Emulsions - Bake Science

In my chocolate chemistry post, I talked about how ganache is an emulsion, and you may have heard a sauce or salad dressing be called an emulsion before, but what does that actually mean? Are emulsions liquids, are they solids, or are they something in between? Are all sauces emulsions? Well, it turns out emulsions are much more common than you might think, and you probably use emulsions every day. But let’s first start at the beginning, what even is an emulsion?

What is an Emulsion?

In the most basic terms, an emulsion is a mixture of multiple liquids (at least one fat and one water-based-liquid), with at least one liquid dispersed throughout another. However, while you may know that many sauces are emulsions, did you know milk is too? If you didn’t, it’s not surprising as many common emulsions seem to act the same as a liquid, most notably milk. While milk may seem very similar to a liquid like water, that is mainly due to its viscosity, or thickness. While many emulsions like mayo thicken significantly, milk does not, giving it the appearance of a liquid.

But how does mayo, a mixture of oil and eggs, become so much thicker than its ingredients? This is due to the process of emulsification, which causes connections between fat and water particles, causing their ability to move to decrease and therefore the viscosity of the emulsion to increase. However, mayo being so much thicker than milk can be attributed to two main factors. The first big factor being the amount of the dispersed liquid present as a larger amount of dispersed droplets creates more connections and therefore more viscosity.

The second factor is dispersed droplet size, following a similar line of reasoning as the larger, smaller amount of globules result in less connections.

Now if we apply these two principles to milk we can start to understand why milk is so much thinner than mayo. Whole milk is about 3.5% fat this means that almost 95% of milk is water with a very small amount of dispersed fat in it, this leads to almost no connections and is why we think of milk as a liquid. Conversely, mayo is almost 80% fat, making it one of the thickest emulsions normally possible.

Oil-in-Water emulsions vs. Water-in-Oil emulsions

Credit:
“Simple emulsion systems oil in water and water in oil” by Onegumas, licensed under CC BY-SA 2.5 PL.
https://commons.wikimedia.org/wiki/File:Simple_emulsion.jpg

However, there are some exceptions. You may have already began to think about another emulsion made from milk even thicker than mayo: butter.

Butter is an interesting emulsion: while it may seem to be thicker because there is so much dispersed fat, butter actually has water dispersed through it, not fat. The real reason butter is so thick is because butterfat contains 30-50% solid fat crystals, with the rest being liquid; this makes butter thick, not due to the properties of an emulsion but the properties of the fat present.

This is also true for milk; although the much smaller amount of fat present makes it far less noticeable. However, emulsions only really start becoming complicated when we start looking at things like whipped cream that act different from liquids.

Colloids

So, what is whipped cream? Is it a liquid? Is it a solid? In reality it’s neither.

As you may know whipped cream is made by incorporating air into a liquid, specifically heavy cream. Now this may sound familiar if you remember how emulsions form, and this is actually because whipped cream and emulsions are actually quite similar as both foams and emulsions fall under a separate larger category called colloids. Colloids are a mixture of one or more substances, dispersed as particles suspended within another set of substances.

While there can be more than two substances in a colloid, these substances are categorized into two phases: the dispersed phase and the continuous phase. The dispersed phase refers to the substances that are dispersed in the other phase, the continuous phase. The continuous phase is made of one or more substances that the dispersed particles are suspended within.

Some common colloids are whipped cream, which, as we talked about earlier, is a colloid made from the dispersion of liquid in a gas, categorized as a foam. Jello is a solid-liquid colloid where liquid is dispersed through a solid creating a gel and smoke is a gas-solid colloid, often called a solid aerosol.

Types of colloids with examples

How to Make an Emulsion

We now know what emulsions are, but how are they formed and why can’t all liquids create emulsions? If you’ve cooked frequently or made a sauce before, you may have heard of an emulsifier, but what really is an emulsifier and what does it do?

Emulsifiers are simply substances that stabilize an emulsion stopping the two phases from separating. However, in order for a substance to be an emulsifier, it must also be classified as a surfactant.

Surfactants are a far more general term, used in many different non-culinary applications to stabilize many mixtures not just emulsions. Surfactants are substances that are amphiphilic, meaning they have a polar (hydrophilic) part, and a non-polar (hydrophobic/lipophilic) part. This property allows emulsifiers to reduce the surface tension between two liquids, stabilizing the emulsion. Some common culinary emulsifiers are egg yolks, mustard, garlic, and xanthan gum.

However, while culinarily all four of these are considered emulsifiers, chemically only the first two are. This is because both garlic and xanthan gum lack the amphiphilic property of surfactants and stabilize emulsions with different processes. In practice, these processes result in similar if not identical outcomes, hence why they are culinarily still classified as emulsifiers.

Why do Emulsions Break?

If all of these emulsifiers work the same way why do emulsions often not form when you make a sauce?

Emulsions breaking can be split into two separate parts: emulsions separating over time due to instability, and emulsions breaking during their formation. Let’s first look at emulsions breaking during their formation, as it tends to be the simpler of the two.

