In this blog post, we will learn about the scientific principles of emulsification (emulsion), in which water and oil mix together, using mayonnaise and vinaigrette as examples, and how it is used in everyday life.
It’s a day to go out to eat with my family. To celebrate, you decide to go to an Italian restaurant for pasta. In keeping with the atmosphere of the restaurant, bread and a strange sauce are served as an appetizer. It is neither cooking oil nor vinegar, but has a bright green color that tempts you to try it. Since it is served with bread, you assume it is a sauce to dip the bread in. Unable to contain my curiosity, I asked the waiter what the sauce was called, and he said it was “vinaigrette.” Still skeptical, I dipped my spoon into the sauce and tried it. It was sweet and sour, not bad. I asked the waiter what the sauce was made of, and he said it was a mixture of balsamic vinegar and olive oil in a 1:3 ratio.
However, when the main dish arrived, the balsamic vinegar and olive oil had separated into two layers, even though they had been well mixed when I first saw it. Is it possible to keep them mixed together? The reason vinegar and oil do not mix is because they have different chemical properties, which cause them to separate by minimizing the area of contact between each other. However, with the help of science, it is possible to mix oil into water and water into oil. If you quickly stir with a hand blender, which is an electric mixer that you hold in your hand and can create bubbles, the friction generates heat, causing the water and oil to mix. This mixture is called an emulsion or emulsified liquid.
Emulsions, which can be interpreted as “combining opposing substances such as water and oil,” can be more technically described as a state in which one liquid is dispersed in another liquid in the form of small particles, even though the two liquids do not dissolve in each other. Since water and oil are two liquids that do not dissolve in each other, there are two types of emulsions: oil-in-water (O/W) emulsions, in which oil is dispersed in water, and water-in-oil (W/O) emulsions, in which water is dispersed in oil.
Another representative example of emulsions found in everyday life is mayonnaise. Mayonnaise is an O/W emulsion in which oil droplets (cooking oil) are dispersed in water (vinegar). However, unlike vinaigrette, how does mayonnaise prevent the vinegar and oil from separating for a long time? As mentioned earlier, it is not because we use the power of science to continuously stir it with a hand blender. The reason is that egg yolk is added as an emulsifier to help emulsify the mayonnaise.
Since it is essentially impossible to maintain a mixture of water and oil, it is necessary to add a third ingredient, a surfactant, to create an emulsion. In this case, the surfactant is called an emulsifier. The added emulsifier causes the oil and water to orient themselves at the interface (this refers to adsorption in which the molecules of a substance adsorbed at the boundary between two phases are arranged in a certain direction) so that the parts that can mix with water are close to the water, thereby reducing the interfacial free energy (the internal energy existing at the interface). In simple terms, it is arranged so that the parts that can mix with water are close to water, allowing water and oil to mix. This reduces the interfacial free energy (internal energy existing at the interface), facilitating emulsification and suppressing the phase separation of water and oil, thereby maintaining the emulsion state.
Emulsifiers continuously change their HLB (the HLB value ranges from 0 to 20, and the smaller the value, the stronger the lipophilicity of the entire molecule, and the larger the value, the stronger the hydrophilicity), which causes the phase of the emulsion to change. This is called phase inversion of emulsions.
In other words, the reason why vinaigrette and mayonnaise, which are mixtures of water and oil, look different is because the way they form emulsions, or the way water and oil mix, is different. Mayonnaise is an O/W emulsion in which oil is dispersed in water. In simple terms, it is a state in which countless oil droplets are scattered throughout the vinegar. The fact that vinegar and oil, which cannot be mixed together, are broken down into small particles and mixed together is thanks to lecithin (a type of phospholipid) contained in egg yolks. Vinaigrette, on the other hand, is a mixture of oil and water droplets. Unlike mayonnaise, vinaigrette does not contain an emulsifier, so it quickly separates into layers of oil and water.
The principle of emulsification is used not only in food but also in the manufacture of cosmetics. Many ingredients used in cosmetics do not mix well with each other, so emulsification technology is very useful for mixing them together in a single formulation. Various emulsification technologies have been developed along with the development of emulsifiers to stabilize emulsified formulations and change their properties. Starting with the HLB method proposed by Griffin in the 1950s, there are methods such as the PIT method, gel emulsification, and D-phase emulsification. Recently, the use of naturally derived emulsifiers and polymer emulsifiers is increasing as a means of improving skin compatibility and safety for the human body. In addition, nano-emulsification, multiple emulsification, liquid crystal emulsification, and Pickering emulsification have been developed and applied as means to differentiate product appearance and increase the delivery efficiency of active ingredients. Research on the application of nano-structured particles such as liposomes and cubosomes is also actively underway.
Therefore, understanding the scientific principles behind vinaigrette can enrich the conversation at the dinner table while enjoying the dish. It is amazing that such small discoveries in everyday life can provide us with great interest and knowledge. In the future, it would be nice to talk about these principles while enjoying a meal.
