29C-6 |
Rheology of xanthan/sucrose mixtures at ultrasonic frequencies |
R. SAGGIN and J. N. Coupland. Dept. of Food Science, Pennsylvania State Univ., 103 Borland Lab., University Park, PA 16802-2504 Ultrasonic shear reflectance is related to the shear modulus of fluids and therefore can be implemented to measure dynamic viscosity. The ultrasonic determination of shear properties of food fluids would be extremely valuable as a non-destructive and non-invasive probe of rheological properties. The goal of this research is to investigate the application of shear reflectance to hydrocolloid solutions and their mixtures with sugar syrups. The samples investigated in this work were xanthan gum solutions (0.1-2 wt%), sugar syrups (74-78 wt%) and mixtures of the same sugar syrups with added 0.3 or 0.5 wt% xanthan gum. Their dynamic viscosity was investigated at different temperatures (5-20°C) by ultrasonic shear reflectance or oscillatory viscometry. The shear ultrasonic reflectance was measured by using a 10 MHz broadband shear transducer and used to calculate the dynamic viscosity. The dynamic viscosity was also measured using a controlled stress rheometer operating with parallel plate geometry and conducting an oscillatory frequency sweep test. For the pure sucrose and sucrose-containing samples, there was good correlation (r2>0.97) between the two methods, but poor numeric agreement. The difference between the techniques was greater for the more viscous samples. On the other hand, the xanthan solutions were indistinguishable from water by the ultrasonic methods. The numeric deviation is attributed to the sucrose glass transition and is modeled according to the Maxwell model. Xanthan gum retards the molecular mobility of sucrose and increases its relaxation time approximately tenfold. The high-frequency shear properties of solutions can be measured non-invasively using an ultrasonic technique and are interesting in their own right both in direct applications (e.g., high speed spraying operations) and as a way to study the glass transition without changing temperature.
Session 29C, Food Engineering: Rheology and texture
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