a 4% dispersion. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.