Starch granules are key in determining rheology.

As you know, corn starch is used to thicken foods. Adding a gum modifies the texture of the end product and minimizes syneresis. Xanthan gum produced from Xanthomonas campestris is being used extensively as a thickening agent. Cornell University scientists wanted to determine the effects of concentrations of xanthan gum and three starches on rheological characteristics: the flow behavior and vane yield stress of certain mixtures.

Xantham gum, with an intrinsic viscosity of 112.3 L per g, was used over a concentration range of 0% to 1%. Dispersions of various starches, waxy corn starch (WCS), cross-linked waxy maize (CLWM) and cold water swelling (CWS) corn with a concentration of 6% were utilized. Starch-xanthan gum dispersions were heated at 80 C for 10 minutes. Then they were cooled in a water-ice bath. Researchers used a six-blade vane (diameter of 4 cm and a height of 6 cm) attached to a commercial viscometer to determine: the static yield stress (YSS), the flow curve and the dynamic yield stress (YSD) at 20 C. YSS and YSD were determined at 0.05 s-1.

Generally, all of the dispersions were shear-thinning fluids with yield stress whose behavior could be described using the Herschel-Bulkley model. The yield stress of WCS-0.35% xanthan gum combination was higher than that of the WCS alone. Increasing the concentration of xanthan gum from 0.35% to 1.0% did not result in a large increase in yield stress.

The mixture of WCS-0.35% xanthan gum had a higher angular deformation than those mixtures with 0.5%, 0.7% and 1% xanthan gum. The yield stress of the CLWM-xanthan gum mixtures decreased dramatically with xanthan gum concentration. In contrast, the angular deformation of YSS and YSD increased. The static yield stress of CWS-gum mixtures was about 30% higher than that of the control.

The scientists found that increasing the amount of gum in the mixtures did not affect YSS. However, YSD decreased with high concentrations of the gum. The rheological behavior of the corn starch-xanthan gum mixtures depended on the type of starch used and the concentration of the gum. It appears that the state of the starch granule plays an important role in determining the rheology of the mixtures.

Further information. Andy Rao, Department of Food Science and Technology, Cornell University, New York State Agricultural Experiment Station, 630 W. North St., Geneva, NY 14456; phone: [phone]; fax: [phone]; email: [e-mail].