An emulsion is a blend of two or more immiscible liquids or liquid and gas, with droplets of one phase distributed in the other phase. Examples of food emulsions are mayonnaise, milk, and some salad dressings. Emulsions contain both a dispersed and a continuous phase with a boundary between them called the interface. The boundary is where the emulsifier sits to create stability.
Types of Emulsions
- Oil in water,
- Water in oil
- Air in Water
- Air in Oil
Without emulsifiers, these are unstable systems in which the dispersed phase droplets tend to agglomerate or coalescence and separate out. The best example of this is oil and water. When oil and water are mixed together, the oil will quickly agglomerate and float to the surface. The addition of an emulsifier will allow the oil and water to mix and not separate.
Emulsifiers contain both hydrophilic (water-loving) and lipophilic (Fat or Air Loving) parts. The hydrophilic end is easy to hydrate (water soluble) and the lipophilic end is very difficult to hydrate (oil soluble). The emulsifier will arrange itself with the water hydrophilic end pointed towards the water and the lipophilic end pointed at the fat.
Emulsifiers arrange themselves at the interface between oil and water and reduce the surface or interfacial tension, or to allow them to mix, thereby making the emulsion more stable.
In food emulsifiers, the hydrophilic or water loving part typically is glycerol, sorbitol, sucrose, propylene glycol or polyglycerol.
The lipophilic or fat/air loving part is formed by fatty acids derived from fats and oils
Emulsion stability refers to the ability of an emulsion to resist change in its properties over time. There are four types of instability in emulsions:
Flocculation occurs when there is an attractive force between the droplets, so they form flocs, like bunches of grapes.
The droplets rise to the top of the emulsion under the influence of buoyancy
Coalescence occurs when droplets bump into each other and combine to form a larger droplet, so the average droplet size increases over time.
- Ostwald ripening.
Small crystals or sol particles dissolve, and redeposit onto larger crystals or sol particles.
The stability of an emulsion depends on:
- Droplet size
- a smaller droplet size facilitates emulsion stability. The droplet size can be influenced by homogenization.
- Smaller droplets are more stable.
- Viscosity of continuous phase:
- a higher viscosity facilitates emulsion stability. The viscosity of the water phase can be influenced by the addition of thickening agents such as gums and starches.
- Specific density of the two phases:
- if the difference in density between the two phases is small,the emulsion will be more stable.
- Quality of the interfacial film:
- the film can consist of emulsifiers and/or proteins.
Emulsifiers are mixtures
During the esterification process the fatty acids are distributed at random, so a mixture of mono- and diglyceride (esters) are formed. Distillation will reduce but not eliminate the minor components.
Commercial emulsifiers are characterized by the major component, but minor amounts of related molecules will also be present.
Fatty acid composition
The type of fatty acid (the type of fat or oil used) influences the characteristics of the emulsifier.
The fatty acid composition influences the melting point of fats and oils, and in the same way influences the melting point of emulsifiers. Fatty acids with a longer chain length and which are more saturated will result in higher melting points.
Monoglycerides are also characterized by the fatty acid composition, saturated or unsaturated fatty acids. More saturated emulsifiers have a lower iodine value.
Emulsifier Crystal form.
Fats and oils can form different crystals which affect the texture and physical properties of the fat. The three major crystalline forms are referred to as alpha-, beta prime- and beta-crystals. The alpha crystal has the lowest melting point and forms fine and flexible agglomerates. Some emulsifiers are referred to as alpha tending emulsifiers; these are most stable in the alpha crystalline form. The alpha crystalline formation is very effective for whipping properties.
HLB: Hydrophilic Lipophilic Balance
Emulsifiers can be characterized by the Hydrophilic-Lipophilic Balance. The balance is measured on molecular weight and is an indication of the solubility of the emulsifier. The HLB scale varies between 0 and 20.
Emulsifiers with a low HLB value are more soluble in oil and creates water-in-oil emulsions.
Emulsifiers with a high HLB value are more soluble in water and creates oil-in-water emulsions.
