Structure/rheological properties relations of crosslinked potato starch suspensions
Résumé
Some rheological properties of aqueous suspensions of crosslinked hydroxypropylated potato starches were investigated. The crosslinking was performed at a temperature above the gelatinization temperature and the degree of crosslinking varied between 0.05 and 0.75% wt. The samples were characterized by the swelling power Q and the average particle size D and observed by light microscopy and low‐temperature SEM. The swelling power decreases with the extent of crosslinking, whereas the average particle size increases. From these observations, the starch samples can be described as suspensions of deformable particles suspended in a continuous suspending medium constituted of polymer chains plus water. The rheological properties allow us to define different concentration regimes depending on the close packing of swollen particles at a concentration C*, which is close to 1/Q. In dilute conditions, that is, for C < C*, the viscosity is little influenced by the degree of crosslinking and, hence, by the particle size. Two domains can be defined: a “true dilute” one for C < 0.6C* and a semidilute one for C > 0.6C*, in which the viscosity rapidly increases with the concentration. Moreover, an apparent yield stress and elastic properties are observed in the concentrated regime, for C > C*. Two domains can be also defined: a first one for C < 2C*, in which the rheological characteristics sharply increase with the concentration, and a second one, a quasi‐plateau domain, for C > 2C*. The variations of these rheological properties suggest that the rigidity of the swollen particles can be considered, contrary to the viscosity in the true dilute regime.