Emulsification is a process that occurs when a liquid is dispersed in fine droplets without dissolving in another liquid and is basically defined as the process of forming emulsions. Emulsification occurs more easily when the interfacial surface tension between two liquids is reduced. Surfactants (surfactants) support this process, enabling a more homogeneous and stable dispersion of one liquid in the other.

An emulsifier increases the stability of the emulsion by forming globules that repel each other and are evenly dispersed or suspended in the mixture. Emulsifiers prevent the coalescence of droplets by forming a film on the surface of the dispersed phase. Some emulsifiers increase the viscosity of the medium, making it difficult for the globules to settle or rise to the surface and facilitate their suspension.

Emulsions are a colloid system consisting of a mixture of two liquids and consist of two main components: dispersed phase (disperse phase) and dispersion medium (continuous phase). The type of emulsion varies depending on which phase is dispersed in which medium. For example, oil in water (O/W) or water in oil (W/O) emulsions are the most common types.

Some substances used as emulsifiers and stabilisers include hydrocolloids (acacia gum and tragacanth), glycerin and polymer carboxymethyl cellulose (CMC). These ingredients are used to keep the emulsion stable for longer and are widely used in food, pharmaceuticals, cosmetics and many industrial products.

What is Emulsification?

Emulsification is the process of stabilizing a mixture formed by combining two immiscible liquids. Typically, it involves distributing one liquid (such as oil) within another (such as water) to create a uniform mixture. The substances that facilitate this process and help maintain stability are called emulsifiers.

Emulsifiers

An emulsifier is a substance that stabilizes an emulsion by reducing the interfacial tension between oil and water. Emulsifiers are part of a broader group of compounds known as surfactants.

Examples of food emulsifiers include:

• Egg Yolk – The primary emulsifying and thickening agent is lecithin.
• Mustard – Various chemicals in the mucilage surrounding mustard seeds act as emulsifiers.
• Soy Lecithin – Another common emulsifier and thickener.
• Pickering Stabilization – Under certain conditions, particles are used for stabilization.
• Mono- and Diglycerides – These are common emulsifiers found in many food products like coffee creamers, ice creams, spreadable products, breads, and cakes.
• DATEM (Diacetyl Tartaric Acid Ester of Mono- and Diglycerides) – Mainly used as an emulsifier in baking.
• Proteins – Those with both hydrophilic and hydrophobic regions, such as sodium caseinate in processed cheese products.

What is Emulsion?

An emulsion is the process where two liquids that normally do not mix are made to appear as though they are mixed, with the help of another substance. However, this mixture is not truly homogeneous but rather a “heterogeneous structure that appears homogeneous.” In an emulsion, the dispersed phase, which is the liquid that is distributed, is known as the internal phase, while the other part is the continuous phase, or the dispersion medium.

Emulsions are generally classified into two main categories based on their continuous phase:

1. Oil in Water (O/W) Emulsions: In these emulsions, oil is dispersed as small droplets in the water.
2. Water in Oil (W/O) Emulsions: In these emulsions, water is dispersed as small droplets in the oil.

Generally, emulsions create a homogeneous appearance of two liquids. But how is this appearance achieved? In an emulsion, one of the liquid phases is polar (for example, water) and the other is nonpolar (for example, oil). When mixed, a heterogeneous structure is expected. Here, surfactants come into play to bridge the gap. Surfactant molecules are active at the surface rather than within the liquid. These molecules are amphiphilic, meaning they have both a hydrophilic (water-loving) and a hydrophobic (oil-loving) part.

Emulsion Stability

An emulsion is composed of two immiscible liquids, in which one of them is dispersed as droplets into the other liquid named the continuous phase. In the food industry, manufacturers develop many products, such as milk, cream, butter, and margarine, which contain emulsion as part of or the entirety of its matrix. As noted earlier, all emulsions are unstable by nature, and the two phases will eventually separate when they are allowed to stand for long enough time. The instability of emulsions may result in some undesirable effects in food including oiling off and sedimentation, which decrease the product quality and shorten shelf life. Therefore, it is important for investigators to understand the mechanisms that cause emulsion instability and to accurately evaluate the stability of such system.

Emulsion stability refers to the ability of emulsions to resist changes in its physicochemical properties over time. The mechanisms that lead to emulsion instability include gravitational separation (creaming/sedimentation), flocculation, coalescence, Ostwald ripening, and phase inversion. The stability of emulsions is influenced by their compositional materials and processing conditions, from which different characteristics of their containing droplets would be developed. Important droplet characteristics include their concentration, size, charge, interactions, and rheological behavior

 Evaluation of Emulsion Stability

Determining the acceptable shelf life is crucial in the development of emulsion formulations. There is no single method available to measure the instability of an emulsion. To accelerate the stability program, the emulsion is subjected to stressed conditions. The stressed conditions used to evaluate physical instability include aging, temperature, centrifugation, ultracentrifugation, and stirring.

