Food for thought: Why doesn't mayonnaise separate?

Have you ever wondered why French dressing separates on standing, while mayonnaise doesn't? After all, they both typically contain oil and vinegar, which as we know do not mix easily. Well, mayonnaise contains an extra substance, called an emulsifier, which is used to unite the two immiscible ingredients to form a stable, usable emulsion.

The emulsifier is made from a hydrophilic (water-loving) head and a hydrophobic (water-hating) tail, which work together to stop the oil from separating out. In the case of mayonnaise, it's the egg yolk that does this job.

Stable emulsions can be found naturally, and may be either oil-in-water, in which case small oil droplets are dispersed through water (as in milk), or water-in-oil, in which case small water droplets are dispersed through oil (as in butter).

Now, who'd like emulsion with their salad?

Roy Ballam, British Nutrition Foundation