Crystallization, fats nucleation

For the P polymorphic crystals, they assume a spherulitic shape with needle-shaped protrusions radiating from the center. These needle-like protrusions from the center of the crystals are the actual growing sites of the crystals as can be observed under the microscope. The distribution of these needles around the center of the crystal gives the final spherulitic shape of the P polymorphic form. As in the case of the P polymorph, the growing needles of the P polymorph can also be considered as two-dimensional as it is only radiating outward from its own center and upward. From the above considerations, it can be seen that the more commonly observed Avrami exponent of three will be inconsistent with the theoretical considerations for oils and fats nucleation. 

A simple calculation can highlight the reason why the microscopic technique is more sensitive than pNMR and tuibidity measurements. Solids in a 30-mg fat sample (p = 0.90 g/cm ) with an SFC of 0.1% (w/w) occupy a volume of 3.33 10 m. The volume of a spherical nucleus of 0.5 pm diameter is 6.54 10" m. If all of this solid mass corresponded to nuclei, 5.1 10 nuclei would be present in this sample. An SFC of 0.1% is below the detection threshold of a pNMR machine. Two obvious conclusions can be drawn from these calculations. Even at 0.1% SFC, the solids present in the sample cannot solely correspond to nuclei, since their number would be too great. Microscopic observation of a typical 30-mg sample of crystallizing fat (0.1% SFC) should convince any skeptic that 5.1 10 nuclei cannot possibly be present. This suggests that by the time SFC values reach 0.5-1.0%, a typical detectable level in a pNMR machine, significant amounts of crystal growth must have necessarily taken place. Therefore, an induction time of crystallization determined by pNMR does not correspond to the induction time for nucleation. 

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 main objectives of this chapter are to clarify the roles of the hydrophobic emulsifier additives added in the oil phase of O/W emulsions how they modify fat crystallization and where they interact within the emulsion droplets. One may ask why the hydrophobic emulsifiers accelerate the nucleation process. The answer may not be straightforward, because their influences on fat crystallization are controlled by their physical and chemical properties and the nature of the interactions with the fat molecules occurring in the oil phase and at the oil/water interfaces. However, the results we have obtained so far indicate that the addition of hydrophobic emulsifiers in the oil phase has remarkable effects on crystallization. Fat crystals typically form a number of polymorphs, whose crystallization properties are influenced by many factors, such as temperature, rate of crystallization, time evolution for transformation, and impurity effects, as is commonly revealed in various examples [27,28], It is reasonable to expect that these polymorphic properties of fats may interfere with the clarification of the essential properties of the interface heterogeneous nucleation that occurs in O/W emulsions. 

In natural fat crystallization systems, nucleation is initiated by some catalytic impurities such as dust particles and foreign substrates (for example, the inner surface of the container). The presence of such impurities initiates nucleation at lower levels of supercooling [32], As crystallization progresses, nucleation begins to exist at many locations secondary nucleation would be started due to the presence of solid crystals, which are chemically similar to the components of the melt [33]. 

Tempering. The state, or physical stmcture, of the fat base in which sugar, cocoa, and milk soHds are suspended is critical to the overall quaHty and stabiHty of chocolate. Production of a stable fat base is compHcated because the cocoa butter in soHdified chocolate exists in several polymorphic forms. Tempering is the process of inducing satisfactory crystal nucleation of the Hquid fat in chocolate. 

Nucleation tempering of the stiU molten fat is necessary because the cocoa butter, if left to itself, can soHdify in a number of different physical forms, ie, into an unstable form if cooled rapidly, or into an equally unacceptable super stable form if cooled too slowly, as commonly happens when a chocolate turns gray or white after being left in the sun. The coarse white fat crystals that can form in the slowly cooled center of a very thick piece of chocolate are similarly in a super stable form known in the industry as fat bloom. 

Differences in nucleation explain the differences observed between crystallization in the bulk and emulsified states. In bulk fats, only a small number of nuclei are needed to induce crystallization. However, when the same fat is emulsified, each fat droplet must contain a nucleus or impurity in order to crystallize, the probability of which is low. As a result, the emulsified fat requires more supercooling (i.e., to a lower temperature) in order to nucleate (Walstra et al., 1994). 

The rate of crystal growth is determined by the degree of supersaturation, the rate of molecular diffusion to the crystal surface, and the time required for TAG molecules to fit into the growing crystal lattice (Mulder and Walstra, 1974 Walstra, 1987). Compared to nucleation, the driving force required for crystal growth is relatively low (Sato et al, 1989). However, in a multicomponent fat, the supersaturation for each TAG is small (Walstra, 1998). This fact, combined with competition between similar molecules for the same sites in a crystal lattice, means that milk fat crystallization is especially slow (Skoda and van den Tempel, 1967 Knoester et al, 1968 Grail and Hartel, 1992). 

The (3 polymorph is the most thermodynamically stable. It has the highest melting point and is therefore the least soluble in a melt at a given temperature below its melting point. Despite this, nucleation for the a-polymorph is favored. Although the a-crystal is less stable, it has a lower crystal-melt interfacial tension and lower heat of crystallization than the (3 - and (3-polymorphs (Timms, 1995). Nucleation in milk fat typically... 

Boistelle, R. 1988. Fundamentals of nucleation and crystal growth. In Crystallization and Polymorphism of Fats and Fatty Acids (N. Garti, K. Sato, eds.), pp. 189-226, Marcel Dekker Inc., New York. 

The crystallization behavior of milk fat (which contains minor lipids) and a pure triacylglycerol fraction of milk fat were compared by Herrera et al. (1999). The results suggested that minor lipids delay nucleation but promote crystal growth. Other workers who examined the effects of added phospholipids on palm oil, suggested that some phospholipids delayed nucleation while others had more significant effects on the rate of growth of fat crystals (Smith, 2000),... 

Tietz and Hartel (2000) studied the effects of removing or adding minor components naturally present in milk fat on the crystallization of milk fat-cocoa butter blends. They suggested that at low concentrations, minor lipids act as sites for nucleation and promote the rate of crystallisation and at higher concentrations inhibit crystallisation. They concluded that the presence of minor lipids, at the concentrations naturally occurring in milk fat, were sufficient to affect crystallisation rates, chocolate microstructure and fat bloom formation in chocolate. 

During fractionation of fats, secondary nucleation is undesired because the small crystals, formed in the presence of larger ones means that subsequent separation is not efficient. Thus, stirring or agitation during fractionation is usually kept to the minimum needed to facilitate heat transfer. 

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