Milkfat crystallization

Mulder and Walstra (1974) showed that fat crystal morphology can substantially affect rheology. For example, they reported that slowly crystallized milkfat, with an SFC greater than 20%, contained large spherulites and was pourable. On the other hand, acceptable spreads with SFCs of—10-15% can be manufactured (Chrysam, 1996 deMan et al., 1995). Hence, the formation of the fat crystal network is strongly influenced by the structure of the individual crystals or crystal aggregates (e.g., spherulites). 

Black, R. G. 1975. Partial crystallization of milkfat and separation of fractions by vacuum filtration. Aust. J. Dairy Technol. 30, 153-156. 

Buchheim, W., Abou El-Nour, A.M. 1992. Induction of milkfat crystallization in the emulsified state by high hydrostatic pressure. Fat Sci. Technol. 94, 369-373. 

Peters-Erjawetz, S., Ulrich, J., Tiedke, M., Hartel, R.W. 1999. Milkfat fractionation by solid-layer melt crystallization. J. Am. Oil. Chem. Soc. 76, 579-584. 

Wright, A.J., Marangoni, A.G. 2002a. The effect of minor components on milkfat crystallization, microstructure and rheological properties. In Physical Properties of Lipids (A.G. Marangoni, S.S. Narine, eds.), pp. 125-161, Marcel Dekker, New York. 

In mixmres of two or more natural fats, as often occurs in processed foods (e.g., milkfat and cocoa butter in chocolate), it is even more difficult to characterize the true phase behavior for crystallization of fats. One approach that has been used to characterize compatibility of fat mixtures is the isosolids diagram (22). SFC melting curves are obtained (by NMR) for various mixmres of the two fats, as seen for cocoa butter and milkfat in Figure 6. Lines of constant SFC for different temperature and composition are calculated and plotted on an isosolids diagram (Figure 7). 

Eutectic behavior is seen where the SFC of a mixture falls below the SFC for either of the two individual components, as seen between 30% and 70% milkfat in Figure 7. Isosolids diagrams allow phase compatibility to be studied (4), but they do not provide a thermodynamic measure of driving force for crystallization. Again, because the crystal phase composition may be different at different temperatures (and mixture ratios), isosolids diagrams do not represent true phase diagrams. 

The crystallization rate constant k) is a combination of nucleation and growth rate constants, and is a strong function of temperature (47). The numerical value of k is directly related to the half time of crystallization, ti/2, and therefore, the overall rate of crystallization (50). For example, Herrera et al. (21) analyzed crystallization of milkfat, pure TAG fraction of milkfat, and blends of high- and low-melting milk-fat fractions at temperatures from 10°C to 30°C using the Avrami equation. The n values were found to fall between 2.8 and 3. 0 regardless of the temperature and type of fat used. For temperatures above 25°C, a finite induction time for crystallization was observed, whereas for temperatures below 25°C, no induction time was... 

Crystallization of milkfat is an important process for fractionation of its contents and production of butter, whipped cream, and ice cream. As the quality of these products strongly depends on polymorphism of milkfat, physical chemical properties of milkfat have been studied by many researchers (98). 

As for the polymorphism of milkfat, a and p forms frequently appear, and p form appears under special conditions when HMF and milkfat are stored for long duration (99-101). In regard to the effects of thermal treatment and emulsification on the polymorphic crystallization of milkfat, Lopez et al. recently performed synchrotron radiation X-ray diffraction and DSC studies, using anhydrous milkfat... 

It is assumed that the crystallization behavior of milkfat is different between emulsion and bulk, and the lack of nucleation centers in the emulsion droplets may delay the nucleation, making less stable a form nucleated in the first. The occurrence of multiple forms of double-chain-length and triple-chain-length structures may be caused by segregated crystallization of multicomponent TAGs exhibiting complicated mixing behavior, but its details are open to fumre study. 

Figure 5. Crystallization and crystal growth of anhydrous milkfat at controlled cooling rates of 0. PC/min, 1°C/min, and 5°C/min monitored using SFC by pNMR.

Polymorphism and Milkfat High cooling rates (>l°C/min), or high levels of supercooling (>15°C), lead to the rapid formation of metastable a nuclei (34). The persistence of these unstable nuclei is dependent on thermal treatments that occur after crystallization. These nuclei may remain in the a form or convert... 

TABLE 2. Rheologically Determined Fractal Dimensions (Dr) and Pre-Exponential Terms X) for Anhydrous Milkfat Crystallized at Various Rates of Cooling and Storage Times at 5°C. 

Figure 12. Polarized light micrographs of anhydrous milkfat cooled at 0. PC/min, PC/min, and 5°C/min. Images were acquired during crystallization in the range of 30°C to 5°C at intervals of 5°C.

Currently, dry fractionation of anhydrous milkfat is performed by two conventional systems—Tirtiaux and De Smet (both from Belgium)—which are bulk crystallization processes. The widely used Tirtiaux dry fractionation process enables one-step or up to hve-step fractionation of anhydrous butter oil at any temperature, ranging from 50°C to 2°C (37, 110-113). The milkfat fractions thus obtained can be used as such or the fractions can be blended in various proportions for use as ingredients in various food-fat formulations. The major shortcoming inherent in this system is the long residence time (8-12 h) for nucleation and crystal growth. 

Confectionery-Liquors and Liqueur. In chocolate confectionery and for pastry creams, it is the physical properties linked to the fusion and the crystallization of the fat that are essential. For milk chocolate, for coating or in bars, AMF can be used in proportions that depend on its compatibility with cocoa butter, whose properties of hardness and rapid fusion at 35°C cannot be altered. Thus it is currently accepted that AMF with high fusion levels obtained by the fractionation technique can be used. In general, milkfat has an interesting characteristic it inhibits the appearance of fat bloom (133). 

Another parameter that influences the overall properties of the bulk emulsion is the physical state of the lipid droplets in an emulsion (17, 19, 28-31). Crystallization of lipid droplets in emulsions can be either beneficial or detrimental to product quality. Margarine and butter, the most common water-in-oil emulsions in the food industry, are prepared by a controlled destabilization of oil-in-water emulsions containing partly crystalline droplets. The stability of dairy cream to mechanical agitation and temperature cycling depends on the nature and extent of crystallization in milk-fat globules. It should be noted that because the density of the phases can change as crystallization occurs, the rate at which milkfat droplets cream can be altered as droplets solidify. Emulsion manufacturers should therefore understand which factors influence the crystallization and melting of emulsified substances, and be aware of the effect that droplet phase transitions can have on the properties of emulsions. 

The addition of milkfat to cocoa butter (4) results in marked lowering of the melting point, adversely affecting the crystallization behavior and the hardness as shown in Table 8. An obvious decrease is clearly evident in the solid fat content and a deterioration in solidification properties as shown in the values of the Jensen curve. These results are further confirmed by comparing the curves for milkfat and cocoa butter in various proportions. There are two reasons for this strong decrease in hardness (5, 6) ... 

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