Emulsion Flocculation and Creaming

Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, UK 

An emulsion may be defined as a heterogeneous system, consisting of at least two immiscible liquids or phases, one of which is dispersed in the form of droplets in the other. Emulsions are generally unstable with respect to their component bulk phases. Rearrangement from the droplet form to the two bulk liquids will occur with a net reduction in interfacial area and this is energetically favourable. However, it is relatively simple to erect kinetic barriers to this process to achieve metastable states that are for all practical purposes completely stable. 

When the surfactant or polymer layer separating the droplets fails, intimate 

For example, when creaming occurs, this extra motion (in addition to Brownian motion) may increase the rate at which droplets encounter one another. This is 

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Y. Hemar, D. N. Pinder, R. J. Hunter, H. Singh, P. Hebraud, D. S. Home 2003, (Monitoring of flocculation and creaming of sodium-caseinate-stabilized emulsions using diffusing-wave spectroscopy), /. Colloid Interface Sd. 264, 502. 

In a recent study by Sun et al. (2007) of 20 vol% oil-in-water emulsions stabilized by 2 wt% whey protein isolate (WPI), the influence of addition of incompatible xanthan gum (XG) was investigated at different concentrations. It was demonstrated that polysaccharide addition had no significant effect on the average droplet size (d32). But emulsion microstructure and creaming behaviour indicated that the degree of flocculation was a sensitive function of XG concentration with no XG present, there was no flocculation, for 0.02-0.15 wt% XG, there was a limited... 

There are three processes by which the number of oil drops in an emulsion is decreased. These are Brownian flocculation, sedimentation flocculation and creaming. But it should be noted that if the absorbed film strength is quite high, flocculation may not necessarily result in coalescence. It is also important to note that flocculation which may be due to any of above three reasons is reversible, but coalescence which follows flocculation is irreversible. 

Proteins also have a major influence on emulsion stability, although various types of instability may occur instability caused by coalescence or flocculation, and cream layer formation caused by differences in density between oil droplets and the aqueous phase. In practice, the latter problem is the most difficult to solve. 

Flocculation is the mutual aggregation of colliding droplets. In stationary emulsions, droplet collisions arise from Brownian motion (small droplets) and/or from the creaming/sedimentation process (larger droplets). In the latter, the mechanism is often referred to as sedimentation/creaming flocculation. Finally, droplet aggregation can also occur in sheared emulsions. It is important to point out that the droplet size distribution is not altered by the flocculation and creaming/sedimentation destabilization mechanisms. 

Tracing the resolved volume fraction of the collected bulk free-water layer over time is also a common means of measuring destabilization (220). Centrifugal forces cause the droplets to flocculate and cream faster, facilitating the drainage of thin liquid films formed between them (221-230). Void and Malefic (231) indicated an Arrhenius type of relationship between centrifugal forces and dosage of demulsifiers for demulsification of an ideal 0/W emulsion. 

Obviously, both flocculation and creaming represent conditions in which drops touch but do coalesce. The key to understanding the true stabihty of emulsions, then, lies on the hne separating the processes of flocculation and coalescence. 

Formation of emulsions has been reviewed by Walstra (1983). In contrast to solid colloids, there appears to be much less control of the sizes of emulsion drops. Mechanical devices called homogenizCTs can reduce fat globules in milk to sizes where they take much longer to flocculate and cream to the top. Homog-enizers capable of making nearly uniform size drops are beginning to appear. 

In practice, double emulsions consist of large and polydispersed droplets that are thermodynamically unstable, with a strong tendency for coalescence, flocculation, and creaming. 

Below we describe the use of the ultrasonic monitor to detect creaming in a polydisperse concentrated emulsion, and to characterise flocculation from the creaming behaviour. The effects of added polymers on the flocculation and creaming processes are also described. 

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. 

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. 

Four major phenomena are associated with the-physical instability of emulsions flocculation, creaming, coalescence, and breaking (Fig. 8) [144]. 

Phase Separation. An approximate estimation of phase separation may be obtained visually. In general, creaming, flocculation, and coalescence have occurred before phase separation is visible, thus sometimes making quantitative evaluations more difficult. Accelerating the separation by centrifugation followed by appropriate analysis of the specimens may be useful to quantitatively determine the phase separation. Details on mechanisms of creaming and phase separation as well as some advances in the monitoring techniques of emulsion stability have been reviewed by Robins [146]. 

Evidence for the flocculation of emulsion droplets is commonly derived from a combination of rheological and creaming stability experiments. For instance, a marked increase in both the viscoelasticity of emulsions of moderately high oil volume fraction (40 vol%) and the rapid serum... 

Example. Cream liqueurs are an example of a food emulsion for which good stability over a period of several years is required. Thus the processes of creaming, flocculation, and coalescence must all be controlled in the formulation. The product must have a cream-like appearance and a relatively high alcohol content. A possible composition might be ... 

Figure 3.27 Breakdown mechanisms of emulsions (from top to bottom creaming, coalescence, flocculation and Ostwald ripening).

Flocculation. Flocculation means an aggregation of emulsion droplets but, in contrast to coalescence, the films of the continuous phase between the droplets survive. Hence, the process may be partially reversible. Both processes, flocculation and coalescence, speed up the creaming of an emulsion due to the increase of the drop size. The process of flocculation is even more important for dispersions of solids than for emulsions because in this case a coalescence is not possible. 

As emulsions are inherently unstable, they eventually revert to the original state of two separate liquids, that is, will break or crack. In the presence of an emulsifier and other additives, this state is approached via several distinct processes, some of which are reversible such as creaming and flocculation and others irreversible such as coalescence and Ostwald ripening. Phase inversion when an oil-in-water emulsion inverts to form a water-in-oil emulsion or visa versa is a special case of irreversible instability that occurs only under well-defined conditions such as a change in emulsifier solubility due to specific interactions with additives or to a change in temperature (Fig. 3). 

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