Fats, crystalline microstructure

Finally, the crystalline microstructure in many foods is not the only microstructural element of interest. Often crystal dispersions are found alongside other structures, such as air cells, fat globules, protein micelles, liquid crystals, and others. The interactions among these structural elements will be the focus of future studies in complex foods. 

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... 

To truly control crystallization to give the desired crystalline microstructure requires an advanced knowledge of both the equilibrium phase behavior and the kinetics of nucleation and growth. The phase behavior of the particular mixture of TAG in a lipid system controls both the driving force for crystallization and the ultimate phase volume (solid fat content) of the solidified fat. The crystallization kinetics determines the number, size, polymorph, and shape of crystals that are formed as well as the network interactions among the various crystalline elements. There are numerous factors that influence both the phase behavior and the crystallization kinetics, and the effects of these parameters must be understood to control lipid crystallization. 

Figure 13.7 Effect of cooling rate on crystalline microstructure (polarized light microscopy) of a milk fat model system (50% high-melting and 50% low-melting component of milk fat) crystallized at 25 °C and agitation rate of 200RPM. Cooling rate (a) 0.20°C/min and (b) 5.50°C/min. (From Herrera and Hartel 2000c with permission.)...

Elena DA, Rodrigues JN, AG L. Effects of crystalline microstructure on oil migration in a semisolid fat matrix. Crystal Growth and Design. 4(4) (2004) 731-736. 

The rate of oil ingress is significantly lower in tempered cocoa butter (see figure 6). It is believed the dense and uniform crystalline microstructure obtained during tempering increases the fat matrix tortuosity and thus reduces the oil migration rate. 

The structure (e.g., number, size, distribution) of fat crystals is difficult to analyze by common microscopy techniques (i.e., electron, polarized light), due to their dense and interconnected microstructure. Images of the internal structures of lipid-based foods can only be obtained by special manipulation of the sample. However, formation of thin sections (polarized light microscopy) or fractured planes (electron microscopy) still typically does not provide adequate resolution of the crystalline phase. Confocal laserscanning microscopy (CLSM), which is based on the detection of fluorescence produced by a dye system when a sample is illuminated with a krypton/argon mixed-gas laser, overcomes these problems. Bulk specimens can be used with CLSM to obtain high-resolution images of lipid crystalline structure in intricate detail. 

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