The ins and out of mayonnaise and a guide on how to make it and fix it if you break it.
What is it?
Mayonnaise is an emulsion formed from eggs and fat seasoned with lemon juice or vinegar and salt.
What’s an emulsion?
If you don’t want to know the science behind emulsions then skip to the next section to learn how to make it.
An emulsion (in the culinary sense) is a mix of water and oil droplets. Since the oil droplets are much larger and slower moving than water molecules their movement is impeded and this creates a thick, creamy consistency. The problem here is that emulsions are made from two liquids that are incommensurable (two liquids that can’t be dissolved into one another). You’ll know this from any experience looking at oil and water. When mixed the oil will disperse into smaller droplets which coalesce into a large mass of oil if left for a few seconds. So emulsions are unstable in nature.
The reason the oil congregates in one mass is due to surface tension. When liquids can’t mix the liquids arrange themselves in a way that minimises contact with each other. They form a single large mass which minimises surface area and therefore has less contact with the other liquid.
So in order to form an emulsion we need to seriously break up that mass of liquid. When you whisk a tablespoon of oil into mayonnaise it’s broken up into roughly 30 billion separate droplets. Those droplets will be approximately 3 thousandths of a millimetre in diameter. A blender can make them smaller and an industrial homogeniser can break them up to smaller than a thousandth of a millimetre across. This is important because smaller droplets are less likely to merge together and cause the two liquids to separate.
There are two things that are important factors in the ease of forming an emulsion. The first is by using a more viscous water base as this drags harder on the droplets and allows the shearing force from the whisk to be transferred from the whisk more effectively. The second is the use of emulsifiers which coat the surface of the droplets and prevent them from merging into one large mass and separating.
Great, why do I care?
Because the science is important for understanding what needs to be done or why something went wrong. There are a few laws that apply to making any emulsion:
- The first material in the bowl should be the water based ingredient and some emulsifying or stabilising ingredients. To this the oil is added as the oil has to be dispersed in the water base.
- The oil should be added very slowly at first. Once the mixture has developed some viscosity the oil can be added more quickly. This is because initially it is easy for large droplets to avoid being dispersed and as you add more oil this will pool with the large droplets causing the liquids to separate as the large droplets merge.
- The proportions of the two liquids need to be kept in balance. The volume of the oil shouldn’t exceed three times the volume of the water base. The more crowded the droplets are the more likely it will be that they will be in continuous contact with each other and might coalesce. When the emulsion becomes stiff it’s a sign to either stop or add more of the water base.
- If it went wrong and didn’t emulsify it’s almost always because:
- The oil was added too quickly.
- The mixture was too hot or too cold.
- Too much oil was added.
Once you’ve made your emulsion you might want to store it. This is done chiefly by keeping it at the right temperature. If it’s too hot the oil droplets will move around energetically making them more likely to coalesce and split the emulsion. At low temperature surface tension increases making neighbouring droplets more likely to coalesce.
How to Rescue Your Emulsion
When an emulsion separates there are two ways to re-emulsify it. The first is to use a blender’s sheer mechanical force to force the droplets apart and break them up. The second more reliable technique is to acquire more of the water base in a separate bowl and slowly whisk the broken sauce into that.
Mayonnaise is the most tightly packed emulsion and as much as 80% of its volume can be oil. This can be achieved as egg yolks are full of natural stabilisers and emulsifiers. When making mayonnaise have everything at room temperature allows the emulsifiers in the yolk to attach to the oil droplets more easily.
You can make mayonnaise from more or less any fat be that olive oil or leftover chicken fat from a roast. Bear in mind that if the fat you’re using solidifies in the fridge then the fat crystals can rupture the emulsifying layer and cause the emulsion to split when brought to room temperature. Be careful with refrigerated mayonnaise otherwise as some oil may have escaped its droplets. Stir gently with a few drops of water to re-emulsify.
Yum yum mayonnaise yum.