Emulsified fats polymorphism

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. 

Crystallization of lard and PSCO emulsions with similar initial droplet size distributions led to partial coalescence under perikinetic and orthokinetic conditions. The application of shear accelerated the destabilization of the emulsions after the achievement of a critical SFC. The SFC was found not to be the sole contributing factor to emulsion destabilization. Crystal morphology and distribution within the droplet are important factors in the destabilization of these emulsified fats. The emulsions are relatively stable in the short term when crystals are small and form quickly in a consolidated mass in the bulk of the droplet. Polymorphic transitions were not detectable as a source of destabilization in this experiment. The observation of the microstructure of bulk and emulsified fats gave insight into the mechanisms of emulsion destabilization. 

Emulsifiers have also assumed valuable roles in products such as chocolates (control of fat polymorphism), toffees and caramels, chewing gums, pharmaceutical preparations, soft and liqueur drinks and meat products, in addition to being used as lubricating, release and cutting aids throughout the food industry. In these applications, emulsifiers can be said to be used in roles not directly related to emulsification. 

Ueno, S., Yano, J., Seto, H., Amemiya, Y., Sato, K. 2000. Synchrotron radiation X-ray diffraction study of polymorphic crystallization in triacylglycerols. In, Physical Properties of Fats, Oils and Emulsifiers (N. Widlak, ed.), pp. 64-78, AOCS Press, Champaign. 

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). 

In this section, the polymorphic properties of natural fats are briefly discussed by highlighting miMat, cocoa butter, and palm oil fraction based on recent research into the effects of external factors on the polymorphic crystallization such as shear stress, ultrasound stimulation, and addition of food emulsifiers. 

Some oil-soluble emulsifiers affect the crystallization process and development of polymorphic forms of fats (4-8). Sucrose fatty acid ester or sucrose polyesters (SPE) and lecithins are well-known food emulsifiers (9,10). The main characteristics of lecithins and SPE useful in food applications are their oil-in-water and water-in-oil emulsifying properties, that result in dispersion with condensed milk and coffee whitener, and prevention of blooming in candy products and chocolate (7,9-11). But there are very few reports about two effects of SPE on the crystallization of fats and oils, i.e., enhancement and inhibition (12,13). 

Stabilization of margarine and other similar food emulsions is achieved by partial adsorption of solid fat particles (P -polymorphs) on to the water-oil interface, bridged by monomeric hydrophobic emulsifiers. The complex stabilization is achieved by wetting the oil phase by solid fat particles and emulsifiers (lecithins and monoglycerides of fatty acids). The concept was re-examined and reconsidered by Bergenstahl and coworkers (54, 55) and the mechanism was somewhat better elucidated. 

The incorporation of lecithin in different amounts (10-50% related to the triglyceride) did not influence the melting or crystallization temperature of the hard fat in the bulk phase neither in the nanoparticles but led to an accelerated polymorphic transition to the stable 6-modification particularly in the nanoparticles. No additional colloidal structures derived from the emulsifiers (lecithin and Solutol) could be detected in the aqueous phase by electron microscopy and the results of H- and P-NMR studies indicate that lecithin and Solutol are nearly completely attached to the particle surface. However, from the results it was also concluded that due to the redistribution of the lecithin from the lipid matrix and its enrichment in the particle interface, neither an increase in drug load (into the solid... 

Crystallization of edible fats and oils in oil-in-water (OAV) emulsions is an important process in many industrial fields such as foods [1-4], cosmetics [5], and pharmaceuticals [6]. In the food industry, crystallization in the 0/W emulsion phase contributes to the de-emulsifying process in whipped creams, the freezing of ice creams, and coagulation of the 0/W emulsions at chilled states. Therefore, the production, quality, and stability of fat products in the emulsion state are highly influenced by crystallization of the oil phase [7], so much recent research has been aimed at the exploration of fat crystallization in O/W emulsions [8-13], Fat crystallization results in complex phenomena in O/W emulsions and affects such parameters as the rate and extent of crystallization, influences of emulsion droplet sizes, effects of emulsifiers, droplet-droplet interactions, polymorphism, and effects of cooling rate and subsequent temperature history. 

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. 

As an application to real food fats, the acceleration effects of the additives on the heterogeneous nucleation of palm oil, PMF, and PKO droplets may be used to control fat crystallization in food emulsions. As for PMF, the results clearly indicated that polymorphism was modified by the addition of hydrophobic emulsifiers. The preferential crystallization of P was explained by assuming that the nature of the acyl chain packing of templates and the shape of their polar headgroup influence the degree of P nucleation. The preferred crystallization of P by the additives may be applied to food fats. 

Aronhime, J, S Sarig, N Garti. (1990). Emulsifiers as additives in fats Effect on polymorphic transformations and crystal properties of fatty acids and triglycerides. Food Struct 9 337-352. 

Emulsifiers are key players in polymorphic transformation studies. They alter the fat surface properties, resulting in changes in crystal size and nature. Early reviews by van den Tempel [52] and Garti [53] showed that many types of emulsifiers tend to reduce the crystal growth rate of natural fat blends. Since then, further work has been performed on the effects of different emulsifiers on fats not only in bulk but also in emulsion systems. Garti and Yano [54] discuss in great detail the progress made in this field in recent years. 

Case 1. A limited amount of emulsifier is miscible in a fat system. The small amount of emulsifier acts as an impurity and results in imperfect fat crystals. It will promote or retard polymorphic tfansformation of fat... 

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... 

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