Texture of fats, polymorphism and

The texture of fat is influenced by a number of factors, including fatty acid and glyceride composition, solid fat content, crystal size and shape, nature of die crystal network, polymorphism, mechanical treatment, and temperature history (1). Many of these factors are interrelated, making it difficult to establish the independent effect of each. It is well recognized diat different fats preferentially occur in either P or p form (Table 1). 

Control of texture and morphology of fat-based products by influencing neutral lipid polymorphism, i.e., the nature and size of triglyceride crystals... 

The sensory properties, especially texture and appearance, of milk fat-based products such as butter, cream, cheese, ice cream and milk chocolate are largely dependent on the physical properties of the product, especially properties governed by the phase change behavior of the fat, used here to mean melting and crystallisation behavior, crystal polymorphism and microstructure (Birker and Padley, 1987 O Brien, 2003). The same may be said of the functional properties of milk fat, milk fat fractions and milk fat-based products when these are used as food ingredients. 

Many interrelated factors influence the texture of plastic fats. Fatty acid and glyceride composition are basic factors in establishing the properties of a fat. These factors, in turn, are related to solid fat content, crystal size and shape, and polymorphic behavior. Once the crystal network is formed, mechanical treatment and temperature history may influence the texture. 

The dispersion of the crystalline fat phase in a material determines the physical and textural properties of a lipid-based product. For example, the hardness, snap, and glossy appearance of chocolate is caused by crystallization of cocoa butter in the form of numerous, very small (1 pm or less) crystals of the most stable polymorph (p form). The size distribution (mean size and range of sizes), polymorphic form, and shape of the fat crystals, as well as the network formed among the crystals, all play important roles in determining physical attributes of lipid-based products. 

Fat crystallization has been extensively studied in bulk fats and, to a lesser extent, in emulsified fats. It has been shown that the crystallization behavior of a fat will proceed quite differently, depending on whether it is in bulk or emulsified form (4,5). Authors have examined the effect of the state of dispersion on the crystallization mechanisms (nucleation, crystallization rate) and polymorphic behavior (6-11) of partial- and triglycerides in bulk and emulsified form. Understanding the mechanisms of emulsion nucleation and crystallization is one of the first steps in understanding the destabilization of emulsions and partial coalescence, e.g., stabilization of liquid fat emulsions by solid particles (fat) or control of the polymorphic form of crystals during the process of partial coalescence to control the size of aggregates and textural properties. 

The distribution of fatty acids in the TAG of natural fats also determines dieir polymorphic behavior and, therefore, the texture of these fats. This is exemplified by the difference between lard and tallow (Table 17). Lard has none of the symmetric SUS glycerides, whereas in tallow there is 21%. The result (Table 18) indicates that lard crystallizes in the P form and tallow in die P form. The levels of TAG 54 and 16 0 in the high-melting glycerides of these fats are roughly similar. 

Oils and fats go through a series of increasingly organized crystal phases upon cooling. This multiple form of crystallization (polymorphism) is an important characteristic of fats and oils because it greatly influences the textural and functional properties of fats and fat-based products. 

Similarly, cocoa butter (CB) will develop a texture of good consistency and uniform matrix in chocolate if the fats solidify from the melt into a crystalline IV or V polymorphic modification of the cocoa butter. Severe fat migration (fat bloom) will take place after prolonged storage or temperature fluctuations if the CB modification is crystallized or transformed in situ into the VI form. Several additional products suffer from similar problems. It is well documented that the formation of crystal structure and the solution- or solid-mediated transformations can be delayed, slowed, or enhanced by small amounts of specific amphiphiles dissolved or dispersed in the fat during the crystallization stages. 

Proper control of the crystalline microstructure leads to products with the desired textural properties and physical characteristics. For example, tempering of chocolate prior to molding or enrobing is designed to control crystallization of the cocoa butter into a large number of very small crystals that are aU in the desired polymorphic form. When controlled properly, the cocoa butter crystals in chocolate contribute to the desired appearance (shine or gloss), snap, flavor release, meltdown rate upon consumption, and stability during shelf life (fat bloom). Similar... 

Fats that crystallize in the p polymorphic form tend to be more coarse textured with large granular crystals. They are poor aerators yet they function well in pie crust applications. Lard crystals tend to be large and grainy, but pie crusts formulated from it have earned wide acceptance because of their flaky texture. 

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