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C. YU, Food and Bioprocess Engineering Laboratory, Biological System Engineering Department, University of Wisconsin-Madison, 460 Henry Mall, Madison, WI 53706 and S. GUNASEKARAN.

The steady flow viscosity data of food materials (such as yogurt, cheese etc.) are required for predicting their performance in many practical applications such as mixing and extrusion processing. These applications usually involve high temperatures and high deformation rates. Under these conditions, it is extremely difficult to get such data directly using standard rheological methods. Polymer rheologists have established the Cox-Merz rule and modified Cox-Merz rules to deal with this kind of difficulty. These rules are based on the similarity between the shear rate dependence of steady flow apparent viscosity and frequency dependence of dynamic viscosity. Therefore, the Cox-Merz rule and its modified forms allow for estimating steady flow viscosity data from the more readily obtainable dynamic rheological properties data. In addition, useful information on polymer structure can also be obtained.

Our objective was to investigate if Cox-Merz rule (or its modified forms) is applicable to food materials.

Different food materials (condensed milk, mayonnaise, yogurt and Mozzarella cheese) were investigated. Their steady shear viscosity and dynamic complex viscosity were determined by rheological measurements at two temperatures using a Bohlin-CVO rheometer.

Based on experimental data, shear rate dependence of steady flow apparent viscosity and frequency dependence of dynamic viscosity was established and compared. It was found that for condensed milk and mayonnaise, a modified Cox-Merz relation can be established and for yogurt, a deviation from the Cox-Merz rule was found. For Mozzarella cheese, a very interesting phenomenon was found, at low shear rate (<2 s^-1), Cox-Merz rule was applicable; at high shear rate (> 10 s^-1), a modified Cox-Merz relation was established with a shift factor.

These results suggest that for some foods, Cox-Merz rule (or its modified form) is applicable, thus we can use much more reliable SAOS (small amplitude oscillatory shear) test data to predict rheological behavior under steady flow condition. This will give the necessary information to achieve a better quality control in processes such as mixing and extrusion.