Most ingredients in the cook's repertoire occasionally need to be thickened. For example, milk might be thickened as part of a sauce, in a custard, or as part of a dessert preparation. People with dysphagia -- difficulty swallowing -- might need to thicken milk so they can drink it without choking. Several suitable techniques exist for thickening milk, depending on the intended purpose.
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One of the commonest ways to thicken any liquid, including milk, is to simmer it until most of its moisture content has evaporated. This makes the milk thicker for the simple and obvious reason that it is less watery. Its fats, proteins and sugars become more concentrated, giving the thickened, reduced milk a rich, distinctive flavor. Although Western cooking seldom uses this technique, Indian cooking, especially Indian desserts, make heavy use of reduced milks.
In sauces and soups, you would usually thicken milk by adding a starch such as flour or cornstarch. For example, in a classic bechamel sauce, the cook first prepares a roux by stirring butter and flour together in a pan. This precooks the flour, removing its starchy taste. When you have added the seasoned milk, the rough thickens it to a velvety-smooth texture. Quick-mixing flour is precooked at the factory, then dried and milled to a smooth powder. It is mixed into cold water to form slurry, and then stirred into the hot milk where it gels quickly. Cornstarch works the same way, but as a purified starch it provides more thickening power.
Many custards, especially dessert custards, work by thickening milk or cream. In custards, the thickening agent is eggs. The eggs must be whisked thoroughly into the milk and heated gently, which causes their proteins to congeal just as they would in a frying pan. Because the strands of protein are dispersed in the milk, they set to a soft gel, rather than the familiar firm texture of a fried or boiled egg. Some custards, such as pastry cream, also incorporate a starch to provide a firmer, more reliable thickening effect.
Thickening for Dysphagia
Thickening milk for people with dysphagia is a rather different prospect, because culinary thickeners require that you heat the milk. That changes its flavor, and makes it less appealing as a beverage. The specialized thickeners used for that purpose use gum-based ingredients such as xanthan gum, which are widely used in manufactured foods as thickeners and gelling agents. You can stir these into milk, fruit juices and other beverages, and thicken them without altering their flavor or appearance.