The main reason that emulsions like mayo break is due to an improper incorporation of the dispersed liquid. For example, when making mayo, an oil-in-water emulsion (meaning oil is the dispersed liquid), if the oil is poured into the egg yolks too quickly, the emulsion will break. But why does this happen?

Well as we already learned, mayo is quite a high-fat emulsion as it contains about 80% oil. This percentage of dispersed phase to continuous phase is one of the highest percentages possible – once an emulsion has any more than about 80-85% fat, the continuous phase is no longer able to hold the dispersed particles and the emulsion will turn from an oil-in-water emulsion into a water-in-oil emulsion (inverting).

However, that is only in theory, in reality what will usually happen is what you have probably had happen if you have had to emulsify a lot of sauces: the sauce breaks. This is because the oil is not actually able to disperse itself as droplets in the water due to an improper formation of the emulsion. To stop this as, you might have guessed, simply pour in the dispersed liquid more gradually, and make sure your emulsion does not contain more than ~80% dispersed liquid.

Emulsions separating over time tends to be the more difficult type to stop as it is a combination of natural processes occurring over time. However, there are some ways to slow down these processes, mainly by increasing the stability of the emulsion.

Emulsion stability is exactly what it sounds like, the frequency of instabilities occurring in an emulsion that cause phase separation, or how quickly an emulsion will separate. But to know how to stop these instabilities, we first must know what they are.

Five Types of Emulsional Instability

There are 5 types of instability that occur in an emulsion: coalescence, flocculation, creaming, sedimentation, and Ostwald ripening.

Coalescence, the most common type of instability, is the process of multiple dispersed droplets bumping into each other and combining to form larger droplets. This causes the average droplet size to increase causing the dispersed droplets to gather into a layer and the emulsion to separate.

Coalescence is often easy to spot as it tends to form three distinct layers as opposed to two, meaning in an oil-in-water emulsion, where the oil is the dispersed particle, oil would be at the top while water would be at the bottom, and in the middle there would be an emulsified layer. However, coalescence often does not occur individually as it often occurs due to the second type of instability, flocculation.

Flocculation is when an attractive force between the dispersed droplets causes them to form bunches, or flocs. The proximity of the particles in these flocs causes a drastic increase in coalescence, leading to separation.

The third type of instability, creaming, is most common in dairy and dairy alternatives like soy or almond milk and occurs when dispersed droplets float to the top of the emulsion due to the buoyancy or a lower density compared to the continuous phase. Unlike coalescence, creaming only causes 2 layers to form and is usually only a small layer of liquid that separates from the emulsion that sits on top of the denser emulsion.

While I said there are 5 types of instability many consider the fourth type, sedimentation to not be a distinct type of instability as it is simply the opposite of creaming, causing the two to be grouped.

Sedimentation occurs when the dispersed droplets are denser than the continuous phase, leading to a similar separated layer, but simply at the bottom of the emulsion.

The final, and most complicated sounding form of instability is Ostwald ripening. First described by Wilhelm Ostwald in 1896, Ostwald ripening occurs when smaller dispersed particles dissolve and then redeposit onto larger particles causing a similar process to occur as during coalescence, where the average particle size increases and the mixture begins to separate into layers.

Types of Emulsion Instability (+ Inverting):
A: Creaming B: Sedimentation C: Inverting (O/W -> W/O) D: Flocculation E: Coalescence F: Ostwald ripening

Credit: “Emulsion destabilzation.jpg” by Onegumas, CC0 1.0 Universal (public domain), via Wikimedia Commons.
https://commons.wikimedia.org/wiki/File:Emulsion_destabilzation.jpg

However, the reason that understanding these different instabilities is important is that it lets us know why and how our emulsions break, and more importantly how to solve them.

One important fact about Ostwald ripening for example is that it occurs more frequently in water-in-oil emulsions. Meanwhile, flocculation tends to occur in oil-in-water emulsions more frequently.

This is important because the stabilizers we use to prevent these instabilities are more effective at preventing certain instabilities but not others. To see this lets look at one of the most confusing stabilizers in cooking.

The Weirdest Ingredient

Xanthan gum is a very weird ingredient not just because it is not chemically an emulsifier, but because it can act culinarily as both an emulsifier and a stabilizer, even though chemically it is only a stabilizer. As with other purely culinary emulsifiers this is because it does not have the amphiphilic property required to be classified as a surfactant.

However, it also is one of the reasons why understanding emulsion instability is so critical, because when stabilizing emulsions with xanthan gum you must be careful of knowing what type of emulsion you have created. This is because the biggest enemies of emulsions are instabilities, and certain stabilizers like xanthan gum and other gums when used with many emulsifiers can actually increase the frequency of Ostwald Ripening in emulsions that have low stability to Ostwald Ripening.

Because of this it is helpful to understand what type of emulsion you are making and the best stabilizers for it as in this case Ostwald Ripening is far more common in water-in-oil emulsions.

Most importantly this also doesn’t mean that gums like xanthan gum cannot be used in water-in-oil emulsions as xanthan gum is one of the best and most common stabilizers, but simply that it may not be ideal in large amounts for water-in-oil emulsions – especially those that will be stored and not consumed immediately, or should be used in conjunction with other stabilizers.