A salad without a dressing, or a well-turned restaurant meal without its sauce, is an oddly anticlimactic affair. The flavorful liquid adhering to the food -- if chosen correctly -- adds to the overall experience, without overpowering or obscuring the flavors of the food itself. The dressings served with salads and the various sauces accompanying other foods serve similar purposes. In truth, salad dressings are simply one specific family within the larger world of sauces.
A Basic Definition
Wayne Gisslen's "Professional Cooking," the textbook of the Le Cordon Bleu cooking schools, defines a sauce as a "flavorful liquid, usually thickened, that is used to season, flavor and enhance other foods." Although the culinary world contains a number of exceptions and variations on that basic definition, it's a good starting point for any discussion of sauces and salad dressings.
The most basic of salad dressings are simple vinaigrettes, consisting of oil, an acidic ingredient such as vinegar, and other flavorings. Although you might not think of vinaigrette as thickened, simply shaking it before adding it to your salad accomplishes that feat temporarily. The agitation disperses the oil and vinegar droplets, creating a short-lived mixture that's thicker than either ingredient separately that clings more effectively to your salad's leaves. Sweeteners such as honey and flavoring ingredients like mustard can help make the mixture thicker and longer-lasting, but truly thick salad dressings require a more robust thickener or emulsifier.
Thickened Dressings and Emulsified Sauces
Some commercial dressings are thickened with starches or vegetable-based gums, but, more commonly, thick dressings rely on eggs or other emulsifiers. Emulsifiers are substances that act as molecular matchmakers, overcoming the proverbial unwillingness of oil and water to remain mixed. Traditional Caesar dressing is made in in the salad bowl, by mixing an egg yolk with lemon juice and garlic and then whisking in the oil. Mayonnaise is the ultimate example of an emulsified dressing, both in its own right and as the basis for other dressings such as Russian or Green Goddess. Interestingly, mayonnaise-based sauces such as remoulade and tartar sauce are often used with hot foods, and famous sauces such as Hollandaise use the identical technique with hot ingredients.
Most of the sauces comprising the traditional Western canon are made from a base of butter, cream, broth, pan drippings and other flavorful ingredients such as wine. Broth- and cream-based sauces are sometimes thickened simply by simmering them until they thicken on their own, but most are thickened by some form of starch. Classic restaurant sauces often incorporate a roux as the thickener -- a mixture of flour and butter cooked together -- but lighter modern sauces often employ cornstarch or arrowroot instead. The desired end result is always the same, a sauce that's thick enough to lightly coat and moisten the food.
Dessert sauces sometimes follow a similar pattern, with starch thickeners proving especially useful for fruit-based sauces. Natural thickeners such as gelatin and pectin -- which would melt, in a hot sauce -- can also sometimes be employed in fruit-based sauces, which are typically served on cold dishes. Caramel sauce and sugar-based syrups and glazes rely on sugar's tendency to form strong chemical bonds, when it's highly concentrated. Some of the most elegant sauces, such as creme anglaise, are thin custards made by simmering eggs or egg yolks in cream or milk. The egg proteins solidify, as they would in a fried egg, but, because they're diluted by the cream, the end result is a soft and delicate texture.