What do these things have in common: milk and butter? Of course they're delicious dairy foods. How about milk, butter and balsamic vinaigrette? Still delicious foods, but no longer dairy. What if we add Polysporin to the list (or hand lotion or hair conditioner)? No longer foods and probably not delicious, though — full disclosure — I've never tasted Polysporin. But what they do have in common is that they're all emulsions.
An emulsion is a mixture of two liquids that don't usually mix. Small droplets of the one liquid are suspended in the other. Oil and vinegar are a good example. You've probably done this: you make a salad dressing using three parts oil and one part vinegar by whisking them together, or shaking them in a bottle. This makes a nicely blended mixture. But not for long. The dressing soon separates into an oil layer floating on a vinegar layer. When you pour some of that on your salad, you get all oil and no vinegar. That's because the emulsion isn't stable.
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| A layer of liquid II floating on liquid I |
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| All shook up: Droplets of II are dispersed in I. This is an emulsion. |
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| But an unstable emulsion separates into the original two layers. |
Milk, by contrast, is a stable emulsion of oil droplets in water. And butter is a stable emulsion of water droplets in oil. Most commercial salad dressings are stable emulsions of vinegar in oil.
So how do you get your salad dressing to be stable? You introduce a chemical —an emulsifier—which will stop the two liquids from separating. The most common way the emulsifier works is this: it's a molecule, one end of which binds to oil, and the other end binds to water. So the emulsifier is active at the oil/water surface and prevents the drops of water (or vinegar) from separating out of the oil.
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| With an emulsifier attached to the droplets of II, this is a stable emulsion. |
While adding a chemical emulsifier sounds distasteful, but it isn't. Mayonnaise is a great example of a very stable emulsion. The secret emulsifier is lecithin, which is in the egg yolk that's used to make mayonnaise. Lecithin also occurs
in soya beans, and soy lecithin is a common emulsifier. So too, are
mono- and diglycerides. While these may sound like nasty, synthetic
chemicals, they occur naturally in seed oils, which is normally the raw material
from which they are produced (though sometimes animal fats are used
instead: vegetarians and vegans beware).
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| Molecular model of soy lecithin |
Other common emulsifiers for home-made salad dressings are mustard and honey. Now you know why those ingredients are often included in recipes for salad dressings. It's not just because of the taste.
Emulsion Factoids
White Milk and Blue Milk
I wrote earlier that milk is a stable emulsion of oil in water. That's only partly true. If you let unprocessed milk stand, it will partially separate: cream will rise to the top. Literally. To make milk totally stable it has to be homogenized. This is a purely mechanical (not chemical) process. The milk is forced through a small nozzle which ensures that the droplets of fat are all small. That's enough to stabilize the emulsion. Full fat and even 2% milk appears white. Milky white. That's because the suspended fat particles are dense enough to reflect the full spectrum of ambient light. Anything that reflects all the colours appears white.
But skim (or non-fat) milk doesn't have fat particles. The (relatively) big fat globules are gone, and what's left are mostly casein proteins suspended in water. This mixture doesn't reflect all wave lengths (colours) equally. The "Tyndall Effect" describes what happens: shorter wave length (blue) light is reflected more than longer wave length (red) light. Since more blue light is reflected, skim milk appears to have a blueish tinge.
Ole Blue Eyes
Unexpectedly, blue eyes don't have blue pigment in the iris. Irises have a translucent layer with particles of melanin suspended. And you guessed right. The particles of melanin reflect shorter wave length blue light more than longer wave length red light. And that's why blue eyes are blue.
The Ouzo Effect
Ouzo is an anise-flavoured liquor, common in Greece. Although it's a clear liquid, when you pour water into it, it becomes cloudy white. Here's why. Ouzo contains anise oil which dissolves in alcohol, but not in water. When the water is added to the Ouzo (alcohol/anise oil mixture), the alcohol itself dissolves in the water. That gives the anise oil a problem, so it leaves the water/alcohol mixture, forming tiny droplets of anise oil suspended in the water/alcohol—a stable emulsion. This phenomenon is called spontaneous emulsion. The emulsion is dense enough to reflect all light, and so the mixture looks white. We also know that if you keep diluting it, it will get a blue tinge.
