Understand the difference between a stable and a temporary emulsion, and how thickeners help bind and thicken sauces.
Stable and temporary emulsions
Why you have to shake a dressing
Not all emulsions are equally stable. Mayonnaise is a stable emulsion: thanks to the yolk, it holds for a long time without separating. But a simple oil-and-vinegar salad dressing (a vinaigrette) is a temporary emulsion: shake it hard, and for a minute the oil breaks into droplets in the vinegar, but with no strong emulsifier the droplets soon merge and the layers separate again. That's why you shake a vinaigrette just before using it.
To make a dressing hold longer, add a little emulsifier — most often a spoonful of mustard or a touch of honey. They don't make the emulsion permanent, but they noticeably stabilize it, so the dressing stays smooth longer. The classic vinaigrette ratio is about 3 parts oil to 1 part acid, but that's a guide for taste, not a law.
Thickeners are a separate story. Sometimes a sauce needs thickening more than emulsifying. That's where starch (flour, cornstarch) and gelatin come in. Starch heated in liquid swells and binds water, and the sauce thickens (that's how gravies and puddings are thickened). Important: don't dump starch straight into hot liquid, or it will form lumps; first stir it into a little cold water, then pour that in.
The principle: understand the nature of a sauce, and you'll know how to handle it. A stable emulsion needs an emulsifier; a temporary one, a shake before serving; to thicken, add a thickener the right way.
Lesson notes
Why you have to shake a dressing
Not all emulsions are equally stable. Mayonnaise is a stable emulsion: thanks to the yolk, it holds for a long time without separating. But a simple oil-and-vinegar salad dressing (a vinaigrette) is a temporary emulsion: shake it hard, and for a minute the oil breaks into droplets in the vinegar, but with no strong emulsifier the droplets soon merge and the layers separate again. That's why you shake a vinaigrette just before using it.
To make a dressing hold longer, add a little emulsifier — most often a spoonful of mustard or a touch of honey. They don't make the emulsion permanent, but they noticeably stabilize it, so the dressing stays smooth longer. The classic vinaigrette ratio is about 3 parts oil to 1 part acid, but that's a guide for taste, not a law.
Thickeners are a separate story. Sometimes a sauce needs thickening more than emulsifying. That's where starch (flour, cornstarch) and gelatin come in. Starch heated in liquid swells and binds water, and the sauce thickens (that's how gravies and puddings are thickened). Important: don't dump starch straight into hot liquid, or it will form lumps; first stir it into a little cold water, then pour that in.
The principle: understand the nature of a sauce, and you'll know how to handle it. A stable emulsion needs an emulsifier; a temporary one, a shake before serving; to thicken, add a thickener the right way.