The Rheological Behavior and Thixotropy of a Fatty Plastic Body

The fatty plastic body such as butter or margarine possesses the remarkable non-Newtonian character in its rheological properties. The quantitative study of the viscosity, elasticity and yield value of butter has been performed under various conditions of temperature and shearing stress. The apparatus employed in the measurements involve an oscillating plate viscometer, a cone-plate viscometer and a parallel plate plastometer. The viscosity, elasticity and yield value decrease remarkably when the specimen is subjected to the kneading on the roll mill, but they recover gradually to their original values after setting the specimen for a long time. The phenomenon can be called as a kind of thixotropy. The volume dilatometry of the specimen has been also carried out continuously for a setting period of two months. From the comparison of results, the magnitude of viscosity or yield value of butter could be successfully connected to the degree of crystallization of fat in butter which had been evaluated from the specific volume of the specimen. The origin of the thixotropic softening and hardening of butter should be sought for, therefore, in the mechanism of rupture and formation of the three dimensional network of fat crystal in the specimen.