Info

FIGURE 7.10 Influence of pH and ionic strength on the stability of 20% corn-oil-in-water emulsions to flocculation. Extensive flocculation is observed near the isoelectric point of the proteins.

The ionic strength of an aqueous solution depends on the concentration and valency of the ions it contains (Israelachvili 1992). As the ionic strength is increased, the electrostatic repulsion between droplets is progressively screened, until eventually it is no longer strong enough to prevent flocculation (Section 3.11.1). The minimum amount of electrolyte required to cause flocculation is known as the critical flocculation concentration or CFC. The CFC decreases as the surface potential of the emulsion droplets decreases and as the valency of the counterions increases. It has been shown that CFC ^ W^/z2 (where is the surface potential and z is the counterion valency) for droplets with relatively low surface potentials (i.e., < 25 mV) (Hunter 1986). These low surface potentials are found in many food emulsions which are susceptible to flocculation. Under certain conditions, is inversely proportional to the valency of the counterions, so that CFC ^ 1/z6, which is known as the Schultz-Hardy rule (Hunter 1986). This relationship indicates that a much lower concentration of a multivalent ion than a monovalent ion is required to cause flocculation. The Schultz-Hardy rule can be derived from the DLVO theory by assuming that the CFC occurs when the potential energy barrier, which normally prevents droplets from aggregating, falls to a value of zero due to the addition of salt. Consequently, when two droplets collide with each other, they immediately aggregate into the primary minima. In practice, significant droplet flocculation occurs when the potential energy barrier is slightly higher than zero, and therefore the Schultz-Hardy rule is expected to slightly overestimate the CFC (Hunter 1986).

Some types of ionic species are capable of specifically binding to oppositely charged emulsifier groups and can therefore alter the surface charge. Specifically adsorbed ions may either decrease, increase, or reverse the charge on a droplet depending on their valency and the nature of the specific interactions involved (Section 3.4.2.1). In addition to their influence on surface charge, specifically adsorbed ions are often highly hydrated and therefore increase the short-range hydration repulsion between droplets (Section 3.8).

The presence of multivalent ions in the continuous phase of an electrostatically stabilized emulsion has a strong influence on its flocculation stability (Dickinson et al. 1992; Agboola and Dalgleish 1995, 1996a), first because multivalent ions are much more effective at screening the electrostatic interactions between droplets than monovalent ions and, second, because they are capable of forming electrostatic bridges between two droplets which have the same charge (Section 3.4.4). For example, the addition of calcium ions (Ca2+) to a protein-stabilized emulsion that contains negatively charged droplets can cause droplet flocculation because the ions shield the electrostatic repulsion between the droplets and because they form protein-calcium-protein cross-links between the droplets (Dickinson et al. 1992; Agboola and Dalgleish 1995, 1996a). These multivalent ions may be simple ions (e.g., Ca2+, Mg2+, or

Al3+), small organic ions (amino acids), or charged biopolymers (e.g., proteins or polysaccha-rides). The influence of multivalent ions on emulsion stability can be reduced by ensuring that they are excluded from the formulation or by adding ingredients which form complexes with them (e.g., EDTA in the case of calcium ions).

The flocculation stability of emulsions that contain electrically charged droplets can be controlled in a variety of ways depending on the system. To prevent flocculation, it is necessary to ensure that the droplets have a sufficiently high surface potential under the prevailing environmental conditions (which often requires that the pH be controlled) and that the electrolyte concentration is below the CFC. In some important types of food, it is necessary to have relatively high concentrations of electrolyte present for nutritional purposes (e.g., mineral-fortified emulsions used in infant, elderly, and athlete formulations). In these systems, the influence of the electrolyte on flocculation stability can be reduced by changing to a nonionic emulsifier or by incorporating ingredients which complex the mineral.

