The chemistry of mayonnaise
In its most basic form, mayonnaise consists only of oil, egg yolk, vinegar and salt, although a pinch of mustard is a common addition. It’s simple enough to make: combine yolk, salt and vinegar, then slowly add the oil while whisking vigorously to create an emulsion. It’s amazing when you think about it: how does clear, golden oil become a thick, creamy, whitish sauce? Of course, the oil is still there, only in a different form: it has been divided into millions of microscopic droplets that reflect light, giving mayonnaise its whitish appearance. The smaller the droplets, the thicker and whiter the mayonnaise. The droplets of oil are suspended in the small amount of water supplied by the yolk and vinegar. They’re so tightly packed, they can barely move, but stay separated from one another because they’re coated with a thin layer of emulsifiers. Emulsifiers in the egg yolk do their work by bonding part of their molecule to the water and another to the oil, stabilizing the droplets. Without them, the droplets would flow into one another (in scientific jargon, they would coalesce) and a pool of oil would rise to the surface of the mayonnaise.
Three essentials of mayonnaise
Any oil will work, but canola, sunflower and corn oil have the virtue of neutral flavour. Make sure the oil is fresh, as the slightest trace of rancidity will spoil the taste of the mayonnaise. You can replace some of the oil with olive, walnut or sesame oil to produce a more distinctive taste. It can be harder to make mayonnaise successfully with olive oil, as it tends to separate more easily because of certain molecules (monoglycerides) found in unrefined oils.
The oil droplets have to be suspended in an aqueous medium—water. About half of the water in a classic yolk-based mayonnaise comes from the egg yolk, while the rest comes from the vinegar. In whole-egg mayonnaise, additional water is provided by the egg white. If you’re feeling creative, substitute freshly squeezed lemon or lime juice or any other juice or flavoured liquid for the vinegar. To make yolk-based mayonnaise, be sure there is at least 15 ml (1 tablespoon) of liquid for every 250 ml (1 cup) of oil. Otherwise the oil droplets will be packed too tightly, leaving your mayonnaise at risk of separating and developing an oily consistency (see table, p. 15). If you want to add extra liquid to achieve a more distinctive flavour, it’s best to add it to finished mayonnaise.
Not only do emulsifiers form a protective coating around the oil droplets, they also lower the surface tension (inward pull) of the water, allowing it to flow between the droplets, keeping them separate. The most efficient of all emulsifying ingre¬di¬ents is certainly egg yolk. It contains a high concen¬tration of powerful emulsifying molecules, of which the most familiar is lecithin. But egg yolks aren’t the only natural emulsifiers around. Many other protein-bearing foods will do, since proteins can unfold and coat the oil droplets, allowing them to stay separate. For example, it’s possible to make mayonnaise using only egg whites, or even gelatin. Proteins and small amounts of other emulsifying molecules found in poultry, seafood, and fruit or vegetable purées can also do the trick. Chefs who practice molecular gastronomy take advantage of this fact to create new emulsified sauces. Finally, while not a true emulsifier, dry or prepared mustard contains tiny particles that insert themselves between the water and oil droplets, helping to keep them separated.
Rescuing broken mayonnaise
If your mayonnaise suddenly turns into a liquid mess of oil and egg, don’t panic. There are ways to save it.
- Problem: Whisked mayonnaise has broken into a soupy mess.
- Cause: The oil was probably added too quickly, preventing it from forming small, stable droplets.
- Solution: For the broken mayonnaise, mixture into a measuring cup. Place 5 to 10 ml (1 to 2 teaspoons) of water in a bowl. Add the broken mayonnaise, drop by drop, while whisking vigorously. When the new emulsion begins to thicken, the remaining mixture (and the rest of the oil, if any) can be added in a thin stream while continuing to whisk.
- Problem: Mayonnaise made in a food processor or blender starts to thicken but suddenly turns into a thin, milky liquid.
- Cause: Blenders and food processor are so powerful that if you let them run too long (by taking too much tome to add the oil) or too fast, the blades will disrupt the layer of emulsifier coating the oil droplets, and the oil will coalesce.
- Solution: You can fix the mayonnaise by using the procedure described above. Next time, use the whole egg instead of just the yolk. The white adds important structural support by coating and protecting the oil droplets. Set the blender or food processor to the lowest speed and add the oil fairly quickly in a thin, steady stream.
- Problem: Nearly all the oil has been added. The mayonnaise is very thick, but oil starts pooling in the whisk's wake and the surface of the mayonnaise looks like it is about to break.
- Cause: There is no longer enough water tu suspend and separate the oil droplets.
- Solution: Quickly whisk in 5 to 10 ml (1 to 2 teaspoons) of water, vinegar or lemon juice. The mayonnaise will thin out a bit and lose it's oily appareance. Any remaining oil can be added at this point.
Making perfect mayonnaise
- Contrary to popular belief, the temperature of the ingredients is not critical. They can be cold, room-temperature or at different temperatures.
- Start by mixing the egg yolk (or whole egg) thoroughly with the salt, vinegar or lemon juice and mustard.
- Roll a damp tea towel into a rope and wrap it around the base of the bowl to keep it from sliding while you whisk.
- At the beginning, whisk in the first third of the oil literally drop by drop. Make sure that each addition is thoroughly incorporated before you add any more.
- Once the mayonnaise starts to thicken, the remaining oil can be added more quickly, in a thin stream.
Making mayonnaise in a food processor or blender
- Since these appliances’ blades rotate hundreds of times faster than a whisk, the oil can be added more quickly. They make very thick, white mayonnaise in less than a minute.
- Use a whole egg and not just the yolk. Why? The white provides extra volume to the egg–vinegar mixture (otherwise, before the oil is added, the appliances’ blades might simply spin in thin air) but most importantly, the white also provides additional emulsifying power and support.
- Select the lowest speed and add the oil fairly rapidly, in a thin stream. Turn off the appliance as soon as the emulsion forms.
Nice to know
A single egg yolk has enough emulsifying power to make several litres of mayonnaise. All it takes is a few drops to whip up a typical batch of homemade mayo!