APPLICATION OF AN IN-LINE RHEOLOGICAL CHARACTERIZATION METHOD TO CHEMICALLY MODIFIED AND NATIVE CORN STARCH

The application of an in‐line ultrasonics‐based rheological characterization method for measuring the rheological properties of 6% (w/v) acid‐thinned and native corn starch suspensions and gels was studied. The measurements were performed in steady, laminar pipe flow using a 5 MHz frequency transducer to determine the radial shear rate distribution. Two pressure transducers were used to measure the pressure drop allowing the shear stress distribution to be calculated through the conservation of linear momentum. It was possible to obtain shear viscosity over a range of shear rates, 1–50 1/s, from a single velocity profile. A comparison of the shear viscosity function at different flow rates showed that this method could be a valid method of process rheometry during the manufacturing of starch‐based products. Both ultrasonics and rotational rheometry results showed that acid‐thinned and native corn starch suspensions exhibited Newtonian behavior before heat treatment and non‐Newtonian behavior after heating. Power law consistency index and flow behavior index of the acid‐thinned corn starch gel at 39°C were 0.60 Pa.s ^0.68 and 0.68, respectively; whereas those of the native corn starch were 5.90 Pa.s ^0.37 and 0.37 at 38C. Their consistency index increased and flow behavior index decreased with cooling and a thermoreversible change in the flow behavior index was observed in the native corn starch after storage.