Which component in cooking acts as a thickener by combining fat and flour?

The component in cooking that serves as a thickener by combining fat and flour is known as a roux. A roux is made by cooking equal parts of fat, typically butter, and flour together. This mixture is heated until it reaches a desired color, which can vary from a pale blonde to a deep brown, depending on the desired flavor and use in a dish.

When used in sauces and soups, a roux acts as a thickening agent because the starches in the flour swell and absorb liquid, creating a smooth, thickened consistency. This technique is fundamental in classic French cuisine and is a key building block for many sauces, such as béchamel and gumbo.

The other components listed, while important in cooking, do not perform the same function as a roux. For example, emulsion refers to the mixing of two liquids that do not normally combine, such as oil and water, while paste generally describes a thicker mixture that may not involve cooking fat and flour together. Gelatin, on the other hand, is a protein used in cooking primarily for gelling and setting liquids, but does not involve the thickening process that a roux provides.