Fats, crystallization

EM studies of whipped cream show that the air bubbles are completely surrounded by a layer of fat giobuies which protrude partiaiiy into the air bubbles. These parts of the fat globules no longer have their original membrane layer, but exhibit surface layers of crystallized fats. The fat globules adsorbed around the air bubbles are bonded together with... 

The water content (and hence the oil content) of emulsions may be obtained by measuring the temperature dependence of the velocity of sound. Firstly, it is necessary to melt any crystallized fat by heating it above its melting point. In most food emulsions, such as milk, this requires heating to 40°C when all of the fat will be melted. An equation similar to 13 gives the water content, w from a measurement of the temperature coefficient of the velocity of sound ... 

Figure 8-42 Structure Formation in Particulate Systems. (A) Flocculation of an emulsion. (B) Network formation in crystallized fat.

Figure 3. Typical signal decay for a partially crystallized fat, following a 90° r.f. electromagnetic pulse. Parameters required for measurements of solid fat content (SFC) are shown.

Solid Fat. The consistency and the emulsion stability of margarine and most other table spreads depends on crystallized fat. Freeze-fracture electron microscopy of deoiled margarine shows the crystalline nature of the water droplet interface as well as a continuous fat matrix that appears to be an interconnected network... 

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. 

Relating Bulk-Fat Properties to Emulsified Systems Characterization of Emulsion Destabilization by Crystallizing Fats... 

The oil-in-water emulsions studied in this experiment contained sufficient levels of crystallized fat to lead to droplet destabilization via partial coalescence. The role of intradroplet crystallization and its effects on emulsion stability and partial coalescence were determined by examining the evolution of droplet size distribution [volume weighted particle size distribution as a function of time. All freshly... 

Shear-induced crystallization had a much greater effect in bulk systems than emulsified systems (Fig. 6) and resulted in an accelerated rate of crystallization. Prior to, and during, the initial stages of crystallization, intradroplet fat is protected from interdroplet crystallization by the spherical shape and pressure of the droplet and is not directly available to the shear field, i.e., no protruding crystals. This observation is consistent with microstructure work where limited destabilization was observed in droplets with no visible crystals. Initially, droplet interfaces in the PSCO system showed that the crystallized fat was not available at the surface, limiting the occurrence of crystal-induced flocculation and coalescence. Droplets remained stable until their interfaces were disturbed by the shear fleld or crystal interaction. 

The most important functional properties of margarines and spreads are spreadability , oiliness and melting property. These properties relate to fat level and type and stability of the emulsion. Spreadability can be predicted by SFI and penetration measurement. Oil-off refers to the phenomenon when fine fat crystals no longer form a stable network to trap the liquid oil. Consistency and emulsion stability depend on the amount and type of crystallized fat. During rapid cooling, the most unstable a crystals form but they quickly transform to the ( / form, which is relatively stable and consists of a very fine crystal network capable of immobilizing a large quantity of oil. These ( / crystals may also transform into the most stable (> form, which has a coarse and sandy texture and... 

A typical ice cream consists of about 30% ice, 50% air, 5% fat and 15% matrix (sugar solution) by volume. It therefore contains all three states of matter solid ice and fat, liquid sugar solution and gas. The solid and gas are small particles - ice crystals, fat droplets and air bubbles -in a continuous phase, the matrix. To understand the creation of the microstructure during the manufacturing process we must first introduce some concepts from the physical chemistry of colloids, freezing and rheology (the study of the deformation and flow of materials). 

The rheology of ice cream is much more complex than that of a simple liquid. The matrix is a solution of small (sugar) and large (stabilizer) molecules, in which particles of other phases (ice crystals, fat droplets and air bubbles) are suspended. We must first look at the effects of each of these, and temperature, in order to understand the rheology of ice cream. 

The determination of the solid fat content by pulsed NMR is based on the fact that the transverse magne tization of solid fat decays much faster than that of oil. The spin-spin relaxation time (T2) of solid fat is about 10 ps, and that of oil is about 100 ms. The NMR signal, derived from the amplitude of the FID, of par tially crystallized fat after a 90° r.f. pulse is schemati cally shown in Fig. 13. The magnetization of the solid fat decays very fast. As a consequence, its contribution to the signal is far less than 0.1% of the initial value after about 70 ps. The decrease in the liquid-oil... 

Figure 13 Signal of partially crystallized fat after a 90° r.f pulse.

Only the pulsed NMR signal of the liquid fat is measured. The total liquid content is obtained by melting and measuring the sample at 60 °C. The pulsed NMR signal at other temperatures of the partially crystallized fat is proportional to the total liquid measurement at 60 C. An olive oil liquid reference is measured at all temperatures to compensate for instrument and sample differences. 

Butter contains 81-85% fat, 14—16% water, 0.5-4.0% fat-free solids and 1.2% NaCl in the case of salted butter. The composition generally must meet legal standards. Butter is an emulsion with a continuous phase of liquid milk fat in which are trapped crystallized fat grains, water droplets... 

Nonionic surfactants and emulsifiers are compounds whose molecules have both aqueous (polar) and alkane (nonpolar) compatible sectors. Their molecules have regions that are sufficiently similar to become part of either system, with the other sectors sufficiently dissimilar to interrupt continuation of that system. For example, when added to a crystallizing fat, some nonionic surfactants may orient themselves to become part of the crystal, and will prevent further replication thus limiting crystal size. Likewise, some will react wth gelatinized starch in aqueous systems and prevent its recrystallization (retro-gradation). Further, surfactants may orient around discrete droplets to stabilize water in oil (W/O) or oil in water (O/W) emulsions. When this occurs, the compatible end becomes associated with the discrete droplet, leaving the other end turned outward to associate with the compatible continuous phase. 

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 this chapter we define the structural hierarchy of the network formed by a crystallizing fat (after Ref. 1), highlights the major developments in quantification of the various levels of structure, and summarizes the application of fractal geometry to the quantification of the microstructural level of structure. Furthermore, we make the case that the microstructure, by virtue of its proximity to the macroscopic world, provides important predictive information on the value of the elastic modulus of the network. Models we have developed to relate the struc-... 

Ziegleder, VG. (1990). DSC thermal analysis and kinetics of cocoa butter crystallization. Fat Sci Technol 92 481-485. 

Thompson et al. [109] studied emulsification and demulsification related to cmde oil production, in particular the role of temperature. The stability of crude oil emulsions was strongly temperature-dependent and in fact demanded the presence of wax crystals. Fat crystals ranged in size from submicrometer to 50 (Am needles. In the temperature range 30-50°C, emulsion instability substan-... 

In his doctoral thesis, Lucassen-Reynders [73] showed that monoolein can adsorb onto tristearin. Johansson and coworkers [74-76] carried out detailed work on model fat systems dispersed in oil. They studied the adsorption of various emulsifiers to the crystals (fats and sugar) dispersed in oils. The adsorbed amount, the strength of the adsorption, and their relationship to the character of the emulsifiers, crystals, and oils were obtained. 

To achieve crystallization at any temperature it is necessary for the concentration of the crystallizing species to exceed its solubility or saturation in the lower melting species at that temperature. This does not imply immediate crystallization. Solutions can exist indefinitely in the supersaturated state without forming crystals. Fats can also remain liquid or supersaturated at temperatures below those at which they would be expected to begin crystallizing. 

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