Fat crystal network structure

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). 

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). 

JM deMan, AM Beers. Fat crystal networks Structure and rheological properties. J Text Stud 18 303-318, 1988. 

Tempel (1961) surmised that fat crystal network structure consisted of an assembly of chains, each chain being an assembly of a linear array of closely aligned particles. The chains were considered to be branched and interlinked to form a three-dimensional network, with liquid fat filling the voids. Unfortunately, this theory did not successfully account for the nonlinear dependence of rheological phenomena such as yield stress and elastic modulus on the proportion of solid fat. Fat crystal networks are now viewed as being composed of haphazardly interlinked aggregates (Tempel, 1979). 

Viscoelasticity studies provide valuable data that can be correlated with fat crystal network structure. Parameters derived from such studies include the storage (solidlike or elastic) modulus (G ) and the loss (liquidlike or viscous) modulus (G") (Davis, 1973). Small-deformation measurements take place below a critical value of deformation (yield point). In plastic fats, G is substantially higher than G" (Rousseau et al., 1996c Drake et al., 1994). 

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. 

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

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. 

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).

The fractal dimension D is used to quantify the micro structure of the fat crystal networks, where d is the Euclidean dimension, x is the backbone fractal dimension that is estimated between 1 and 1.3. The backbone fractal dimension describes the tortuosity of the effective chain of stress transduction within a cluster of particles yielding under an externally applied stress (Shih et al. 1990 Kantor and Webman 1984). 

Marangoni s group has since advanced the fractal theory applying Shih et al. s weak link regime with Vreeker s rheological findings to develop a fractal theory for fat crystal networks. Fat crystal networks are considered as cross-linked fractal clusters formed by aggregating fat crystals. Self-similarity is assumed to exist within the clusters, from the primary fat crystals to the clusters. If the force-constant of the links between micro structures was expressed as k/, then the macroscopic elastic constant K (in one dimension) of the network could be modeled as ... 

The structure within the microstructure is fractal in nature therefore the diameter of the micro structure (or aggregates) is related to the particle volume fraction of the fat crystal networks Ot as ... 

Table 17.7 summarizes the effects of the microstructural factors on the microscopy fractal dimensions, Dj, y, and Zlpr- Different fractal dimensions reflect different aspects of the microstructure of the fat crystal networks and thus have different meanings. It is necessary to define which structural characteristic is most closely related to the macroscopic physical property of interest (mechanical strength, permeability, diffusion) and then use the fractal dimension that is most closely related to the particular structural characteristic in the modeling of that physical property. 

This book describes the science and practice behind the materials in foods that impart their desirable properties. The first part of the book describes those physicochemical aspects that intervene in the organization of food components from the molecular level to actual products and methods used to probe into foods at different length scales. The second part explains how food structures are assembled during processing in order to achieve desirable and recognizable properties. Processed foods are mostly metastable structures in which water, air, and lipids are immobilized as dispersed phases within a polymeric matrix of proteins, polysaccharides, or a fat crystal network. The last section of the book presents specific examples of how structures of familiar products are obtained by processing and describe some new developments. 

Solid Fat Content and the Fat Crystal Network The solids content of a fat crystal network is of critical importance to the final physical properties of the system. Generally, an increase in SFC leads to an increase in fat firmness. The SFC measurement has been widely used as a determinant quantity for the structural properties of fat systems. Estimations for commercial plastic fats, including butter, predict firmness increases of 10% for every percent increase in SFC (45). As a result, models used to describe the rheological properties of fats incorporate references to SFC values. 

---