EMULSIFIER: EMULSION PROCESS, MOLECULAR ASPECTS, APPLICATIONS - CHEMISTRY - 2025

An emulsifier or emulsifier is any compound capable of stabilizing an emulsion for a considerable time. Its function is essentially that of "fraternizing" two liquids or phases that under normal conditions cannot unify; that is, to form a mixture of homogeneous appearance.

The classic example to address what an emulsifier means is that of a glass of oil and water. The oil will be positioned at the top, with a lower density, while the water will be at the bottom. Both liquids cannot coalesce, they are immiscible (they cannot be mixed), since their intermolecular affinities differ from each other; water is polar, and oil is nonpolar.

Peanut butter owes in part its consistency and thickness to its added emulsifier. Source: Pixabay.

However, if an egg yolk is added to the heterogeneous and biphasic mixture of water (w) -oil (o), shaking vigorously, a water-in-oil emulsion (w / o) will form, if the oil is dispersed and to a lesser extent; or oil in water (o / w), if now it is the water that is dispersed. Thus, the egg yolk becomes the emulsifier.

From the above, adding other additives, we obtain the mayonnaise. Like mayonnaise, other food products like margarine, peanut butter, milk, tomato sauces, dressings, etc., are made thanks to the addition of an emulsifier.

Emulsion process

Process of formation of an emulsion. Source: Gabriel Bolívar.

The emulsifier thus allows an emulsion to be formed from two immiscible liquids or mixtures. The w / o and o / w emulsions are the two most important cases. The image above shows in a simplified way what was explained above.

Note that we have two liquids: a bluish one and a yellow one, which make up a biphasic heterogeneous mixture. They are immiscible, so they do not coalesce to form a homogeneous mixture. However, when the emulsifier is added (either solid or liquid), it intervenes in such a way that globules or dispersed particles of the yellow liquid are formed.

If the yellow particles managed to coalesce, we would have a yellow phase again as in the beginning. The smaller these particles, the smoother and more uniform the resulting mixture will be. Therefore, from this hypothetical mix we would see a green mix; but that the yellow particles would be easily visualized under the microscope.

This is where the stabilizers come in, compounds that are responsible for preventing the coalescence of the dispersed particles, and further lengthen the homogeneity of the emulsion; that is, it will not "cut" into two phases.

Molecular aspects of the emulsion

Molecularly, the emulsion process is quite dynamic and there are several theories that try to explain the action of the emulsifier. However, they all share something in common, and that is the fact that the emulsifier must be one or more amphiphilic (or amphipathic) molecules; These are those that have both apolar and polar character.

The amphiphilic molecule can be compared to a lollipop: the head is polar, hydrophilic; while the rod or tail is apolar, hydrophobic. Two liquids are immiscible because essentially their polarity difference is very large. The emulsifier interacts with both liquids at the same time.

Its polar head is oriented in the direction of the polar liquid. On the other hand, the apolar tail seeks to interact with the apolar liquid. Depending on the predominant liquid or phase, amphiphilic molecules tend to form micelles; say, capsules, within which molecules of the dispersed liquid are enclosed.

For example, the yellow globules would come to be surrounded by the amphiphilic molecules of the emulsifier, whose outer part interacts with the continuous phase or liquid (of greater proportion, blue), and also repels the other globules. However, the micelles move, which will cause that sooner or later they will end up separating and giving rise to the appearance of the yellow phase.

Applications of emulsifiers

Emulsifiers contribute to the creamy textures of ice cream. Source: Pexels.

Without the existence of emulsifiers or emulsifiers, it would not be possible to make emulsions, which are of enormous importance in the food and pharmaceutical industry. While the formulation of such products also incorporates thickening agents and stabilizers, emulsifiers help mixtures develop body and texture.

The viscosities of the emulsions obtained can be higher than those of the original immiscible liquids. Mayonnaise proves this point. But the final viscosity can also be lower, the resulting mixture being smoother. Thus, emulsifiers are key in the consistency of food and, therefore, in its flavors.

Products that require emulsifiers

Among some of the food products that require emulsifiers we have:

-Milk, being an o / w emulsion, capable of acting as an emulsifier thanks to its protein content.

-Butter and margarine, both w / o emulsions.

-Pan, for its hardening and freshness.

-Directions.

-Chocolates, where they modify their viscosity during their industrial production in bars or molds.

-Ice creams, since milk proteins stabilize the combination of fats and water, in addition to the addition of extra emulsifiers regroup them (partially destabilize) so that the mixture can incorporate air.

-Cookies.

-Facial creams.

-Lipstick.

-Ointments.

-Cheese.

-Cakes.

Examples of emulsifiers

It was mentioned that the molecules of the emulsifiers must be amphiphilic in principle. Surfactants, on the other hand, also possess this characteristic. However, this does not imply that an amphiphilic molecule is a surfactant (as is the case with proteins).

Therefore, surfactants are a good choice when choosing an emulsifier; although there are salts that perform the same function. The feasibility of using any of them will depend on the formulation and chemical properties of the product.

Likewise, not all emulsifiers are amphiphilic, since it may be that they interact with the components of a mixture (proteins, fats, sugars, etc.), thus achieving the homogenization of the whole. Therefore, and finally, some examples of emulsifiers will be listed:

-Fatty acid esters

-Monoglycerides

-Diglycerides

-Lecithin (found in egg yolk)

-Gum arabic

-Pectin

-Rusty starch

-Jelly

-Polyethylene glycol

-Maltitol

-Calcium citrate

-Sodium and potassium lactates

-Sodium alginate

-Agar

-Karaya rubber

-Cellulose

-Ethoxylated alcohols

- Sodium and calcium stearoyl lactylate

-Polysorbates 20, 40, 60 and 80 (food grade)

-Lactitol

As can be seen, there are many emulsifiers available, and each one has a functionality, whether for food, creams, syrups, detergents, lotions, etc.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Wikipedia. (2020). Emulsion. Recovered from: en.wikipedia.org
3. The Editors of Encyclopaedia Britannica. (March 20, 2019). Emulsifier. Encyclopædia Britannica. Recovered from: britannica.com
4. EU Specialty Food Ingredients. (2020). Emulsifiers. Recovered from: specialtyfoodingredients.eu
5. Guest Author. (2015, April 10). Emulsifiers at Work: Applications Across Industries (Infographic). Recovered from: knowledge.ulprospector.com
6. Ruben. (February 1, 2012). Emulsifiers in ice cream. Recovered from: icecreamscience.com