Rheology of milk fat

Setting, Spreadability, Hardness, Work Softening and Thixotropy 

Several rheological characteristics of milk fat and butter have practical significance. Setting, spreadability, hardness, work softening and thixotropy are affected by the rheology of milk fat. Setting refers to the continued increase in the firmness of newly manufactured butter. Increases in firmness 

Hardness and spreadability are inversely related. They can be investigated by sensory evaluation (deMan et al., 1979 Dixon and Parekh, 1979 Mortensen and Danmark, 1982b Pokorny et al., 1984 Rohm and Ulberth, 1989) or instrumental methods as discussed previously. 

In cases where the region of elastic deformation is very small compared the total deformation applied, the yield stress is sufficient to characterize the behavior of a material. Essentially, such a material is classified as a rigid 

The rheological properties of many dairy products are strongly influenced by the amount and melting point of the fat present. The sensory properties of cheese are strongly influenced by fat content but the effect is even greater in butter in which hardness/spreadability is of major concern. The hardness of fats is determined by the ratio of solid to liquid fat which is influenced by fatty acid profile, fatty acid distribution and processing treatments. 

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De Man (1983) has reviewed this property of fats. Consistency is defined as (1) an ill-defined and subjectively assessable characteristic of a material that depends on the complex stress-flow relation or as (2) the property by which a material resists change of shape. Spreadabil-ity, a term used in relation to consistency, is the force required to spread the fat with a knife. The definition is similar to that for hardness the resistance of the surface of a body to deformation. The most widely used simple compression test in North America is the cone penetrometer method (AOCS Method Cc 16-60, 1960). More sophisticated rheological procedures are also available. Efforts have been made to calibrate instrumental tests with sensory response. With the cone penetrometer method, penetration depth is used as a measure of firmness. Hayakawa and De Man (1982) studied the hardness of fractions obtained by crystallization of milk fat. Hardness values obtained with a constant speed penetrometer reflected trends in their TG composition and solid fat content. 

Colloidal interactions between emulsion droplets play a primary role in determining emulsion rheology. If attractions predominate over repulsive forces, flocculation can occur, which leads to an increase in the effective volume fraction of the dispersed phase and thus increases viscosity (McCle-ments, 1999). Clustering of milk fat globules due to cold agglutination increases the effective volume fraction of the milk fat globules, thereby increasing viscosity (Prentice, 1992). 

Methods used to Determine the Rheological Properties of Milk Fat... 

Large Deformation Rheological Testing of Milk Fat and Butter... 

Compression testing is also useful for evaluating the technological and end-use properties of milk fat and butter (Davis, 1937 Scott-Blair, 1938 Dolby, 1941a Mohr and Wellm, 1948). In this approach, a uniform stress is applied to the top and bottom of a sample (typically a cylinder or prism) placed between two flat plates. Because of the uniform stress field, it is easier to extract rheological information than in penetrometery. 

Solidified milk fat displays non-Newtonian behavior. It acts as a plastic material with a yield value (Sone, 1961 deMan and Beers, 1987). Throughout its wide melting range, milk fat, like butter, exhibits viscoelasticity, possessing both solid and liquid-like characteristics (Sone, 1961 Shama and Sherman, 1968 Jensen and Clark, 1988 Kleyn, 1992 Shukla and Rizvi, 1995). Several models to describe the complex rheological behavior of milk fat have been proposed. Figure 7.12 shows the corresponding stress-strain curves for the models discussed. 

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