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Everyone knows that, left to their own devices, oil and water don’t mix. But for many recipes, you mix oil-based and water-based liquids—and want them to stay that way. Often, egg yolks come to your rescue by creating an emulsion. Most food emulsions are known as the oil-in-water type, which means that oil (or fat) droplets are dispersed throughout the water. Put oil and water in a jar, shake it vigorously, and you’ll disperse the oil. To prevent the oil droplets from coalescing, however, a substance known as an emulsifier is required. Egg yolk contains a number of emulsifiers, which is why egg yolks are so important in making foods such as hollandaise and mayonnaise. Many proteins in egg yolk can act as emulsifiers because they have some amino acids that repel water and some amino acids that attract water. Mix egg proteins thoroughly with oil and water, and one part of the protein will stick to the water and another part will stick to the oil. Lecithin is another important emulsifier found in egg yolk. Known as a phospholipid, it’s a fatlike molecule with a water-loving “head” and a long, water-fearing “tail.” The tail gets buried in the fat droplets, and its head sticks out of the droplet surface into the surrounding water. This establishes a barrier that prevents the surface of the fat droplet from coming into contact with the surface of another fat droplet. An emulsion is a mixture of two liquids that normally can't be combined. Oil and water is the classic example. Emulsifying is done by slowly adding one ingredient to another while simultaneously mixing rapidly. This disperses and suspends tiny droplets of one liquid through another. Chemically, emulsions are colloids, heterogeneous mixtures composed of tiny particles suspended in another immiscible (unmixable) material. These particles are larger than molecules, but less than one one-thousandth of a millimeter (.001mm). Small particles like this do not settle out and will pass right through filter paper. The particles in a colloid can be solid, liquid or bubbles of gas. The medium that they are suspended in can be a solid, liquid or gas (although gas colloids cannot be suspended in gas). Emulsions are liquid-liquid colloids, tiny liquid droplets suspended in another liquid. Emulsions are usually thick in texture and satiny in appearance. As an example, mayonnaise is made by combining lemon juice or vinegar with egg yolks. Eggs (containing the emulsifier lecithin) bind the ingredients together and prevent separation. Then, oil is added drop by drop as the mixture is rapidly whisked. Adding oil too quickly (or insufficient, rapid whisking) will keep the two liquids from combining (emulsifying). But, as the sauce begins to thicken, oil can be added more rapidly. |
