Creaming or sedimentation

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 dispersed system of two immiscible phases. Emulsions are present in several food systems. In general, the disperse phase in an emulsion is normally in globules 0.1-10 microns in diameter. Emulsions are commonly classed as either oil in water (O/W) or water in oil (W/O). In sugar confectionery, O/W emulsions are most usually encountered, or perhaps more accurately, oil in sugar syrup. One of the most important properties of an emulsion is its stability, normally referred to as its emulsion stability. Emulsions normally break by one of three processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation originate in density differences between the two phases. Emulsions often break by a mixture of the processes. The time it takes for an emulsion to break can vary from seconds to years. Emulsions are not normally inherently stable since they are not a thermodynamic state of matter. A stable emulsion normally needs some material to make the emulsion stable. Food law complicates this issue since various substances are listed as emulsifiers and stabilisers. Unfortunately, some natural substances that are extremely effective as emulsifiers in practice are not emulsifiers in law. An examination of those materials that do stabilise emulsions allows them to be classified as follows ... 

The sequence, flocculation - coalescence — separation, is complicated by the fact that creaming or sedimentation occurs and that Ibis process is determined by the droplet size. 

Problems with large or dense particles. If the particles are particularly large or have a large density contrast with the surrounding medium, there may be problems due to creaming or sedimentation. In these situations, it is necessary to increase the viscosity of the suspending liquid and/or to use a stirring device to ensure that the particles are uniformly distributed (Lines, 1996). 

Creaming or sedimentation. Creaming or sedimentation is one of the principal instability mechanisms seen in emulsions. Emulsion... 

Probably the most important physical property of an emulsion is its stability. The term emulsion stability can be used with reference to three essentially different phenomena - creaming (or sedimentation), coagulation and a breaking of the emulsion due to droplet coalescence. 

One of the most important properties of an emulsion is its stability. Emulsions normally break by one of three different processes creaming (or sedimentation), flocculation or droplet coalescence. Creaming and sedimentation have their origin in density differences between the two phases, and emulsions often break by a mixture of the three main processes. The time it takes for an emulsion to break can vary from seconds to years. 

Creaming or sedimentation occurs when the dispersed droplets or floccules separate under the influence of gravity to form a layer of more concentrated emulsion, the cream. Generally a creamed emulsion can be restored to its original state by gentle agitation. This process, which inevitably occurs in any dilute emulsion if there is a density difference between the phases as a consequence of Stokes law, should not be confused with flocculation which is due to particle interactions resulting from the balance of attractive and repulsive forces. Most oils are less dense than... 

Particles will still collide, but the frequency or the impact of the collisions can be minimised. What happens when the particles do come into close contact The encounters may lead to permanent contact of solid particles or to coalescence of liquid droplets. If they are allowed to continue unchecked, the colloidal system destroys itself through growth of the disperse phase and excessive creaming or sedimentation of the large particles. Whether these collisions result in permanent contact or whether the particles rebound and remain free depends on the forces of interaction, both attractive and repulsive, between the particles, and on the nature of the surface of the particles. 

The very small droplets prevent any creaming or sedimentation, as the Brownian diffusion is sufficient to prevent separation by gravity. 

Case (b) represents emulsions consisting of monodisperse droplets with radius > 1 pm. In this case, the emulsion separates into two distinct layers with the droplets forming a cream or sediment and leaving the clear supernatant hquid this situation is seldom observed in practice. 

Case (c) is that for a polydisperse (practical) emulsions, in which case the droplets will cream or sediment at various rates. In this last case a concentration gradient build-up occurs, with the larger droplets staying at the top of the cream layer or the bottom of the sediment ... 

Creaming or Sedimentation Rates 10.6.1.1 Very Dilute Emulsions [cp < 0.01)... 

