Why: why do oil and vinegar dressings separate, stay blended, get thicker, get hard?

Sunset, July, 1994 by Linda Lau Anusasananan

As days heat up, salads become a daily ritual--and so do questions about the curious behavior of oil and vinegar dressings. The answers have a lot to do with the way oil reacts to various conditions.

"Why does my olive oil and vinegar salad dressing sometimes turn very thick after mixing it [in a blender]?"--Franklyn W. Meyer, Honolulu

When you shake oil and vinegar in a jar or mix the two with a fork or whisk, you separate the oil into small drops that are dispersed throughout the vinegar, making a short-lived, opaque blend called an emulsion. On standing, the lighter oil quickly floats to the top of the vinegar, the oil drops rejoin, and the mixture separates.

If you whirl oil and vinegar in a blender, the power and speed of mixing make the oil drops very tiny and the vinegar can keep them apart longer, creating a thicker dressing that clings more readily to salad. But even with mechanical help, this marriage slowly comes apart.

Binders that help give additional body to salad dressings and slow down separation include mustard (dry or prepared), mayonnaise, and whipping cream.

"Why does oil and vinegar dressing turn white and solid when refrigerated?"--Anita Shank, Somers, New York

Fats and oils are made of a combination of three kinds of fatty acids: saturated, monounsaturated, and polyunsaturated. If there are enough saturated fatty acids, the fat is solid at room temperature or colder; heated, it usually melts. If a monounsaturated (single) fatty acid dominates, the fat is a liquid or an oil at room temperature or warmer, but it will partially solidify when it is chilled. When polyunsaturated (two) fatty acids dominate, the fat is a liquid and inclined to stay so even when it is chilled, because it has a more mobile molecular structure.

Refrigeration gradually slows the molecular activity of monounsaturated oils--olive, peanut, avocado, and macadamia--and they will become firm (although not as hard as butter), and turn cloudy or opaque. Returned to room temperature or heated, they quickly liquefy.

If your salad dressing is made with a monounsaturated oil and stored for a week or more in the refrigerator, chances are the oil will turn paler in color and get lumpy or solidify; but at room temperature it returns to liquid and is fine to use.

Why does oil get rancid?

It's oxygen that does the dirty deed. Fat's flavor deteriorates when oxygen hooks onto the bonds (especially the double bonds) that link together oil's basic elements: carbon and hydrogen. Oxygen then begins to break apart the fatty acids. A by-product of this decomposition is rancidity, a nasty taste and smell.

Because polyunsaturated oils have the most double bonds, they are the easiest for oxygen to attack and become rancid much more quickly than monounsaturated oils. Safflower oil and sunflower oil, for example, are high in polyunsaturated fatty acids and more prone to spoilage than extra-virgin olive oil, which is high in monounsaturated fatty acids--and which, if stored well, remains fresh for many months.

Heat and light make it easier for oxygen to get at an oil; so does salt (as in a dressing) and metal (most metal cans for oil have a protective coating). To protect oils, store them airtight in a cool, dark place. Once an oil container has been opened for use, the oil will keep best if tightly covered. If you buy oil in large containers, you can extend the oil's longevity by pouring the balance of the opened oil into smaller glass or lined metal containers and keeping them tightly closed.

More questions?

We would like to know what kitchen mysteries you're curious about. Send your questions to Why?, Sunset Magazine, 80 Willow Rd., Menlo Park, Calif. 94025. With the help of Dr. George K. York, extension food technologist at UC Davis, Sunset food editors will try to find solutions. We'll answer the questions in the magazine.