The Accidental Scientist: Science of Cooking

recipe: Hollandaise Sauce
What’s Going On?

Why are oil and water so difficult to mix—and how does adding an egg yolk help?

Water molecules like to stick together. That’s why water drops bead up when you spray them on a waxy surface—and it’s also why oil and water don’t mix.

Water molecules stick together because of their chemical structure. Each water molecule is made up of two hydrogen atoms and an oxygen atom. (That’s why water is sometimes called H2O.) Each hydrogen atom is made of a positively charged proton and an negatively charged electron. Within the molecule, the hydrogen atoms form bonds with the oxygen atom by sharing their electrons. But the oxygen atom tends to hog the negatively charged electrons, becoming negatively charged and leaving the hydrogen atoms positively charged.

Molecules like water, which have some areas that are positively charged and some that are negatively charged, are called polar molecules. Polar molecules tend to attract other polar molecules. The positively charged area on one molecule will attract the negatively charged area on another, pulling the molecules together.

The molecules that make up fats and oils are nonpolar molecules. In these molecules, the electrical charge is evenly distributed. Because polar water molecules like to crowd together, they tend to push out any nonpolar oil molecules. And the oil molecules have an affinity for each other as well.

Although stirring or shaking can temporarily mix oil and water, a substance known as an emulsifier is needed to stabilize this state of affairs. One end of an emulsifier doesn’t like water and will become embedded in a drop of oil, while its other, water-loving end, sticks out into the water and prevents the oil droplets from merging together, creating an emulsion. In addition to proteins that can act as emulsifying agents, egg yolk contains an abundance of lecithin, a fatlike molecule known as a phospholipid.

For more information about emulsions, click here.

On the labels of many foods, you will find the ingredients lecithin and soy lecithin. Lecithin extracted from soybeans is used as an emulsifier in many food products.

Other molecules can also serve this function. Detergent cleans your greasy dishes because each detergent molecule has one end that attracts oil and one end that attracts water. The detergent molecules stick to the oil and lift it off the dishes.

Why does the kind of saucepan matter?

If you use an aluminum or iron saucepan, metal oxides can discolor the lovely yellow of your sauce.

Why do I have to keep the heat low?

In their natural state, egg proteins are long molecules that are all curled up into a ball. When you heat them, these proteins uncurl and form bonds with other proteins.

When you are cooking the egg yolks for hollandaise sauce, you want to heat them enough to cause the proteins to uncoil—but not so much that they form many bonds with other proteins. The hotter you get the proteins, the more they are likely to bond to other proteins. Heat the yolks too much, and you’ll end up with scrambled eggs!

Why do I have to keep whisking? Why can’t I just dump all the butter in at once?

You keep whisking the mixture as you add the melted butter because you want to break it up into tiny, tiny drops. Each tiny drop ends up surrounded by emulsifiers. But to give the emulsifiers a helping hand, you need to keep the butter from gathering in a big glob. You do that by constantly whisking while you add the butter, and by adding the butter slowly.

Can curdled hollandaise sauce be saved?

“A broken emulsion isn’t a complete disaster. Providing you haven’t overcooked the egg yolks, it can be formed again. Simply whisk the broken hollandaise gradually into another beaten egg yolk. Then serve it immediately. It won’t be quite as delicate, but most guests are so impressed with homemade hollandaise, they probably won’t notice.

“Don’t try to store this sauce, as it’s the ideal medium for bacterial growth. And because oil and water expand at different rates, extremes in temperature also cause hollandaise to separate. Refrigeration doesn't work. The butter solidifies and separates. And reheating causes curdling. Once you’ve made hollandaise sauce, serve it as quickly as possible.”

—Anne Gardiner and Sue Wilson, The Inquisitive Cook


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