Determine the Rheological Properties of Milk Fat

Methods used to Determine the Rheological Properties of Milk Fat 

Most of the methods used to characterize the rheological behavior of butter are empirical and attempt to imitate certain sensory perceptions. They typically involve penetrometry, extrusion or sectility tests (Prentice, 1972). In these tests, the structure of the material is destroyed in order to probe its response to an applied stress or deformation. These methods mostly serve a quality control function. Their results provide an index of consistency to adjust milk-blending operations or to regulate a step in the butter-making process. While the results have practical significance, they often have no theoretical basis. Therefore, attempts have also been made to study the intrinsic properties of plastic fats. In many such cases, small deformation tests, in which the structure of the sample remains intact have been used to probe milk fat rheology. 

Large Deformation Rheological Testing of Milk Fat and Butter 

Cone penetrometry has the advantages of being simple and economical to use. Also, its results correlate well with testing by sensory panels (Dixon, 1974 Rousseau and Marangoni, 1999). In addition, standardized tests and commercial standards of design are available. The most widely used method is that of the American Oil Chemists Society (AOCS) (Cc 16-60) (AOCS, 1960). According to this method, the depth (d) (in increments of 0.1 mm) to which the cone penetrates the sample is read (AOCS, 1989). This depth is an indicator of consistency and can be related to some structural parameter of the material. Penetrometry results, for example, are often translated into spreadability or hardness values. For example, spreadability (S), as 

Converting penetration depth to hardness has the advantage of normalizing consistency values so that they are less dependent on the penetration load. This is the rationale behind hardness testing in metallurgy. In these cases, the contact pressure as defined by hardness in Equation 2 is used to deduce the yield stress of a material (Tabor, 1996). However, the yield stress is the resistance to an applied shear stress, but it is not the only resistance to a penetrating body. The elastic properties of a fat, and the coefficient of friction between the cone and the fat sample will also impede the penetration of the cone (Tabor, 1948). Kruisher et al. (1938) tried to eliminate friction effects and advocated the use of a flat circular penetrometer with concave sides. 

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