Flocculation emulsion concentrates

If the free polymer concentration is increased above a certain limit, phase separation may occur and the flocculated emulsion droplets may cream or sediment faster than in the absence of the free polymer. 

The change of the flocculation degree is particularly considerable with increasing concentration of oil-soluble surfactants, such as Span-80 (see Fig. 6.10). While at a Span-80 concentration of 0.4 wt% the emulsion is practically completely flocculated, increasing concentration to 1.15 wt% leads to a flocculation degree less than 0.1 %. The same rule of the change of the flocculation degree is observed for emulsions stabilized by pentol. At 0.3 wt%... 

The colloidal properties of emulsions are responsible for the quality of many foods. Ultrasound is sensitive to most of the properties of interest and can be used as both a research and a process-control tool by food scientists. As a research tool, ultrasonic measurements are particularly powerfid as they can be used to generate information not readily available by other methods - importantly, physical state, particle size, concentration, and flocculation in concentrated and optically opaque emulsions. In a process environment, ultrasonic measurements can be effected noninvasively in process lines and are therefore compatible with the stringent hygiene and cleaning requirements of food production. 

At low surfactant concentrations it is observed that an attraction dominates at short separations. The attraction becomes important at separations below about 12 nm when the surfactant concentration is 0.01 mM, and below about 6 nm when the concentration is increased to 0.1 mM. Once the force barrier has been overcome the surfaces are pulled into direct contact between the hydrophobic surfaces at D = 0, demonstrating that the surfactants leave the gap between the surfaces. The solid surfaces have been flocculated. However, at higher surfactant concentrations (1 mM) the surfactants remain on the surfaces even when the separation between the surfaces is small. The force is now purely repulsive and the surfaces are prevented from flocculating. Emulsion droplets interacting in the same way would coalesce at low surfactant concentrations once they have come close enough to overcome the repulsive barrier, but remain stable at higher surfactant concentrations. Note, however, that the surfactant concentration needed to prevent coalescence of emulsion droplets cannot be accurately determined from surface-force measurements using solid surfaces. 

No significant changes were detected in the droplet size distribution and viscosity of the emulsions with salt concentrations between 0 and 300 mM. An alteration of the droplet size distribution is not expected in weakly flocculated emulsions as the samples are agitated before the measurement, leading to redispersion of aggregates. All emulsions were very vulnerable to freeze-thaw cycling. Emulsions containing salt split after one cycle, whereas emulsions without salt split after two cycles. 

Coalescence requires that the molecules of liquid within two or more emulsion droplets coming into direct contact. Droplets therefore need to be in close proximity, which is for example the case in highly concentrated emulsions, flocculated emulsions, or creamed layers. In a subsequent step, a disruption of the interfacial membrane must occur to allow the liquid molecules to come into direct contact. The rate at which coalescence proceeds, and the physical mechanism by which it occurs, is thus highly dependent on the nature of the emulsifier used to stabilize the system. Improving the stability of an emulsion to coalescence may... 

Figure 5.6 Dependence of emulsion viscosity on droplet concentration for non-flocculated and flocculated emulsions. The viscosity increases with increasing droplet concentration.

Another method of reducing creaming or sedimentation is to induce weak flocculation in the emulsion system. This may be achieved by controlling some parameters of the system, such as electrolyte concentration, adsorbed layer thickness and droplet size. These weakly flocculated emulsions are discussed in the next section. Alternatively, weak flocculation may be produced by addition of a free (non-adsorbing) polymer. Above a critical concentration of the added polymer, polymer-polymer interaction becomes favourable as a result of polymer coil overlap and the polymer chains are squeezed out from between the droplets. This results in a polymer-free zone between the droplets, and weak attraction occurs as a result of the higher osmotic pressure of the polymer solution outside the droplets. This phenomenon is usually referred to as depletion flocculation [59] and can be applied for structuring emulsions and hence reduction of creaming or sedimentation. 

Hima and Kitamori [5] monitored the particle size changes in emulsions of oily droplets (liquid paraffin, olive oil, and orange oil) in water stabilized with arabic gum. They found that the observed flocculation depends on the oil droplet concentration, the initial size distribution, and the concentration of electrolytes. The results (Fig. 7) show that the floe size distribution curve migrates to the larger sizes with an increase in emulsion concentration. 

The repulsion between oil droplets will be more effective in preventing flocculation Ae greater the thickness of the diffuse layer and the greater the value of 0. the surface potential. These two quantities depend oppositely on the electrolyte concentration, however. The total surface potential should increase with electrolyte concentration, since the absolute excess of anions over cations in the oil phase should increase. On the other hand, the half-thickness of the double layer decreases with increasing electrolyte concentration. The plot of emulsion stability versus electrolyte concentration may thus go through a maximum. 

For example, van den Tempel [35] reports the results shown in Fig. XIV-9 on the effect of electrolyte concentration on flocculation rates of an O/W emulsion. Note that d ln)ldt (equal to k in the simple theory) increases rapidly with ionic strength, presumably due to the decrease in double-layer half-thickness and perhaps also due to some Stem layer adsorption of positive ions. The preexponential factor in Eq. XIV-7, ko = (8kr/3 ), should have the value of about 10 " cm, but at low electrolyte concentration, the values in the figure are smaller by tenfold or a hundredfold. This reduction may be qualitatively ascribed to charged repulsion. 

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... 

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