Vinaigrettes are only temporary emulsions. When you mix oil and vinegar, nothing actually happens to bind them chemically; that is, they don't change. They just get broken up into tiny drops, so it LOOKS as though they're mixed. But they're not. (BTW, I'm saying "vinegar" here, but I'm really talking about any acid that you might use, including lemon, lime, orange, or even tomato juice.) You can add other ingredients to help them hold together, but you only want to add something that tastes good, or something that doesn't have any taste at all: AIR. :lips:
When you use a blender, you're chopping the drops of each into teeny tiny drops, and mixing in a lot of air. The drops are so small that you may think the oil and vinegar are actually mixed together, but really all they are doing is hanging onto the air that you mixed in. If you use an immersion (stick) blender or regular canister blender, the drops have more air to hang onto, and the vinaigrette holds together better than if you mixed it by hand with a whisk. But they still can't hang on for very long, so the emulsion breaks, and the tiny oil drops gather together to make big oil drops, and the vinegar drops do the same with other vinegar drops. Still, that's a good way to make a vinaigrette that holds together longer than if you mixed it by hand.
If you really feel you need to add something to hold the emulsion together, please please PLEASE don't add arrowroot, or any of those commercial binders. :eek: That may give you something similar to the stuff in the bottle, but ugh. Use a good prepared mustard, such as Dijon, which will give you both flavor and holding power, and make the vinaigrette with an electric mixer. Put the mustard into the bowl, add some of the vinegar (or whatever acid you're using), and whip it until it's really light-colored and fluffy. Then start to add the oil very slowly, just as if you were making mayonnaise. Once you've got a good amount of oil mixed in, you can alternate adding oil and vinegar. The point is to break up the drops and whip in a LOT of air for those tiny drops to cling to. I used to make a couple of quarts of sherry vinaigrette this way (just Dijon mustard, olive oil, sherry vinegar, salt, and pepper), and it would hold for a couple of days. :bounce:
"Notorious stickler" -- The New York Times, January 4, 2004