The Physics and Chemistry of Emulsions and Blender Mayonnaise
I am a big fan of mayonnaise. I like it on my fries, on my burgers, and combined with different ingredients to create a variety of amazing sauces. Even if you don’t like mayo, I think you should agree with me that there is something magical about how egg yolk and oil blended together in just the right way can result in something with a texture so different from either base ingredient. And if you ARE a fan like I am, the good news is that mayonnaise is really easy to make at home and is is waaay better than anything you can buy in a jar.
I’ve made a video quickly explaining and demonstrating how to make mayonnaise using an immersion blender (also called a stick blender or hand blender):
If you want more mayo science, read on! Otherwise, skip to the bottom for the recipe.
Unlike other sauces such as tomato sauce, gravies, or curries which are thickened by dispersing materials like protein or starch molecules in a liquid, mayonnaise is thickened by filling a water-based liquid with droplets of oil. This dispersion of one liquid into another is called an emulsion. An emulsion can only be made with two liquids that won’t dissolve into each other (so, alcohol and water cannot form an emulsion). Milk and egg yolk are examples of natural emulsions, liquid fats dispersed in water.
As you’ve probably seen for yourself, if you were to just pour oil and water into a bowl together, the liquid for separate, continuous layers–this is because when two liquids won’t mix for chemical reasons, the molecules of each will arrange themselves to minimize contact with the other. A single continuous layer exposes the least amount of surface area and also results in the force of surface tension. In order to form emulsions, cooks must use energy to break this surface tension. This is where whisking or blending comes into play. According to Harold McGee in On Food and Cooking, when you “beat a single tablespoon of oil into mayonnaise, you break it up into about 30 billion separate droplets,” each droplet no more than 3 thousandths of a millimeter across. That’s pretty tiny!
In addition to oil and water, most mayonnaise is made with egg yolks. Yolks are important because they contain a phosopholipid called lecithin, which has the wonderful property of lowering the surface tension of one liquid dispersed in another. One end of a lecithin molecule is attracted to oil, and the other is attracted to water, so it helps to hold the emulsified liquids together.
By the way, if you’re making mayo and it comes out too thin, the ingredient you want to add more is actually oil, not egg yolk. It feels counter-intuitive to add a liquid to make something thicker, but this is part of the cool physics of mayonnaise. Remember how I said earlier on that mayo is formed by filling water with droplets of oil. The more oil, the more tightly-packed the emulsion, and the more viscous it will be.
Traditionally, mayonnaise is made by continuously whisking the egg yolk and water mixture while pouring in a thin, steady stream of oil. You have to add the oil very slowly; otherwise large drops will be able to avoid the whisk and won’t get broken up. If this happens, the mixture “breaks:” the oils stays separate and you end up with something greasy and not delicious. Even if you do it right, it takes a lot of whisking and your arm is very tired by the end of it. Imagine putting in all that work and not having it turn out!
Thankfully, someone figured out a better way to form those billions of droplets, using modern technology! A stick blender is the perfect tool: You add all of your ingredients to the blender cup at once, and because of its lower density, the oil floats on top. Start blender from the bottom of the cup, and the oil gets sucked down gradually into the water and yolk mixture, giving you a perfect emulsion every time.
2-Minute Blender Mayonnaise (from Serious Eats’ The Food Lab)
1 large egg yolk
1 tablespoon lemon juice
1 tablespoon water
1 teaspoon Dijon mustard
1 cup vegetable or canola oil
- Add yolk, lemon juice, water, and mustard to a cup that just fits the head of your immersion blender.
- Pour the oil over slowly, so it sits on top of the other ingredients.
- Lower the blender to the bottom of the cup and then turn on. Raise up through the mixture very slowly, until a thick emulsion is formed.
- Use a spatula to fold in salt to taste.
- Store in a sealed container in the fridge for up to two weeks.