Summer is the perfect time for salads, and what makes them even better is a good dressing. Most salad dressings are emulsions, which are temporary mixtures of oil and water. But how do these emulsions form and how can we improve them?
Oil and water are known to not mix well together due to their distinct chemical properties. If you’ve ever tried making a salad dressing by shaking oil and vinegar (which is mostly water) together, you may have noticed that they quickly separate. This is because there is a high energy cost to breaking apart and mixing the water and oil layers. To blend them together, an extra ingredient called a surfactant or emulsifier is needed.
Surfactants, derived from “surface active,” work at the surface or interface to bridge the interactions between oil and water. They function similarly to how detergents remove grease from dishes. Many vinaigrette recipes call for emulsifiers like mustard and garlic, which contain mucilage that acts as an emulsifier. So if your vinegar/oil dressings are separating, make sure you’re adding enough of these ingredients.
Commercial salad dressings also contain naturally sourced emulsifying carbohydrates, often listed as “vegetable gum” or similar on the ingredients label. However, there have been concerns about synthetic emulsifiers used in processed foods, as studies in mice suggest potential health risks. It’s still too early to determine the exact implications for humans.
Mixing is crucial for dispersing oil in water. While shaking a jar is convenient, using a whisk or food processor will result in a more complete emulsion. The white or opaque color of many emulsions is due to the formation of microdroplets that scatter light. Mechanical mixing methods are especially important for permanent emulsions like mayonnaise.
Mayonnaise is an emulsion of oil in water, with egg yolk serving as the key emulsifier. Egg yolks contain phospholipids that can interact with both the oil and water layers. Mayonnaise is a stable emulsion, allowing it to be sold in a shelf-stable form. However, heating the emulsion can cause it to split. Recovering split emulsions may involve adding more emulsifier and re-mixing.
Preparing hollandaise sauce is notoriously challenging. The traditional method involves whisking egg yolk, water, and lemon juice over low heat, then slowly adding melted butter. The key to a successful hollandaise emulsion is separating the butter into fine droplets without overcooking the egg yolk. Using an immersion blender and controlling the temperature of the melted butter can help achieve a consistently emulsified sauce.
Emulsions are not limited to salads and sauces. They are also used in medicated creams, cosmetics, lotions, insecticides, acrylic paints, vaccines, inks, ice cream, margarine, and hair products. Understanding the chemistry behind emulsions can lead to delicious results in the kitchen and beyond.