Ever tried to make a simple dressing of olive oil and balsamic vinegar? Noticed that it’s impossible to keep these two mixed without continuously stirring or adding something like mustard? Wondered how food manufacturers are able to do this?
If you’ve looked on labels of various dressings and sauces you might have come across the word ’emulsifying agent’ or ’emulsifier’. This is the ‘secret’ to their stable dressings, it’s these components that prevent their mixtures from splitting. Even though their name might sound pretty chemical, emulsifiers have been used for a long period of time and the word emulsifier is simply used to cover a range of different components that all do a similar job: keep two components mixed that don’t want to be mixed.
What is an emulsion?
Emulsifiers stabilize emulsions, so in order to understand an emulsifier, we should first introduce an emulsion. An emulsion is an example of a dispersion, it is a mixture of two umiscible liquids. These liquids can be mixed together by force (a strong whisk or homogenizer), however, over time they will spontaneously split again. In an emulsion one of the two liquids will be the continuous phase and the other liquid will float around that continuous phase in separate droplets.
In food the most common example of an emulsion is that of water (e.g. balsamic vinegar) and oils (e.g. olive oil). You can mix them by hand, but once you leave it alone they will split again.
In order to store emulsions for longer, thus extend their shelf life, an emulsion will have to be stabilized. In other words, the oil particles shouldn’t be able to sit together, nor should the water particles. There are several ways an emulsion can be stabilized:
- Surface modifiers; these components will sit on the surface of the droplets floating through the continuous phase, preventing them from merging, for the rest of this post, we’ll call these emulsifiers
- Thickening agents: by increasing the viscosity of the continuous phase, thus by making it thicker, the particles won’t be able to move as easily to one another, slowing down the split of the two liquids
- Weighting agents: if the density of the two liquids is very different, sedimentation or floating will be another driving force behind the separation of the two phases, the lighter phase will want to float on top of the denser phase. This can be prevented by adding these weighting agents.
A short note on the definition of emulsifier
The term emulsifier is used in various ways. I’ve come across cases where emulsifiers are all components that somehow stabilizes an emulsion. In other cases, emulsifiers only refer to components that stabilize an emulsion by modifying the surface (which is the definition we’ll be using here). An again more limiting defintion though is the use of emulsifier to describe ‘surfactants’ only. We’ll look into surfactants later on, but we’ll be using the term emulsifier a bit more broadly here.
In this post we’ll describe emulsifiers as components which stabilize an emulsion by sitting on the surface between the two phases. There are different types of components again that can sit onto this surface.
The first and most commonly mentioned emulsifier type is the ‘amphiphilic’ molecule. This type is especially important for oil/water emulsions. Amphipilic means that the molecule has both hydrophobic as well as hydrophilic properties. In other words, part of the molecule love to sit in water, whereas other parts do not. Those hydrophobic parts instead prefer sitting in an oil for instance.
Emulsion can also be stabilized by non-amphilic components though. This is a relatively new area with the field of emulsifier research and amongst others looks into using particles to stabilizing an emulsion. These particles don’t necessarily prefer to sit in one phase or the other. The way they stabilize emulsions is also focussed on preventing the particles floating in the continuous phase to merge together. We will not focus on this type of emulsifier, an interesting, but advanced level, article on the topic can be found here.
How do amphiphilic emulsifiers work?
Amphiphilic molecules have parts of them that prefer to sit in the water phase (hydrophilic) and parts that prefer to sit in the fatty phase (hydrophobic). In an emulsion the molecules will arrange there hydrophibic sides into the fat and their hydrophilic sides in the water. See below for a simplified illustration.
There are a lot of these amphiphilic emulsifiers, but we’ll discuss two main categories in more detail.
Proteins are large complex molecules built up of a long chain of amino acids. Each amino acid has a side chain and this side chain can be hydrophilic or hydrophobic. Through all these different amino acids proteins will have sections which are hydrophobic and sections which are more hydrophilic.
In order for proteins to properly be surface active they will have to unfold partly. Some of this can occur spontaneously, but it can also be help along, for example by whipping proteins (which is done when making an egg white foam). Since these proteins are quite large and bulky, their bulkiness helps stabilize an emulsion. They form some sort of a layer around the particles which makes it harder for like particles to find each other.
Surfactants work in the same way as emulsifiers, they sit in both phases, but they have a very different structure. Whereas proteins are generally very large molecules, surfactants tend to be very small. They are similar to the emulsifiers in the image above. They have a very hydrophilic head and hydrophobic tail. This tail is often some sort of a fatty acid (or similar structure) which is why it’s long and pretty thin.
Common emulsifiers in food
There are a lot of different emulsifiers which are used in food. We will focus on just a few here to give you some examples:
- An egg yolk is an emulsifier mostly thanks to the lecithin inside which acts as a surfactant
- Egg proteins can help stabilize a custard
- Mono- and diglycerides
- Various esters involving fatty acids
- Polysorbates (surfactants)
Choosing an emulsifier
When developing a product it is important the right type of emulsifier is chosen, because there are a lot of different types. When choosing an emulsifier you should start with asking what your continuous phase is and which are the particles floating in that phase. For example, do you want to make a water-in-oil emulsion or an oil-in-water emulsion?
You should choose an emulsifier that prefers to sit in the continuous phase just a little more than in the discontinuous phase.
This is where the HLB-value comes in handy, it describes the hydrophilic/lipophilic balance of an emulsifier. A higher value indicates it prefer to sit in the water, whereas a lower value makes it relatively more oil loving. This is calculated based on the relative sizes of the hydrophobic and hydrophilic regions.
Water-in-oil emulsifiers tend to have an HLB-value of 3-7 whereas those for oil-in-water emulsions tend to be higher, 8-16. That said, the exact required number strongly depends on the emulsifier itself and the exact ingredients in your product. Maybe there are already some naturally occuring stabilizers present as well, which lowers the need for a strong emulsifier.
A good comparison between surfactants and proteins as emulsifiers (Proteins and emulsifiers at liquid interfaces, 2004, P. Wilde).