Effect of shear on the rheology and crystallization of palm oil.

UNLABELLED: This article reports on the impact of shear on crystallization upon cooling of palm oil. Samples were cooled down under shear from 70 to 10 degrees C, then kept at this temperature, while performing rheological measurements using a controlled shear rate rheometer and rheo-optical observations using optical microscopy and small-angle light scattering. Shear rates between 1 and 300 s(-1) were investigated. Two crystallization steps were observed, characterized by associated viscosity increases. The effect of shear on these 2 crystallization processes was investigated. Shear was shown to influence almost all of the steps of the structuring process of the crystallizing palm oil. The spherulite size and growth rate during the 1st crystallization are affected by shear. The onset time of the 2nd crystallization process strongly depends on the extent of shear. The steady state structures after the 1st and 2nd crystallization processes constituted of a suspension of aggregates of spherulites are controlled by the applied shear rate. PRACTICAL APPLICATION: The texture of crystallized vegetal fats and subsequent end product properties depend on the structure developed during the crystallization process. This structuring process is strongly influenced by the thermo-mechanical history applied to the product (cooling rate, degree of undercooling, annealing time, application of flow). This article shows how the shear rate as well as extent of shear affects the different steps of the crystallization and aggregation processes in the case of palm oil after the 1st crystallization.