Aging and Temperature: The shelf life of an emulsion is determined by storing it at high temperatures for different periods. Temperature changes affect the physical and chemical stability of emulsions. Specifically, emulsions stored at high temperatures show a significant decrease in viscosity. Moreover, temperature changes can affect the hydration of polymers and colloidal substances, the distribution of emulsifiers, phase transitions, and the crystallization of certain lipids. The temperature cycling between two temperatures is important for shelf-life evaluation and should be between 4-45°C. High temperatures increase the rates of coalescence and creaming, and changes in viscosity are also observed.

Centrifugation and Ultracentrifugation: Centrifugation and ultracentrifugation create stress in emulsions, allowing their stability to be evaluated. Phase separation occurs rapidly with centrifugation. This method is useful for predicting the shelf life of emulsions but may not be suitable for very viscous or semi-solid products. The ultracentrifugal force can cause the breakdown of droplet structures in emulsions.

Stirring: In an emulsion, coalescence does not occur unless collisions between emulsion droplets take place due to Brownian motion. Simple mechanical stirring causes two droplets to collide. Emulsions that are stirred too rapidly or excessively can both form emulsions and lead to their breakdown.

Factors Affecting Emulsion Formation

For an emulsifier to be effective at forming small droplets during homogenization, it must possess several physicochemical characteristics:

(i) Surface Activity: Emulsifiers need to be capable of adsorbing to oil-water interfaces, requiring a balanced ratio of polar and non-polar groups on their surfaces.

(ii) Adsorption Kinetics: Emulsifiers must quickly adsorb to droplet surfaces during homogenization, enabling rapid reduction of interfacial tension and preventing droplet aggregation.

(iii) Interfacial Tension Reduction: Adsorbed emulsifiers should effectively reduce interfacial tension, which aids in breaking up droplets within homogenizers.

(iv) Stabilization: Emulsifiers should prevent droplets from aggregating during encounters by generating strong repulsive interactions, such as steric or electrostatic repulsion.

(v) Surface Coverage: The amount of emulsifier needed to stabilize an emulsion depends on surface load, which refers to the mass of emulsifier per unit surface area at saturation. A higher surface load requires more emulsifier to stabilize the emulsion.

Natural emulsifiers can differ significantly in these characteristics, leading to variations in their ability to form stable emulsions

Comparison of Different Natural Emulsifiers

Proteins:

Protein-coated lipid droplets are typically stabilized against aggregation through electrostatic and steric repulsion. Long-range electrostatic interactions primarily protect droplets from flocculation, while short-range steric interactions help prevent coalescence. However, several factors can promote aggregation of protein-coated droplets:

• pH Changes: Protein-coated droplets tend to flocculate near the protein’s isoelectric point due to reduced charge magnitude.
• High Ionic Strength: High salt concentrations, particularly multivalent ions, can screen electrostatic repulsion, promoting flocculation.
• Elevated Temperatures: Emulsions held above the thermal denaturation temperature of proteins may promote flocculation due to increased hydrophobic attraction when non-polar groups are exposed.
• Protease Activity: Proteases, such as gastric pepsin, can hydrolyze the protein layers on the droplets, encouraging aggregation.

Polysaccharides:

Polysaccharide-coated lipid droplets are stabilized mainly through steric repulsion due to the large hydrophilic groups that extend into the aqueous phase. These emulsions tend to be stable under varying pH, ionic strength, and temperature conditions. However, the presence of an electrical charge in some polysaccharides can influence their interactions with other charged substances like transition metals or biopolymers.

Phospholipids:

Lecithin-coated droplets are stabilized against aggregation by electrostatic repulsion from the electrical charge on the phospholipid head groups. At low ionic strengths, lecithin-stabilized emulsions tend to be stable at neutral pH due to the high negative charge. However, they become unstable under acidic conditions due to a reduction in charge and electrostatic repulsion. High salt concentrations may promote aggregation in lecithin-stabilized emulsions by screening electrostatic interactions. Lecithin-coated droplets are relatively stable at high temperatures due to strong electrostatic repulsion.

Saponins:

Saponin-coated droplets are stabilized primarily by their electrical charge, mainly attributed to glucuronic acids. These droplets are highly charged at neutral pH but lose charge as the pH decreases. Saponins are stable at low ionic strengths from pH 8 to 3, but flocculate at pH 2 due to charge loss. They are also stable to droplet aggregation at high temperatures due to strong electrostatic repulsion. Saponins appear to be effective at forming small droplets that remain stable across a wide range of conditions, making them an attractive option for emulsification. Identifying other commercially viable sources of saponins could be valuable.