How Yolks Create Emulsions

• Egg yolk contains several components that allows it to act as an emulsifier. Yolks are high in cholesterol, particularly low-density lipoproteins. They also contain proteins that attract water molecules and other proteins that attract fat molecules. When the protein molecule connects to either fat or water, it prevents another fat or water molecule from bonding with the first. The separated fat and water molecule remain mixed together instead of separating. Temperature plays a role, too. The protein molecules in a room-temperature yolk are able to move more quickly than those in a cold yolk, creating a stronger emulsion.

Process

• You need to follow a certain sequence if you want an emulsion to hold together. Whether you are making a cold sauce such as mayonnaise or a warm sauce such as Hollandaise, combine the yolk with the water-based liquid first. Add the oil or fat to the liquid slowly at first, and whisk the mixture as you pour the oil in. The oil should be completely combined with the liquid and yolk before you add more. When making a warm sauce, keep the heat on low to prevent the egg yolks from hardening.

Troubleshooting

• Emulsion sauces can have a number of issues, even if you use an egg yolk. If you don't mix combine the oil and water correctly when making mayonnaise, you can end up with a runny, thin sauce. Typically, mayonnaise is runny if you pour in the oil too quickly. If that happens, you can help it thicken by mixing together another yolk and a small amount of water, then slowly stirring it into the sauce. If you are making Hollandaise sauce that turns out thin or chunky, the yolk most likely cooked. To fix, strain the sauce, then whisk with another yolk.

Safety Concerns

• Using raw egg yolks to make mayonnaise or lightly cooked yolks to make warm sauces does put you at risk for food poisoning caused by Salmonella. More than 140,000 people become sick due to Salmonella found in eggs every year, according to the Food and Drug Administration. You can reduce your risk of illness by using eggs that are pasteurized in the shell. If you use unpasteurized yolks, you can heat them at home to kill any bacteria, advises food science writer Harold McGee. Combine one yolk with a tablespoon of water, then microwave the mixture until it's nearly boiling. Stir, then heat the mixture again until nearly boiling. Remove the yolk mixture and stir until it cools down.