Over-Mixing

• Although a stable emulsion depends on breaking up the oil droplets small enough for them to suspend themselves in the egg yolk, you can break them up too small and create a diminishing-returns effect. When the shearing force, or the whisking or blending used to break up oil into droplets, increases, more oil incorporates into the egg yolks, making the mayonnaise thicker and more stable. But too much shearing force, such as that which results from blending the egg yolks and oil too long, makes the mayonnaise too thick and causes the droplets to move so close to each other they can't remain separated. The oil droplets then recombine and pool together, cleanly separating from the yolks. You can avoid this type of separation by mixing the yolks until they stabilize then stopping mixing immediately.

Too Much Oil, Too Fast

• Egg yolks can only hold a little oil added to them at a time, no matter how much shearing force you use to break it up into small droplets. You have to add the oil -- referred to as the dispersed component of the emulsion -- to the egg yolks -- which are referred to as the container component of the emulsion -- in a slow, steady stream. Add too much to a container, too fast, and it breaks; add the same amount slowly, and it holds together. After the emulsification begins, you can add the oil in a heavier stream, but not in the beginning. Avoid this type of separation by adding a few drops of oil -- three or four -- at a time in the beginning. Then, whisk vigorously for a few seconds, add a few more drops, and whisk again, slowly building to an uninterrupted stream.

Egg-to-Oil Ratio

• An egg yolk can only emulsify 1 cup of oil, and the one or two droplets that exceed the 1-cup-per-yolk maximum are the droplets that cause the mayonnaise to break, often within a few minutes of finishing the emulsification, a delayed separation of sorts. You also have to whisk the egg yolk fully before adding the oil if you're mixing by hand, or the emulsion won't start. This type of separation is easily avoided by measuring the oil before you begin and whisking the egg yolks before starting.

Emulsifiers

• Even under the best conditions, homemade mayonnaise separates, and sometimes a little extra, natural boost is needed to ensure yours won't break in the middle of lunch. Unlike the world of commercial producers, who have access to every exotic food stabilizer conceived, you'll have to rely on your wits -- and what's in your pantry -- to give your mayonnaise a little separation insurance. To give your mayo a better chance of holding together, add 1 teaspoon of Dijon mustard to the egg yolks. Dijon is a natural emulsifier, and helps sauces like mayo and hollandaise stay together. If you want to make sure your mayo never breaks, take a page from the commercial mayonnaise playbook and add 1/2 teaspoon of soy lecithin per 1 cup while whisking. Soy lecithin is a natural ingredient available in health food stores and supermarkets, and works just like egg yolks in emulsions, except more effectively.