Fat crystal networks

Marangoni, A.G. and Hartel, R.W. 1998. Visualization and structural analysis of fat crystal networks. FoodTechnol. 52 46-51. 

Modeling Fat Crystal Networks and Relating Structure to Rheology... 

Based on this description of a fat crystal network, it makes sense that its macroscopic properties should depend significantly on the nature of the microstructures since this level of structure is closest to the macroscopic world. 

Figure 7.13. Schematic model of a fat crystal network showing particles of diameter a (i.e., small circles, <10 p,m) arranged into clusters of diameter f (i.e., large circles, > 100 p,m) with liquid oil interspersed.

Figure 7.14. Schematic of fat crystal network under extension when the weak-link theory is applicable ( is the diameter of crystal clusters, La is the size of the microscopic system, AL is the extension due to elongational stress, a is the size of a primary particle within a cluster and da is the interfloc distance).

The fractal dimension of the fat crystal network in milk fat decreased from 2.5 to 2.0 when the cooling rate was increased. Concomitantly, the particle-related constant, A, increases. These results demonstrate how a faster cooling rate leads to a less ordered spatial distribution of mass within the microstructural network, which would result in a lower value of D, and a decrease in the average particle diameter, which would result in a higher value of A, as predicted by our model. These microstructural changes were correlated with a much higher yield force value for the rapidly cooled milk fat (64.1 3.3N versus 33.0 3.9N for the samples cooled at 5.0°C/min and 0.1°C/min, respectively). 

Narine, S.S., Marangoni, A.G. 1999a. Relating structure of fat crystal networks to mechanical properties a review. Food Res. Int. 31, 227-248. 

Narine, S.S., Maranongi, A.G. 2001. Elastic modulus as an indicator of macroscopic hardness of fat crystal networks. Lebensm. Wiss. Techonl. 34, 33—40. 

Rye, G., Litwinenko, J., Marangoni, A.G., 2005. Fat crystal networks - structure and rheology. In, Bailey s Industrial Oil Fat Products (F. Shaihidi, ed.), John Wiley, New York (In press). 

Other methods of imaging fat crystals and fat crystal networks (not all of them optical) include confocal laser scanning fluorescence microscopy, multiple photon microscopy, atomic force microscopy and electron microscopy (Narine and Marangoni, 1999). 

The network systems in plastic fats differ from those in protein or carbohydrate systems. Fat crystals are embedded in liquid oil and the crystals have no ionized groups. Therefore, the interactive forces in fat crystal networks are low. The minimum concentration of solid particles in a fat to provide a yield value is in the range of 10 to 15 percent. 

DeMan and Beers (1987) have reviewed the factors that influence the formation of three-dimensional fat crystal networks. The fat crystal networks in plastic fats (Figure 8-44) are highly thixotropic, and mechanical action on these products will result in a drastic reduction of hardness. 

A variety of rheological tests can be used to evaluate the nature and properties of different network structures in foods. The strength of bonds in a fat crystal network can be evaluated by stress relaxation and by the decrease in elastic recovery in creep tests as a function of loading time (deMan et al. 1985). Van Kleef et al. (1978) have reported on the determination of the number of crosslinks in a protein gel from its mechanical and swelling properties. Oakenfull (1984) used shear modulus measurements to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. 

Marangoni, A.G. (2005). Fat Crystal Networks. Marcel Dekker, New York. 

Narine, S.S., and Marangoni, A.G. (1999b). Mieroscopic and rheological studies of fat crystal networks. J. Cryst. Growth. 198, 1315-1319. 

This section attempts to relate the micro structural organization of fat crystals to the mechanical properties. The importance of hierarchies in structural organization will again be stressed in this section in an attempt correlate micro structure to macroscopic properties. Figure 17.7 depicts the hierarchies in a fat crystal network structure. Past work has focused on lipid composition, polymorphism and solid fat content to interpret the mechanical strength of the network (Kamphuis and Jongschapp 1985 Papenhuijzen 1971, 1972 Payne 1964). 

There are several techniques used to image the microstructure of fat crystal networks. (See Chapter 11 on Imaging. ) The most commonly used imaging method is polarized light microscopy (PLM) since fat crystals are birefringent and appear white, while the liquid oil is not and thus appears black. 

In recent years, many other techniques have been employed to elucidate the structure of fat crystal networks including confocal laser scanning fluorescence microscopy (Heertje et al. 1987) and multiple photon microscopy (Marangoni and Hartel 1996). Another advance has been the development of three-dimensional imaging. 

Figure 17.7. Structural hierarchy of fat crystal networks (Narine and Marangoni 2005).

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