Droplets, creaming

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 term stability, when applied to macroemulsions used for practical applications, usually refers to the resistence of emulsions to the coalescence of their dispersed droplets. The mere rising or settling of the droplets (creaming) because of a difference in density between them and the continuous phase is usually not considered instability. Flocculation or coagulation of the dispersed particles, without coalescence of the liquid interior of the particles, although a form of instability, is not considered as serious a sign of instability as coalescence or breaking of the... 

Emulsified systems can be classified aceording to their thermodynamie stability and their droplets size. Macroemulsions (or simply emulsions) are metastable systems, i.e., the system is not in thermodynamic equilibrium, and it will breakdown into two distinct phases if suffieient time is allowed. However, emulsions that keep their kinetic stability for periods of months or years ean be prepared by using appropriate components and amounts (McClements et al., 2007). This is the most common type of emulsion, and it is found in many food systems such as milk and salad dressing. Macroemulsions are usually polydisperse, with droplet sizes in the range of 1-100 pm. The main destabilization mechanisms in macroemulsions are droplets creaming, flocculation, and coalescence. 

In the bidisperse case. Figure 4.4(b), fi ctionation does occur. The large droplets cream faster than the small ones and two sharp boundaries form at the base and rise to die top at two discrete rates. The two creaming rates allow two hydrodynamic sizes to be inferred fiom eqn. (4.1). The rates at which die boundary rises at two volume fiacdons (ordinates yi and 2) are sufficient to define completely the cumulative size distribution of a bidisperse dispersion. Polydisperse dispersions are treated as an extension of the bidisperse case, the number of ordinates examined being increased as required until die size distribution is sufficiendy well defined. However, this simplistic analysis is only applicable to very dilute emulsions, where Stokes law is valid (i.e at infinite dilution in an infinite medium). In closed concentrated emulsions, droplets will interfere with one another and the effect of back-flow by the continuous phase becomes significant. 

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

As an example figure B 1.14.13 shows the droplet size distribution of oil drops in the cream layer of a decane-in-water emulsion as determined by PFG [45]. Each curve represents the distribution at a different height in the cream with large drops at the top of the cream. The inset shows the PFG echo decay trains as a fiinction of... 

Figure Bl.14.13. Derivation of the droplet size distribution in a cream layer of a decane/water emulsion from PGSE data. The inset shows the signal attenuation as a fiinction of the gradient strength for diflfiision weighting recorded at each position (top trace = bottom of cream). A Stokes-based velocity model (solid lines) was fitted to the experimental data (solid circles). The curious horizontal trace in the centre of the plot is due to partial volume filling at the water/cream interface. The droplet size distribution of the emulsion was calculated as a fiinction of height from these NMR data. The most intense narrowest distribution occurs at the base of the cream and the curves proceed logically up tlirough the cream in steps of 0.041 cm. It is concluded from these data that the biggest droplets are found at the top and the smallest at the bottom of tlie cream.

The sequence, flocculation — coalescence — separation, is compHcated by the fact that creaming or sedimentation occurs and that this process is determined by the droplet size. The sedimentation velocity is monitored by the oppositely directed forces which form the buoyancy and the viscous drag of the continuous phase on the droplet ... 

An emulsion is a mixture of oil and water. Some emulsions, such as butter and margarine, have tiny droplets of water in the oil. Others, like cream or mayonnaise, are droplets of oil in water. 

Polysorbate 80 is an emulsifying agent that is often used in ice cream to prevent milk proteins from completely coating the fat droplets. This allows them to join together in chains and nets, to hold air in the mixture, and to provide a firmer texture that holds its shape as the ice cream melts. 

Some detergents and surfactants are used as emulsifying agents. An emulsifier keeps oil droplets and water droplets from joining together, so a thick mixture of oil and water will not separate. Examples of emulsions are mayonnaise, butter, cream, homogenized milk, and salad dressings. 

Reconstituted milks with natural milk fat globules (CREAM) or emulsified milk fat droplets stabilized by jS-casein (BCAS), /i-lactoglobulin 5g/L (BLG5), skim milk proteins (MP). 

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]. 

In terms of measuring emulsion microstructure, ultrasonics is complementary to NMRI in that it is sensitive to droplet flocculation [54], which is the aggregation of droplets into clusters, or floes, without the occurrence of droplet fusion, or coalescence, as described earlier. Flocculation is an emulsion destabilization mechanism because it disrupts the uniform dispersion of discrete droplets. Furthermore, flocculation promotes creaming in the emulsion, as large clusters of droplets separate rapidly from the continuous phase, and also promotes coalescence, because droplets inside the clusters are in close contact for long periods of time. Ideally, a full characterization of an emulsion would include NMRI measurements of droplet size distributions, which only depend on the interior dimensions of the droplets and therefore are independent of flocculation, and also ultrasonic spectroscopy, which can characterize flocculation properties. 

Figure 4.5.8 shows the spatially-resolved droplet size distribution of a fully creamed isooctane-in-water emulsion obtained using the PGSTE pulse sequence shown in Figure 4.5.5(b). It can be seen that this method is able to provide droplet size distributions with spatial resolution. 

Fig. 4.5.16 Schematic drawing of a boundary layer mixing mechanism. It is proposed that a thin layer with thickness 8 has a linear velocity profile with average velocity V/2. Material with bulk droplet volume fraction ( >in is drawn into the creamed layer (area Ac) and material with average creamed layer volume fraction (j)ou, is swept out. The remainder of the emulsion (inside the dashed circle) is stagnant.

Fig. 4.5.17 Plot of average droplet diameter as a function of time at the top of the creamed layer (y/h = 1).

Studies of flow-induced coalescence are possible with the methods described here. Effects of flow conditions and emulsion properties, such as shear rate, initial droplet size, viscosity and type of surfactant can be investigated in detail. Recently developed, fast (3-10 s) [82, 83] PFG NMR methods of measuring droplet size distributions have provided nearly real-time droplet distribution curves during evolving flows such as emulsification [83], Studies of other destabilization mechanisms in emulsions such as creaming and flocculation can also be performed. 

M. Heidenreich, R. Kimmich 1999, (Magnetic-resonance determination of the spatial dependence of the droplet size distribution in the cream layer of oil-in-water emulsions Evidence for the effects of depletion flocculation) Phys. Rev. E 59, 874. 

Finely divided solid particles that are wetted to some degree by both oil and water can also act as emulsifying agents. This results from the fact that they can form a particulate film around dispersed droplets, preventing coalescence. Powders that are wetted preferentially by water form O/W emulsions, whereas those more easily wetted by oil form W/O emulsions. The compounds most frequently used in pharmacy are colloidal clays, such as bentonite (aluminum silicate) and veegum (magnesium aluminum silicate). These compounds tend to be adsorbed at the interface and also increase the viscosity of the aqueous phase. They are frequently used in conjunction with a surfactant for external purposes, such as lotions or creams. 

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