Polymeric Steric Interactions. Many food emulsifiers prevent droplet flocculation through polymeric steric repulsion (Section 3.5). This repulsion must be sufficiently strong and long range to overcome any attractive interactions (Section 3.11.3). Sterically stabilized emulsions are much less sensitive to variations in pH and ionic strength than electrostatically stabilized emulsions (Hunter 1986). Nevertheless, they can become unstable to flocculation under certain conditions. If the composition of the continuous phase or the temperature is altered so that polymer-polymer interactions become more favorable than solvent-solvent/solventpolymer interactions, then the mixing contribution to the steric interaction becomes attractive and may lead to droplet flocculation. A sterically stabilized emulsion may also become unstable if the thickness of the interfacial membrane is reduced (Section 3.11.3), which could occur if the polymeric segments on the emulsifier were chemically or biochemically cleaved (e.g., by acid or enzyme hydrolysis) or if the continuous phase becomes a poor solvent for the polymer segments. Short-range hydration forces make an important contribution to the flocculation stability of many sterically stabilized emulsions (Israelachvili 1992, Evans and Wennerstrom 1994). In these systems, droplet flocculation may occur when the emulsion is heated, because emulsifier head groups are progressively dehydrated with increasing temperature (Israelachvili 1992, Aveyard et al. 1990).

Biopolymer Bridging. Many types of biopolymer promote flocculation by forming bridges between two or more droplets (Lips et al. 1991). Biopolymers may adsorb either directly to the bare surfaces of the droplets or to the adsorbed emulsifier molecules that form the interfacial membrane (Walstra 1996a). To be able to bind to the droplets, there must be a sufficiently strong attractive interaction between segments of the biopolymer and the droplet surface. The most common types of interaction that operate in food emulsions are hydropho-bic and electrostatic (Dickinson 1989, 1992).

When a biopolymer has a number of nonpolar residues along its backbone, some of them may associate with hydrophobic patches on one droplet, while others associate with hydro-phobic patches on another droplet. This type of bridging flocculation tends to occur when a biopolymer is used as an emulsifier and there is insufficient present to completely cover the oil-water interface formed during homogenization (Walstra 1996a). Bridging may occur either during the homogenization process or after it is complete (e.g., when a biopolymer is only weakly associated with a droplet, some of its segments can desorb and become attached to a neighboring droplet). This type of bridging flocculation can usually be prevented by ensuring there is a sufficiently high concentration of biopolymer present in the continuous phase prior to homogenization (Dickinson and Euston 1991, Dickinson 1992, Stoll and Buffle 1996).

FIGURE 7.11 Influence of temperature on the flocculation stability of 20% corn-oil-in-water emulsions stabilized by whey protein isolate (pH 7, 0 mM NaCl). Flocculation is observed when the emulsions are heated above 70°C because of protein unfolding and exposure of hydrophobic groups.

Bridging flocculation can also occur when a biopolymer in the continuous phase has an electrical charge which is opposite to that of the droplets (Pal 1996). In this case, bridging flocculation can be avoided by ensuring the droplets and biopolymer have similar charges or that either the droplets or biopolymer are uncharged.

Hydrophobic Interactions. This type of interaction is important in emulsions that contain droplets which have nonpolar regions exposed to the aqueous phase. Their role in influencing the stability of food emulsions has largely been ignored, probably because of the lack of theories to describe them and experimental techniques to quantify them. One of the clearest examples of their importance in food emulsions is the effect of heat on the flocculation stability of oil-in-water emulsions stabilized by globular proteins (Hunt and Dalgleish 1995, Demetriades et al. 1997b). At room temperature, whey protein stabilized emulsions (pH 7, 0 mM NaCl) are stable to flocculation because of the large electrostatic repulsion between the droplets, but when they are heated above 70°C, they become unstable (Figure 7.11). The globular proteins adsorbed to the surface of the droplets unfold above this temperature and expose nonpolar amino acids which were originally located in their interior (Monahan et al. 1996, Dalgleish 1996a). Exposure of these nonpolar amino acids increases the hydrophobic character of the droplet surface and therefore leads to flocculation because of the increased hydrophobic attraction between the droplets (Hunt and Dalgleish 1995, Monahan et al. 1996, Demetriades et al. 1997b).

Hydrophobic interactions are also likely to be important in emulsions in which there is not enough emulsifier present to completely saturate the surfaces of the droplets. This may occur when there is insufficient emulsifier present in an emulsion prior to homogenization or when an emulsion is so diluted that some of the emulsifier desorbs from the droplet surfaces.

Flocculation due to hydrophobic interactions can be avoided by ensuring that there is sufficient emulsifier present to completely cover the droplet surfaces or by selecting an emulsifier which does not undergo detrimental conformational changes at the temperatures used during processing, storage, or handling.

