Salad Dressing and Mayonnaise

When oil is mixed with vinegar (an aqueous solution of acetic acid) we call it salad dressing. Throw in some spices and we call it Italian Dressing. However, Italian Dressing separates and, therefore, must be stirred or shaken before use.

If a small amount of egg whites are added to the mix, the egg whites (which are soap-like because they have a polar "head" and a non-polar tail) form micelles with the oil trapped in the middle. If enough egg white is added to the mix so that all of the oil is trapped in micelles, the mixture will not separate. The micelles will interfere with light passing through however, making the mixture appear cloudy. We call it “creamy” Italian dressing.

If we add more and more oil and egg whites the mixture gets thicker and thicker until it becomes “solid.” At that point we call it mayonnaise.

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