The HLB value of an emulsifier can be used as an indication of its possible use.
An emulsifiers HLB can vary within one group or type. For example sucrose esters, and lecithins can be made with very high or very low HLB values
Typical HLB values of common food emulsifiers
Glycerol Monostearate 3.8
Sorbitan Monooleate 4.3
Sorbitan Monostearate 4.7
Sorbitan Monopalmitate 6.7
sodium Stearoyl Lactylate 10-12
Polysorbate 60 14.9
Polysorbate 80 15.0
Sucrose Esters 1-16
Emulsifiers concentrate to the surface between the oil and water phase, two liquid phases. Emulsifiers will also concentrate on any surface of two immiscible phases such as:
- the interface of gas and liquid as air bubbles in a whipped cream or cake batter.
- the interface of solids and liquids as ice crystals in ice cream or sugar crystals in chocolate.
Emulsifiers influence the consistency, the viscosity and the texture of many multi-phase food systems.
Interaction with starch
Emulsifiers with fully saturated long fatty acid chain form complexes with amylose starch. The fatty acid chain penetrates the amylose helix and prevents starch retrogradation or the recrystallization of the starch. Retrogradation is the mechanism responsible for staling of bread Typically a monoglyceride, SSL and Datem are used for starch complexing in yeast raised baked items. The long saturated chain of the fatty acid will complex in the helix of the straight chain starch.
Another attribute of the use of emulsifiers in starch-based products is the reduction of stickiness in reconstituted products such as pasta and instant mashed potatoes, and tortillas.
Interaction with protein
Emulsifiers with an ionic structure can interact with proteins, particularly wheat gluten. This interaction strengthens the gluten network in yeast raised dough resulting in increased volume, tolerance to freezing and improved crumb structure. Typically SSL and Datem are used to increase gluten strength. This type of protein interaction is the disulfide bond in the gluten matrix. Ethoxylated monoglycerides also reinforce the gluten matrix, but they do with hydrogen bonding.
Ionic emulsifiers such as SSL can interact with proteins in sauces and gravies to improve mouthfeel.
Interaction with fat
Emulsifiers are fat-like substances and they influence fat by promoting or inhibiting crystallization, influence the crystal shape of the fat, and improve the dispersion of fat crystals inside the food product. Fats can exist in more than one crystalline form. The number of forms will vary by fat. Most fats can exist in alpha, beta, and beta prime. Cocoa Butter can exist in many more crystal forms. Certain crystals are preferred in certain products and emulsifiers can be used to promote and retain the desired crystal structure.
For example, chocolate bloom is a type of undesirable crystal change that occurs that will give the chocolate a dull appearance or mottling. Lactic acid esters, sorbitan esters, and polysorbates can delay this crystal change.
Alpha Crystal State - Melts at a lower temperature than beta or beta prime. Typically this form does not give the best texture. It is also a very short lived crystal.
Beta Prime - This is typically the desired crystalline state. It provides a nongrainy and creamy texture.
Beta- This is commonly found in confectionery to provide shine and gloss. It also helps promote flaky layers in biscuits and danish.
Emulsifiers can also act as:
lubricant and release agent, lubrication of extruded products but also lubrication of food processing equipment. Lecithin is commonly used as a flow and release agent. Monoglycerides are commonly used to aid in extrusion of pasta and cereals
Release or anti-sticking agent, for de-molding purposes during food processing or anti-sticking to confectionary packaging. Lecithin is commonly used in pan release sprays.
Optimal emulsifier functionality is determined by the correct food processing conditions. Common emulsions are unstable and do not form spontaneously. Energy input in the form of mixing, shaking, stirring or homogenizing is needed to form the emulsion. Emulsifiers also need to be heated above their melting temperature to become functional.
When it is not possible to provide enough heat or shear in the process a pre-hydrated or melted emulsifier is needed. Emulsifiers can be melted into shortening to create an emulsified shortening or they can be melted/pre-hydrated in water. Pre-hydrated an emulsifier before use can increase the functionality significantly.