Depletion Interactions. The presence of nonadsorbing colloidal particles, such as biopolymers or surfactant micelles, in the continuous phase of an emulsion causes an increase in the attractive force between the droplets due to an osmotic effect associated with the exclusion of colloidal particles from a narrow region surrounding each droplet (Section 3.6). This attractive force increases as the concentration of colloidal particles increases, until eventually it may become large enough to overcome the repulsive interactions between the droplets and cause them to flocculate (Aronson 1991; Jenkins and Snowden 1996; Dickinson and Golding

0.0 0.005 0.0075 0.01 0.015 0.02 0.04 0.06 %XANTHAN

FIGURE 7.12 Influence of xanthan concentration on the stability of 20% corn-oil-in-water emulsions to depletion flocculation. Flocculation is observed above the critical flocculation concentration (i.e., % xanthan > 0.0075 wt%).

0.0 0.005 0.0075 0.01 0.015 0.02 0.04 0.06 %XANTHAN

FIGURE 7.12 Influence of xanthan concentration on the stability of 20% corn-oil-in-water emulsions to depletion flocculation. Flocculation is observed above the critical flocculation concentration (i.e., % xanthan > 0.0075 wt%).

1997a,b; Dickinson et al. 1997). This type of droplet aggregation is usually referred to as depletion flocculation (Walstra 1996a,b). The lowest concentration required to cause depletion flocculation is referred to as the critical flocculation concentration by analogy to the CFC used to characterize the effect of salt on the stability of electrostatically stabilized emulsions. The CFC decreases as the size of the emulsion droplets increases (Section 3.6.3). The flocculation rate initially increases as the concentration of nonadsorbing colloidal particles is increased because of the enhanced attraction between the droplets (i.e., a higher collision efficiency). However, once the concentration of colloidal particles exceeds a certain concentration, the flocculation rate may actually decrease because the viscosity of the continuous phase increases so much that the movement of the droplets is severely retarded (i.e., a lower collision frequency). This is clearly illustrated in Figure 7.12, which shows the dependence of the height of the creamed layer in a series of corn-oil-in-water emulsions containing different concentrations of xanthan (Basaran et al. 1998). Xanthan is a nonadsorbing biopolymer which is normally used as a stabilizer or thickening agent in food emulsions (BeMiller and Whistler 1996). In the absence of xanthan, the emulsions are stable to creaming over a 24-h period. At xanthan concentrations around 0.01%, there is a net attraction between the droplets which causes them to flocculate and therefore cream rapidly. At higher xanthan concentrations, there is still a strong attraction between the droplets, but they are unable to move because of the large increase in the viscosity of the continuous phase.* Similar observations have been obtained for emulsions to which ionic or nonionic surfactant micelles have been added (Bibette 1991, Aronson 1992, McClements 1994, Jenkins and Snowden 1996). For ionic colloidal particles, such as sodium dodecyl sulfate (SDS) micelles or charged biopolymers, the CFC is expected to be strongly dependent on electrolyte concentration because of its ability to reduce the electrostatic repulsion between colloidal particles and therefore reduce their effective size.

Hydrodynamic Interactions. The efficiency of the collisions between droplets is also determined by the strength of the hydrodynamic interactions between them (Davis et al. 1989, Dukhin and Sjoblom 1996). As two droplets approach each other, a repulsion arises because

* Even when the droplets do flocculate at higher xanthan concentrations, the movement of the flocs themselves will be retarded, and therefore no creaming is observed.

of the resistance associated with the flow of the continuous phase from the thin gap between them. The magnitude of this resistance decreases as the droplet surfaces become more mobile, leading to an increase in the collision efficiency (Section 3.10). On the other hand, the collision efficiency may be reduced when the droplet surfaces are stabilized by small-molecule surfactants because of the Gibbs-Marangoni effect (Walstra 1996a,b).

Influence of Droplet Size. It should be noted that the magnitude of the colloidal interactions between emulsion droplets usually increases with droplet size, which will cause an increase in the height of any energy barriers and in the depth of any energy minima. As a consequence, altering the droplet size may either increase or decrease the stability of an emulsion, depending on the nature of the system.

Was this article helpful?

0 0
Atkins Low Carb Diet Recipes

Atkins Low Carb Diet Recipes

The Atkins Diet is here. Dr Atkins is known for his great low carb diets. Excluding, Dr Atkins carb counter and Dr Atkins New Diet Revolution.

Get My Free Ebook


Post a comment