Commercial mayonnaise is a familiar but not especially interesting product, a blandly pleasant sandwich spread that's also useful as the base of a thick salad dressing. Making mayonnaise from scratch produces a startlingly different result, a smoother and creamier dressing filled with subtle flavors. The basic mayonnaise-making technique, while finicky, is not difficult if you're patient and have a steady hand.
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When you make mayonnaise, you force oil and water to overcome their proverbial inability to mix. Lecithin and other emulsifiers in the egg yolks make this possible by providing a chemical bond between the lipids in your vegetable oil and the water molecules in the other ingredients. As you whisk the oil into your mixture, it disperses in fine droplets throughout your water-based ingredients, eventually forming the thick and familiar texture. That thick emulsion might not result the first few times you try this, but the mayonnaise can usually be saved.
Mayonnaise By Hand
To fully experience the process, try making your mayonnaise first by hand. Separate an egg yolk into the bottom of a mixing bowl, and add roughly a teaspoon of vinegar or lemon juice. Add a pinch each of salt and pepper, then rest your bowl on a damp kitchen towel so it doesn't slide around as you work. Set a cup of cold water nearby and measure a cup of oil. Whisk the egg yolk until it's pale and frothy, then add a few drops of oil. Whisk the initial drops until they're fully incorporated, then add a few more drops. Once you've incorporated a quarter of the oil, you can start pouring it in a very thin stream. Continue until the oil is incorporated, adding a few drops of water if it seems too thick.
Making mayonnaise the old-fashioned way will leave your forearms very tired, so even professionals frequently use a mixer, blender, small food processor or stand mixer to speed the process. With a mixer or stand mixer, the process is identical to the manual technique, except you're not whisking by hand. In a blender or food processor, you'll pulse the egg and lemon juice until they're mixed, then add your oil slowly through the feed tube or blender's cap. Full-sized food processors have difficulty coping with a single-egg batch of mayonnaise, so use your small bowl or reserve a full-size food processor for large batches.
The most common error for novices is adding the oil faster than it can be emulsified into the egg mixture. When this happens, the oil will remain unabsorbed for several moments and then the mixture will separate into its oil and egg components. If you catch it at the beginning, adding a few drops of cold water will help pull the emulsion back together. If it breaks completely, start over with a fresh egg yolk and add your broken mayonnaise drop by drop until it's all incorporated. The egg yolks also represent a potential salmonella risk, so if you don't have access to pasteurized eggs, it's best to "coddle" or par-boil your egg before separating it. The finished mayonnaise is highly perishable, and should be refrigerated immediately and used within a day.
Using Your Expertise
Once you've mastered the basic mayonnaise-making technique, you'll find other opportunities to use it. Upscale aioli, for example, is simply mayonnaise made with olive oil, plenty of garlic, and optional ingredients such as saffron for an exotic flavor. Scratch-made Caesar dressing is just a thin, garlicky mayonnaise with plenty of Parmesan cheese added. Even rich, luxurious hollandaise sauce and its derivatives are very similar. The only difference is that it's made over a double boiler, by whisking cubes of cold butter into the egg mixture.