Crystal structure, fats polymorphism

In many instances, TGs exist in polymorphic forms. Crystal structure is very important to the properties of margarines, shortenings, and specialty fats. The very unstable a form is readily transformed to the more stable ft form, which in some TGs has a higher melting point (more stable) than the /3 form. In single-acid TGs, the order is a — f3 — a, but some mixed TGs show a lower-melting (less stable) (3 form. This is further complicated by the existence of multiple / and /3 forms, depending upon the detailed TG structures at hand. 

Localized NMR spectroscopy, which is often called as MRS in comparison with MRI, is not so familiar technique in food science, because a specific pulse sequence such as ISIS and a facility which can precisely follow the pulse sequence without any contamination from other position is needed for localization of position. The localized NMR is usually used together with NMR imaging. The study of solid/liquid ratios, fat structure and polymorphism and the kinetics of fat crystallization was reviewed [24], The potential of applications in food process development and control was offered. The localized spectra of sausages in areas of 0.3 mm X 0.05 mm (thickness of sample =1.5 mm) were obtained by the spin echo 2DFT method [113], in which the difference in the tissue structure was discussed with relation to the process and original materials. McCarthy et al. determined mobility of water in foams by using a localized spectroscopy [114]. T2 relaxation time varies in the foam as function of diameter and its variation was analyzed by the classic 2-state fast exchange model. 

Precht, D. 1988. Fat crystal structure in cream and butter. In Crystallization and Polymorphism of Fats and Fatty Acids (N. Garti, K. Sato, eds.), pp. 305-361, Marcel Dekker Inc., New York. 

Hemqvist, J.W. (1988). Crystal structures of fats and fatty acids. In N. Garti K. Sato (Eds.), Crystallization and polymorphism offats fatty acids. Marcel Dekker, pp. 97-138. 

The three common polymorphic forms that exist in fat crystal networks are the suba, a, (), and (3 modifications. These polymorphic modifications and their characteristic crystallization patterns are shown in Figure 6. The subot and ot forms are metastable (34). Each polymorphic form yields different crystal structures dependent on the magnitude of the crystallization driving force. The polymorphic modifications also have varying thermodynamic stability, which determines their lifetime within a crystal matrix, with the tendency toward greater stability in the order a to (3 to (3 (24, 34). 

It has been reported extensively that fats solidify in more than one crystalline type (2-23). Triglycerides exhibit three main crystal types—ot, p, and p—with increasing degrees of stability and melting point. The molecular conformations and packings in the crystal of each polymorph have been reported. In the ot form, the fatty acid chain axes of the triglyceride are randomly oriented and the ot form reveals a freedom of molecular motion with the most loosely packed hexagonal subcell structure. 

In this chapter, molecular factors affecting structural behavior of fat polymorphism are discussed in terms of internal influences of the TAG molecules. In particular, the influences of fatty acid compositions and their positions connected to glycerol carbons on the polymorphism of fat crystals are of primary concern. It has been known that the fats with simple and symmetric fatty acid compositions tend to exhibit typical oc, P, and P forms, whereas those with asymmetric mixed-acid moieties often make the P form more stable (1,9). In the mixed-acid TAG containing unsaturated fatty acid moieties, the number and conformation of the double bond, cis or trans, give rise to remarkable influences on the polymorphic structures (10-12). The TAG containing different saturated fatty acids with different chain-lengths also revealed quite diversified polymorphism (13-15). Therefore, it may be worthwhile now to discuss the molecular aspects of the polymorphism of fats. This consideration may also be a prerequisite for molecular design of structured fats, in combination with nutritional and metabolic properties. 

Koyano, T., I. Hachiya, and K. Sato, Fat Polymorphism and Crystal Seeding Effects on Fat-Bloom Stability of Dark Chocolate, Food Structure 9 231-240 (1990). 

Sato, K., Fat Polymorphism How Its Structures and Kinetic Behavior Are Modified, Conference on Crystallization and Solidification Properties of Lipids, American Oil Chemists Society, Toronto, Canada, 2000. 