The rate of creaming or sedimentation becomes a complex function of 4>, as illustrated in Figure 10.24, which also shows the change of relative viscosity with [Pg.189]

The data in Figure 10.24 also show that when

. In practice, most emulsions are prepared at Rvalues well below

[Pg.189]

Since the gravity force is proportional to R, then if R is reduced by a factor of 10, the gravity force is reduced by 1000. Below a certain droplet size (which also depends on the density difference between oil and water), the Brownian diffusion may exceed gravity and creaming or sedimentation is prevented. This is the principle of formulation of nanoemulsions (with size range 20-200 nm) that may show very little or no creaming or sedimentation. The same applies for microemulsions (size range 5-50 nm). 

These are high-molecular-weight polymers, either natural or synthetic, such as xanthan gum, hydroxyethyl cellulose, alginates, and carrageenans. In order to understand the role of these thickeners, the gravitational stresses exerted during creaming or sedimentation should be considered ... 

Simple calculation shows that is in the range 10" to 10" Pa, which implies that for prediction of creaming or sedimentation it is necessary to measure the viscosity at such low stresses. This can be achieved by using constant stress or creep measurements. 

The above-described thickeners satisfy the criteria for obtaining very high viscosities at low stresses or shear rates. This can be illustrated from plots of shear stress a and viscosity tj versus shear rate y (or shear stress), as shown in Figure 10.25. These systems are described as pseudoplastic or shear thinning. The low shear (residual or zero shear rate) viscosity tj(0) can reach several thousand Pa s, and such high values prevent creaming or sedimentation [24, 25]. 

The above-described behaviour is obtained above a critical polymer concentration (C ), which can be located from plots of log tj versus log C, as is illustrated in Figure 10.26. Below C, the log //-log C curve has a slope in the region of 1, whereas above C the slope of the line exceeds 3. In most cases a good correlation between the rate of creaming or sedimentation and //(O) is obtained. 

The above phenomenon of weak flocculation may be applied to reduce creaming or sedimentation, although in practice this is not easy as the droplet size has also to be controlled. 

The above weak flocculation can be appHed to reduce creaming or sedimentation, although it suffers from the following drawbacks ... 

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 use of a rheology modifier such as HEC or xanthan gum that produces a viscoelastic solution which prevents not only creaming or sedimentation but also entrapment of the oil droplets in between the suspension particles, or the suspension particles in between the emulsion droplets. 

The very small droplet size causes a large reduction in the gravity force, and the Brownian motion may be sufficient for overcoming gravity this means that no creaming or sedimentation wiU occur on storage. 

For smaller particles, smaller stresses are exerted. Thus, in order to predict sedimentation it is necessary to measure the viscosity at very low stresses (or shear rates). These measurements can be carried out using a constant stress rheometer (Carrimed, Bohlin, Rheometrics, Haake or Physica). Usually, a good correlation is obtained between the rate of creaming or sedimentation, v, and the residual viscosity rj 0), as will be described in Chapter 21. Above a certain value of ri(0), v becomes equal to 0. Clearly, in order to minimize sedimentation it is necessary to increase rj 0) an acceptable level for the high shear viscosity must be achieved, depending on the application. In some cases, a high rj[0) may be accompanied by a high rj (which may not be acceptable for apphcation, for example if spontaneous dispersion on dilution is required). If this is the case, the formulation chemist should seek an alternative thickener. 

As mentioned above, thickeners reduce creaming or sedimentation by increasing the residual viscosity tj(o), which must be measured at stresses compared to... 

Most fornmlations undergo creaming or sedimentation as a result of the density difference between disperse phase particles and medium [1]. This situation is particularly the case with most practical systems that contain particles with radii R that are large (>1 pm), whereby the Brownian diffusion is not sufficient to overcome the gravity force, that is... 

Sedimentation or creaming is prevented by the addition of thickeners that form a three-dimensional elastic network in the continuous phase. If the viscosity of the elastic network, at shear stresses (or shear rates) comparable to those exerted by the particles or droplets, exceeds a certain value, then creaming or sedimentation is completely eliminated. 

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