Effect of processing conditions on physical properties of a milk fat model system: Rheology

Abstract

The effect of processing conditions on rheological behavior of three blends of 30, 40, and 50% of high-melting fraction [melting point measured as Mettler dropping point (MDP) = 47.5°C] in low-melting fraction (MDP= 16.5°C) of milk fat was studied. The effects of cooling and agitation rates, crystallization temperature, chemical composition of the blends, and time of storage on complex, storage and loss moduli were investigated by dynamic mechanical analysis (DMA). Compression tests were performed on samples using frequency values within the linear viscoelastic range (1 to 10 Hz). Loss modulus was, on average, 10 times lower than elastic modulus and was generally not affected by processing conditions. Samples showed a more solid-like behavior that was better described by storage modulus. Storage modulus varied with all processing conditions used in this study, and even for the same solid fat content, different rheological properties were found. Storage and complex modulus increased with temperature of crystallization (25 to 30°C), even though solid fat contents of samples measured after 24 h at 10°C were the same. Moduli were higher for samples crystallized at slow cooling rate, decreased with agitation rate, and were lower for the 30-70% blend at all processing conditions used. Storage moduli also increased with storage time. Shear storage modulus was calculated from the DMA experimental data, and the results were in agreement with the values reported in literature for butter systems. Fractal dimensions calculated for these systems showed a significant decrease as agitation rate increased in agreement with the softening effect reported for working of butter.In food, an understanding of rheology is critical in optimizing product development efforts, processing methodology, and final product quality (1). The rheological behavior of plastic fats is governed by interactions between fat crystals in an aggregated three-dimensional, solid-liquid matrix (2). The liquid portion of the fat, interspersed throughout the aggregated fat network, serves as a continuous phase and, in conjunction with the solid fraction, is responsible for viscoelastic behavior (3). Of primary importance to the rheological behavior of fat are the amount of crystalline fat and the type of crystals present in the fat crystal network (4,5).Rheological measurements of fats can be performed at low or high deformation. In the latter, the fat crystal network undergoes irreversible deformation, whereas in the former, viscoelasticity is measured below the yield point and any permanent strain remains upon complete release of stress. Materials can be linear elastic, elastoplastic, or nonlinear elastic. Linear elastic materials show a straight line through the origin for stress vs. strain curves. Elastoplastic materials show straight lines until the yield point is reached, and then permanent deformation occurs with higher stress. Nonlinear elastic materials do not show linear behavior in any range of stress. Margarine and butter, at room temperature, may be...

Microstructure and Material Properties of Milk Fat Systems During Temperature Fluctuations

“…In general, freeze-dried cream had a less stiff, softer, and more elastic structure than butter and AMF-based emulsion, with the latter having the most stiff and hardest structure. Among AMF-based emulsion samples, the rheological properties can be different even though the SFC level is similar, which is due to different microstructure of the crystalline network [12]. Variations in size of milk fat crystal clusters can be obtained by manipulating cooling rate or by applying shear [13,14].…”

“…Smaller crystals lead to a crystal network with more contact points, which in turn results in a material with higher stiffness than a material with less contact points [13]. However, larger crystals can be characterized by stronger connections between each other, thus by a higher elastic modulus than systems with smaller crystals and weaker connections [12,15,16]. Therefore, both particle size and nature of the interactions between crystals should be considered when evaluating material properties.…”

Milk fat in structure formation of dairy products: a review

“…For example, the rate of creaming of milk depends on the viscosity of milk plasma [13]. The content of lactose or the whey proteins in milk influence the viscosity of milk only little; fat content has a major influence, although by far the greatest influence is that of the casein content [8,14]. If fat globules are present as separate particles, the fat content is less 40% and the milk fat completely molten, milk and cream behave as Newtonian fluids at intermediate and high shear rates.…”

“…The term polymorphism describes the type of lateral packing of the aliphatic triglyceride chains [38]. The lateral packing is influenced by factors such as cooling rate, agitation temperature, and the mechanical treatment [11,14,18,41].…”

Modeling the Rheological Properties of Fats: A Perspective and Recent Advances

“…The rheological properties of fats are the result of a combined effect of the solid fat content (SFC) and the microstructure of the fat crystal networks, including the shape, size, and spatial distribution pattern of the fat crystals. 1,3,5,6,[8][9][10][11][12][13][14][15][16][17] To fully understand and eventually predict the macroscopic properties of these soft, plastic materials, it is necessary to characterize and define different levels of structure present and their respective relationship to macroscopic properties. 18 From the 1960s onwards, many attempts have been made to reveal the relationship between the elastic modulus of colloidal networks and their structural characteristics, but only a few of them have been partially successful.…”