In products containing lipids, control of the crystal polymorphic form is also necessary. Lipids form different crystalline structures, or polymorphs, depending on the nature of the fat and the processing conditions. Transitions from less stable to more stable polymorphs are also dependent on composition and processing conditions. For example, tempering (or precrystallization) of chocolate is a process through which the chocolate is sequentially cooled and warmed to promote crystallization of cocoa butter into the desired polymorphic form. Controlling crystallization to produce the proper size distribution of this polymorph provides ... 

Solid substances with the same chemical composition and different crystal structure are called polymorphic forms or modifications. Each polymorphic form has characteristic properties (such as specific volume and melting point). The formation of a given polymorphic form depends on many factors (such as temperature, cooling rate of the liquid phase and type of solvent). In the solid crystal state, one polymorphic form is transformed into the other polymorphic forms without previous melting of the crystals. If one of the forms and another form is stable, these two polymorphic forms are called monotropic. The transformation occurs only in one direction, the stable form arises from the metastabile form. All natural fats are monotropic. 

D. Adsorption on Fats and Polymorphism-Crystal Structure Modification... 

Emulsifiers sensitively modify the rates of crystal growth and polymorphic transition of fats through the preferred adsorption at or inclusion in fat crystals [55,56]. The retardation or acceleration of the polymorphic transformation is influenced by the hydrophobic moiety structure. Figure 20 shows the effect of emulsifiers on the a-to-p transformation of tristearin during aging at room temperature... 

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. 

The dispersed state has a considerable effect on fat crystal polymorphism. Lopez et al. (2000, 2001c) observed that crystallization in milk fat globules is more disordered than in bulk fat. On slow cooling, milk fat crystallizes in the a form in cream (Lopez et al., 2001a), whereas in anhydrous milk fat, it crystallizes first in the (3 form and then in the a form (Lopez et al., 2001b). Rapid cooling of cream or anhydrous milk fat from 60 to 4°C leads to the formation of a crystalline structures, which transformed into (3 structures... 

Ollivon, M., Loisel, C., Lopez, C., Lesieur, P., Artzner, F., Keller, G. 2001. Simultaneous examination of structural and thermal behaviours of fats by coupled X-ray diffraction and differential scanning calorimetry techniques application to cocoa butter polymorphism. In, Crystallization and Solidification Properties of Lipids (N. Widlak, R. Hartel, S. Narine, eds.), pp. 34-41, AOCS Press, Champaign, IL. 

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). 

Crystallization from the emulsified state may lead to different nucleation processes than observed for the same fat in bulk liquid form. It has been suggested that nucleation often occurs at the interface of the droplet where surface-active agents are located. The general similarity of the lipophilic components of surfactants oriented at the surface may provide some ordering and structure for the lipid molecules within the droplet and enhance nucleation, as found for example by Kaneko et al. (40) for a hydrocarbon emulsion. Walstra (11) also suggests that formation of compound crystals from emulsions of natural fats may be different than the same fat crystallized from bulk liquid. The initial polymorph formed may also be different, with more stable polymorphs more likely to form in the emulsion (38). 

As for the effects of the shear stress, it was shown by a Synchrotron radiation X-ray diffraction study that transformations from metastable to more stable forms, especially to Form V, were accelerated by high shear stress (110). Figure 25 shows the time variation of relative intensities of X-ray diffraction peaks of CB crystals formed after cooling from 50°C to 18°C at a rate of 3°C/min. In the case of no shear. Form III appeared at first after the temperature reached at 18°C, and then Form IV crystallized at the expense of Form III. On the other hand, applying the shear stress at 1440 s caused accelerated transformation from Form III to Form V, without the occurrence of Form IV. The same result was observed with lower shear rates (19), and the persistence time of Form III was reduced as the shear rate was increased. Mazaanti et al also observed that the orientation of CB crystals are aligned with the shear flow (110). These results indicated that temperature and shear treatments are the tools for tailoring the desired polymorphic structures of fats. 

Figure 6. Polymorphic forms of fat crystals, including the possible polymorphic transitions, subcell packing structures, stability characteristic, and triacylglycerol stacking conformations.

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