How to Thicken a Sauce

September 17, 2008

Ed. Note: My apartment has been without power since Sunday afternoon, hence the delay in posting.

Sauces are a major part of tradition French cuisine, as I learned from The Making of a Chef. I don’t often make sauces at this point, but I’d like to discuss the basics of how to thicken them.

The Composition of a Sauce

Sauces are composed of a continuous phase, the liquid that surrounds all the other molecules and is the base of the sauce, and the dispersed phase, all the other things in the sauce that are added for flavor or consistency. In most sauces, water is the continuous phase – the exceptions being butters and vinaigrettes, in which fat is the continuous phase. The dispersed phase is anything and everything that floats around in the continuous phase, including spices, thickeners, food molecules, etc.

Sauces are thickened when dispersed phase components are added to block the flow of water molecules. When a starch, oil, or bubbles are added to a water based sauce, “the water molecules can move only a small distance before they collide with these foreign, less mobile substances,” as McGee explains. Blocking water molecules creates a thicker sauce.

Different thickening agents will have different affects on consistency (of course) and texture. Some thickeners block the flow of water, while others will actually bind to the water in some places to impede it. Particle thickeners can create grainy sauces (or smooth, based on the size), oils make sauces creamy, and air bubbles make them light.

Thickening Methods

There are four main ways to thicken sauces: particles, molecules, droplets, and bubbles.

Particles:

Thickening with particles usually means using some sort of pulverized food such as a pureed vegetable. The food is destroyed and its individual cells along with it. When these particles are added to a sauce, called a suspension, they “obstruct and bind the water molecules,” thereby thickening the sauce. The texture of the sauce depends on the size of the molecules, but particle thickened sauces always end up opaque because the particles can block light. Over time a particle suspension will settle and separate from the continuous water phase. This can be prevented by adding other things to the dispersed phase or boiling away much of the continuous phase.

Molecules:

Molecules are much smaller than whole particles, and only certain ones can be separated out of foods to use as a thickener. Common ones include starch, pectin and gelatin. These molecules are generally long and light, so when they are added to a sauce they don’t settle out like particles. Additionally, their length causes them to tangle with each other, which blocks water molecules and thickens the sauce. Therefore longer molecules thicken sauces better than shorter molecules. Because of their small size, these molecules do not reflect light, leaving the sauce translucent.

Droplets:

Droplet thickeners usually come in the form of fat and the product is called an emulsion. To form an emulsion, you need your continuous phase (water), dispersed phase (fat/oil), and an emulsifier. Because fat and water don’t mix, the emulsifier prevents the oil from joining back together by coating and stabilizing the tiny droplets. Emulsifiers can be egg yolks, ground herbs or spices, or fragments of cells. Cream is actually an emulsion, as the fat is forced into tiny droplets and incorporated back into the liquid.

Bubbles:

Bubbles may seem like a funny thickener, but think of different foams like whipped cream (a combination of thickeners!) or the top of a recently poured beer. Air bubbles, when stirred in, obstruct the flow of water molecules and stabilize the sauce. Often gravity causes the liquid to drain to the bottom and destroy the foam, but sometimes other thickeners or stabilizers can be added to prolong the life of the foam.

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