Mayonnaise is an emulsion, which is a mixture of two liquids that normally can't be combined. Combining 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.
However, the two liquids would quickly separate again if an emulsifier were not added. Emulsifiers are liaisons between the two liquids and serve to stabilize the mixture. Eggs and gelatin are among the foods that contain emulsifiers. In mayonnaise, the emulsifier is egg yolk, which contains lecithin, a fat emulsifier.
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.
Emulsions are used in many different ways:
- by pharmacists, as a vehicle for medication
- in photography, to coat plates, film and paper
- in explosives, paints, coatings, make-up and detergents
- in food, including baked goods and confectionery products
Since homemade mayonnaise is uncooked, be sure to use the freshest eggs possible, and ones that you are reasonably sure are free from salmonella. Homemade mayonnaise will last three to four days in the refrigerator.
Commercial mayonnaise, which will last up to six months in the refrigerator, contain (by U.S. law) at least 65-percent oil by weight (except reduced-fat and fat-free mayonnaises). The standard of identity law also requires that all commercial "real mayonnaise" use only egg as an emulsifier. Reduced fat mayonnaise, which isn't considered real mayonnaise, usually contains modified food starch, cellulose gel and other thickeners and emulsifiers.
Mayonnaise is used as the base for other sauces, such as tartar sauce and thousand-island salad dressing. Aioli is garlic-flavored mayonnaise. Another classic emulsion sauce is hollandaise, which is a cooked mixture of butter, egg yolks and lemon juice.
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Making Mayonnaise
If, you add a certain other substance to a mixture
of oil and vinegar,
you can stabilize the mixture
and prevent separation. The result is a stable emulsion.
The substance that keeps
the two liquids from separating is called an emulsifying agent.
Mayonnaise is an emulsion of oil in vinegar.
The emulsifying agent is egg yolks.
The egg yolks are first thoroughly mixed with the water in the vinegar so the water droplets
are dispersed and ready
to receive the oil.
The oil is then added slowly to the yolk mixture. The idea in making mayonnaise is to spread tiny
droplets as they form and prevent them from coming together and forming separate layers.
If you add the oil too fast, or add too much oil at one time, the droplets will come together before the oil can be forced into the egg yolks and the mayonnaise will curdle or separate.
If this happens, you can
correct the situation with fresh egg yolk, but add the curdled mayonnaise to the yolk rather than the other way around.
The egg yolk contains an emulsifying agent, lecithin, which acts as a stabilising emulsion between oil and water. This prevents oil and water mixtures from separation. Lecithin attracts oil and water particles and forms a thin layer around them to keep them from dispersing.
When making mayonnaise, the egg lecithin surrounds the oil particles, preventing them from joining and discharging oil.
Egg yolk is the emulsifying agent. Without it mayonnaise is basically a vinaigrette. If you omit egg yolk entirely your mayonnaise will not emulsify (thicken), if you reduce it by half, it will still work, but may not be as stable, or as easy to make.It is less viscous than that of the original amount.
References:
http://recipes.howstuffworks.com
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