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I've been making a lot of vinaigrette dressing lately. Julia Child and Emeril both say that you need to mix up everything but the olive oil, then add the olive oil in slowly (ideally drop by drop) while whisking.

First, this is a pain because you need 3 hands. Ok you don't really, but it isn't easy to do with 2 hands.

Second, it seems like I still need to shake the dressing a lot before putting it over salad, especially if the dressing has been in the refrigerator.

Finally, the question: what is the upside of all this doing-it-slowly when you add the oil to the mix? As far as I can tell, it makes a tiny tiny bit of difference: there will still be some tiny tiny drops of vinegar suspended in the oil a few days later. Why would Julia and Emeril think this is such a big deal?

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To ease the process- as with mayonnaise- support your bowl with a wet towel coiled around the base and put your oil in a squeeze bottle to better control flow. –  Sobachatina Nov 4 '10 at 15:59
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3 Answers

up vote 8 down vote accepted

Specifically, the reason you whisk it in slowly at first is to create small drops. If you just dump the oil in fast, it will adhere to itself and make it impossible to break up into droplets dispersed in the vinegar (or other water based liquid, such as lemon juice). This is the definition of an emulsion: tiny droplets of one liquid evenly dispersed in another. Once you've got the emulsion started, you can pour in the oil a bit faster because the already suspended droplets reduce the room for a big blob of oil to form, and act sort of as "sandpaper" to break up the incoming stream as you whisk.

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But doesn't whisking break up that big blob pretty easily? Seems like a lot of work for little gain. –  jcollum Nov 4 '10 at 16:15
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No, it doesn't. The surface tension in the big blob is so strong that it becomes very difficult to break up once formed. So you want a good vortex going and a thin stream to give yourself a leg up on creating small droplets. Try it both way sometime, I guarantee you will see the difference. –  Michael at Herbivoracious Nov 4 '10 at 20:41
    
Is this a concern if you use a blender instead? –  Michael Hoffman Jan 15 '12 at 0:03
    
Even with a blender, you'll get better results if you drizzle in the oil through the hole in the top. If you blender is powerful enough, you may be able to add everything at once and let it break the droplets up, but the other way is more of a sure thing and tends to produce a more stable emulsion with consistently sized droplets. –  Michael at Herbivoracious Jan 15 '12 at 16:20
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I assume that the recipes also include a weak emulsifier like mustard.

In the presence of an emulsifier adding the oil slowly will create an emulsion. The oil will be basically dissolved into the vinegar. This will result in a vinaigrette that will be slightly more viscous and will adhere better to the target food.

With weak emulsifiers this is still temporary because the emulsion is not stable. Using a more powerful emulsifier like xanthan gum or lecithin would be permanent but I don't know if that would still be considered a vinaigrette. You would easily cross the line into mayonnaise.

In short- not having tried your particular recipes- if they are recommending the extra work to form a better emulsion then they are probably expecting the vinaigrette to be served immediately and not days later.

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The last paragraph here is important, I think. Every unstable emulsion is going to separate in the refrigerator, no matter how you mix it. But assuming you plan to serve right away, then combining the liquids very slowly will result in something that is temporarily much more stable than if you had just dumped it all into a big bowl. –  Aaronut Nov 4 '10 at 20:21
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The purpose of adding oil slowly is to make a smoother emulsion. It will stay together on its own longer this way, but will always separate eventually. You can add a bit of mustard (an emulsifier) to your vinaigrette to keep it stable.

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