Lipid crystallization

Food products can generally be considered as a mixture of many components. For example, milk, cream and cheeses are primarily a mixture of water, fat globules and macromolecules. The concentrations of the components are important parameters in the food industry for the control of production processes, quality assurance and the development of new products. NMR has been used extensively to quantify the amount of each component, and also their states [59, 60]. For example, lipid crystallization has been studied in model systems and in actual food systems [61, 62]. Callaghan et al. [63] have shown that the fat in Cheddar cheese was diffusion-restricted and was most probably associated with small droplets. Many pioneering applications of NMR and MRI in food science and processing have been reviewed in Refs. [19, 20, 59]. 

The detached amounts of cadmium octadecanoate LB films at the water surface with various temperatures are shown in Figure 18. The detached amount increased linearly with increasing the subphase temperature. The detachment of LB films is concerned with equilibrium spreading pressure (ESP), which represents the equilibrium between bulk lipid crystals and a lipid monolayer on the water surface [45]. ESPs... 

In addition to particle size, the degree of crystallinity and the modification of the lipid are of relevance for drug incorporation and release. Lipid crystallization and a change of the modification can be delayed with very small particles and in the presence of emulsifiers [20,21]. 

When milk is cooled, lipid crystals form and grow. These crystals may have different polymorphic forms, only one of which is stable under a particular set of conditions. Other polymorphic crystalline forms are unstable and are transformed into the stable form (Mulder and Walstra 1974). This phenomenon, known as polymorphism, has been extensively studied (Mulder and Walstra 1974 Brunner 1974), and the... 

Figure 8-9 Differential scanning calorimetric curves for l-stearoyl-2-linoleoyl-sw-glycerol. (A) Crystals of the compound grown from a hexane solution were heated from -10° to 35°C at a rate of 5°C per minute and the heat absorbed by the sample was recorded. (B) The molten lipid was cooled from 35° to -10°C at a rate of 5° per minute and the heat evolved was recorded as the lipid crystallized in the a phase and was then transformed through two sub-a phases. (C) The solid was reheated. From Di and Small.87 Courtesy of Donald M. Small.

It is important to weigh Nile Red accurately. Nile Red modifies lipid nucleation kinetics when used at higher concentrations and, thus, modified crystal images may be obtained if the concentration is too high. The concentration recommended in this protocol has been proven not to modify lipid crystallization kinetics. 

Conductivity, in water activity measurement, 67-70 Confocal laser scanning microscopy to characterize lipid crystals, 575-579 description of, 575, 577 Conjugated dienes and trienes, determination of, 515-517 Conjugated linoleic acid (CLA), fatty acid analysis, 437-438, 445-446 Convection oven, gravimetric measurement of water, 7-8, 10-11... 

The crystallization kinetics of bulk triglycerides and oil-in-water emulsions has been characterized by both NMR imaging and localized spectroscopy. The rate of lipid crystallization in an oil-in-water emulsion was affected by the addition of a second homopolymer (addition of trilaurin to trimyristin in this case). The addition of the second homopolymer of higher chain length was observed to slow the rate of crystallization [26]. 

Lipid-water gel phases were previously regarded as metastable structures that are formed before separation of water and lipid crystals when the corresponding lamellar liquid crystal is cooled. New information on gel phases (see below) reveals that they can form thermodynamically stable phases with very special structural properties. This characteristic makes them as interesting as the lamellar liquid crystals from a biological point of view. 

Adapa, S., Dingeldein, H., Schmidt, K.A., Herald, T.J. 2000. Rheological properties of ice cream mixes and frozen ice creams containing fat and fat replacers. J. Dairy Sci. 83, 2224-2229. Adleman, R., Hartel, R.W. 2002. Lipid crystallization and its effect on the physical structure of ice cream. In Crystallization Processes in Fats and Lipid Systems (N. Garti, K. Sato, eds.), pp. 381-427, Marcel Dekker, New York. 

Wright, A.J., Narine, S.S., Marangoni, A.G. 2001a. Comparison of experimental techniques used in lipid crystallization studies. In, Crystallization and Solidification Properties of Lipids (N. Widlak, R. Hartel, S. Narine, eds.), pp. 120-131, AOCS Press, Champaign, IL. 

In many food products and even some processing operations, it is important to be able to control lipid crystallization to obtain the desired number, size distribution, polymorph, and dispersion of the crystaHine phase. In most foods, it is crystallization of triacylglycerols (TAG) that is most important, although, at times, crystallization of other lipids (i.e., monoacylglycerols, diacylglycerols, phospholipids, etc.) may also be important to product quality. 

There are several aspects of lipid crystallization that make it unique from crystallization of other components in foods (like water, sugars, salts, etc.). These are related to the complex molecular composition of natural fats and the orientation of the triacylglycerol molecules. 

In order to understand and control lipid crystallization, one should know the thermodynamic driving force for crystallization. In a pure system, like a single TAG, the melting point, T, defines the driving force and a temperature below is required to induce crystallization. That is, the subcooling or the melting temperature minus the actual temperature (Tm — T) defines the driving force for crystallization. 

Crystallization data have typically been treated theoretically using either the Fisher-Tumbull model or the Avrami equation. These analyses not only allow lipid crystallization to be modeled but may also shed some hght on the mechanisms of nucleation and growth. However, there is some recent debate about the validity of such models, especially the application of the Avrami equation (42) to accurately depict crystallization of lipids. 

To truly control crystallization to give the desired crystalline microstructure requires an advanced knowledge of both the equilibrium phase behavior and the kinetics of nucleation and growth. The phase behavior of the particular mixture of TAG in a lipid system controls both the driving force for crystallization and the ultimate phase volume (solid fat content) of the solidified fat. The crystallization kinetics determines the number, size, polymorph, and shape of crystals that are formed as well as the network interactions among the various crystalline elements. There are numerous factors that influence both the phase behavior and the crystallization kinetics, and the effects of these parameters must be understood to control lipid crystallization. 

Parameters that affect crystallization may influence either the thermodynamic behavior or the crystallization kinetics (or both). Parameters that influence lipid crystallization include chemical composition, subcooling, cooling rate, agitation, minor components of fats (mono- and diacylglycerols, polar lipids, etc.), and scale of operation. The effects of these parameters on lipid crystallization will be reviewed briefly in this section. More detailed information about the effects of these parameters on lipid nucleation and crystal growth may be found elsewhere (4, 24, 28, 54). 

Controlling lipid crystallization in foods has proven to be a technical challenge over the years. Despite a considerable amount of study, controlling the complex interactions between the various lipid components during crystallization remains essentially an empirical process of studying the effects of various operating parameters on crystal formation. Further work on the fundamental principles of lipid nucleation, growth, and polymorphic transformation is needed to truly control crystallization of lipids in foods. 

Glycerol is a widely used humectant and moisturizer in cosmetics, and considerable research has been done on its effect on the stratum corneum functionality (75). Glycerol was identified as an effective agent in the formation of lipid crystals under conditions of low humidity. The use of alpha- and beta-hydroxy acids on skin desquamation is now well established and widely practiced (76). New lipophilic variants of salicylic acid are reported to affect comeodesmolysis by acting on the whole stmcture of the comeodesmosomes. Alpha-hydroxy acids fractionate the cor-neodesmosomes only (77). 

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