A well-constructed salad consists of more than a basic bowl of greens. At their best, salads represent a range of colors, flavors and textures that few other mainstream dishes can match. The dressing plays a significant role as well, lending richness to the otherwise Spartan vegetables and transforming their diverse flavors into a coherent whole. As with pasta sauce, salad dressing works more effectively if it's thick enough to adhere to the greens. You can thicken your dressing in a number of ways.
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The simplest way to thicken your dressing is physically. When you shake a vinaigrette, its oil separates into tiny globules that disperse throughout the vinegar and other liquids. This forms a suspension, or temporary emulsion, of those two proverbially dissimilar ingredients. This results in a vinaigrette with a creamy texture, thicker than either of its ingredients alone. The effect is transient, and they'll soon separate again, but it lasts while you eat your salad. Mixing the vinaigrette with an immersion or "stick" blender keeps it in suspension longer, but you'll achieve a better effect though added thickening ingredients.
Thicker for Longer
A number of common vinaigrette ingredients slow the separation of your oil and vinegar, preserving its creamy texture. These include commonplace ingredients such as honey and prepared mustard -- often paired together -- and more exotic offerings such as tahini, the sesame paste that's a staple of Middle Eastern cooking. These work in part because they're thicker than the vinaigrette, and simply dispersing them through the dressing makes it richer and thicker. Mustard or fresh garlic also acts as a mild emulsifier, helping bond oil and water molecules together. Dressings thickened this way will still separate eventually but can take hours or even days to do so.
Staying That Way
Some dressings simply don't separate. They're made by combining agitation and emulsifiers in a specific way to make a permanent emulsion. The most common example, mayonnaise, binds the oil and liquid together with the natural emulsifiers in egg yolk. The cook whisks a yolk with lemon juice, vinegar or other acidic liquid, then adds the oil in a very fine stream. Classic mayonnaise has a very thick texture when finished, but other dressings such as Caesar salad dressing thin the emulsion with added liquids to result in a pourable, creamy consistency. Egg-based emulsions are highly perishable, and should be used or refrigerated immediately.
The Heavy Artillery
Most homemade salad dressings follow one of these familiar patterns, but a few delve into territory normally occupied by their commercially manufactured peers. For example, diners with egg allergies might rely on an alternative emulsifier, such as soy lecithin, to make mayonnaise. Low-fat dressings often add a small quantity of guar gum, xanthan gum or other gelling agents to compensate for their reduced oil content, which leaves them thin even after shaking. If you make your own low-fat dressing with chicken or vegetable broth replacing part of the oil, you can achieve a similar effect by thickening the broth slightly with cornstarch before adding it to your vinaigrette.