Creaming or Sedimentation of Emulsions

Since r has to be less than 1, packing constraints imply that a 3oo or Oo/a 5. Thus, the CPP for a spherical micelle is j. 

Surfactants that form spherical micelles with the above packing constraints are more suitable for O/W emulsions. 

When the CPP exceeds j, but is less than 1, spherical bilayers (vesicles) can be produced. When the CPP is 1, the bilayers may remain planar. With CPP 1, inverted micelles are produced. Surfactants that produce these structures are suitable for forming W/O emulsions. 

Case (a) represents the situation for small droplets ( 0.1 p, i.e. nanoemulsions) whereby the Brownian diffusion kT (where k is the Boltzmann constant and T is the absolute temperature) exceeds the force of gravity (mass x acceleration due to 

Whilst the optimum stability of the emulsion was found to be relatively insensitive to changes in the HLB value or the PIT of the emulsifier, its instabiHty was very sensitive to the PIT of the system. 

It is essential, therefore to measure the PIT of the emulsion as a whole (with aU other ingredients). 

As the addition of electrolytes reduces the PIT, an emulsifier with a higher PIT value will be required when preparing emulsions in the presence of electrolytes. Electrolytes cause dehydration of the PEO chains which, in effect, reduces the cloud point of the nonionic surfactant this must be compensated for by using a surfactant with a higher HLB. The optimum PIT of the emulsifier is fixed if the storage temperature is fixed. 

Measurement of the PIT can at best be used as a guide for the preparation of stable emulsions. Any assessments of stability should be evaluated by following the droplet size distribution as a function of time, using a Coulter counter or 

Care should be taken when analysing the rheological results as coalescence leads to an increase in droplet size that is usually followed by a reduction in the viscosity of the emulsion. This trend is only observed if the coalescence is not accompanied by flocculation of the emulsion droplets (which results in an increase in the viscosity). Ostwald ripening can also complicate the analysis of rheological data. 

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Creaming or sedimentation of emulsions with dro plet sizes above 1 pm eauses some experimental diffr culty because of the ehange in the total amount of spins in the NMR-aetive volume of the sample tube diuing the experiment. This can be accounted for by extra reference measurements with no gradient applied before and after each NMR scan at a particular value of 5. In addition, such reference measurements may provide information on the creaming rate which is a useful characteristic of emulsions. 

Polysaccharides increase the viscosity of the continuous phase of the emulsion. One of the main functions of polysaccharides in emulsions is to thicken the continuous hquid. The intended effect is usually to impart a desired texture (increase viscosity or stiffness to the system and reduce buoyancy-driven creaming or sedimentation of the emulsion droplets and other particles in the system). Because of their highly swollen molecular structure in solutions, leading to a high effective volume fraction at low concentrations, most polysaccharides are very effective in providing a high viscosity at low concentration. 

Density differences between the dispersed and continuous phase result in creaming or sedimentation of the droplets. The droplet concentration within the product becomes inhomogeneous but the individual droplets preserve their size. In emulsions of low viscosity and smaller droplet size (< 0.7 p,m), separation under earth gravity will be counteracted by Brownian motion. Creaming and sedimentation processes in very dilute emulsions (volume fraction (p < 0.01) can be described using Stokes law... 

The above-mentioned viscoelastic polymer solutions reduce (or eliminate) creaming or sedimentation of the emulsion, providing they produce an elastic network in the continuous phase that is sufficient to overcome the stresses exerted by the creaming or sedimenting droplets. Such viscoelastic solutions produce a very high zero shear viscosity that is sufficient to eliminate creaming or sedimentation. 

X10" ms" for 10 pm droplets and 4.4 x 0 ms for 1 pm droplets. This means that in a 0.1 m container creaming or sedimentation of the 10 pm droplets is complete in 0.6 hour and for the 1 pm droplets this takes 60 hours. For moderately concentrated emulsions (0.2 < cj) < 0.1) one has to take into account the hydrodynamic interaction between the droplets, which reduces the Stokes velocity to a value v given by the following expression [97] ... 

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 ... 

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 The process of emulsion droplets floating upwards under gravity or a centrifugal field to form a concentrated emulsion (cream) quite distinct from the underlying dilute emulsion. Creaming is not the same as the breaking of an emulsion. See also Sedimentation. 

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... 

Creaming of an emulsion or sedimentation of a given suspension can be reduced in several ways ... 

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 ... 

The data in Figure 10.24 also show that when

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

[Pg.189]

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 functional requirements of practical food emulsions are not complete stability, but rather controlled instability. Destabilizing reactions of food emulsions involve creaming, flocculation, and coalescence. An emulsion would cream or sediment if the dispersed phase is sufficiently different in density from the continuous phase. Creaming can be reduced by increasing the viscosity of the aqueous phase or be enhanced by increasing the particle size of oil droplets or lowering the density of the oil phase. 

Creaming or sedimentation is not a problem in the study of most food emulsions (such as low-calorie spreads), highly concen trated emulsions, or viscous water-in-crude oil emul... 

In many cases the creaming or sedimentation occurs simultaneously with coalescence and is related to emul sion stability. In the next section, we will briefly con sider the assessment of emulsion shelf-life by NMR. 

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