Margarines: A rheological study
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Time-dependent flow properties of both commercial and pilot plant-made margarines were characterized under steady shear. Flow curves were fitted to the kinetic expression τ = τi +a e−k1t +b e−k2t. A first-order kinetic model did not describe the observed destruction process of the margarine structure in an adequate manner. In the proposed model, two structures with different destruction rates are postulated. Each structure contributes to a part of the shear stress necessary to deform the margarine sample. The parameters τia andb are temperature-dependent. Decreasing temperatures produce an exponential increase of τi, a linear increase ofb and an increase ofa up to a point beyond which it remains constant. A physical interpretation of the model is proposed. The role of the aqueous phase was also studied. Greater hardness was detected at higher water content. Parameter τi increased, at any selected temperature, with increasing aqueous phase content of the sample. Parametera increased with decreasing temperatures and higher water content. On the other hand, parameterb was not affected by the amount of aqueous phase. This kinetic model could be employed to perform studies on the influence of different parameters of margarine formulation on its rheologic behavior.
Key wordsDifferent water contents margarine mathematical model physical interpretation rheology texture thixotropic behavior
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