Why should you mix the oil slowly when you add it to the egg and vinegar mixture?
1 Answer
It's mainly down to the viscosity of the oil in comparison to the vinegar/egg mixture.......
Explanation:
Mayonnaise is a stable emulsion essentially of oil and water (basically oil and vinegar, but vinegar is >90% water anyway), stabilized by the proteins in egg yolk.
The key is to form a dispersion of microscopic droplets of oil in vinegar, so that the droplets can then adsorb protein molecules from the egg at the interface between the oil and vinegar interface. However, because oil is higher viscosity than vinegar (vinegar has about the same viscosity as water) if you add it too quickly, it will simply form large macro sized blobs of oil and you will not get efficient mixing.
This can cause the emulsion to "break", i.e. the fact that there are large blobs of oil rather than the required colloidal sized micro droplets gives and imbalance in the stabilizing system, which then causes the egg proteins to coalesce instead of acting as a stabilizer.
