When it comes to thickening soups and sauces, both cornstarch and flour are valuable tools in a cook’s arsenal.

Rather than one being superior to the other, each has distinct properties that make it ideal for specific culinary applications.

Flour vs. Cornstarch: The Differences

Understanding the differences between these common thickeners can elevate your soup from good to exceptional.

Flour provides a stable thickness that holds up well through reheating and imparts a subtle wheaty flavor that complements hearty stews, though it creates an opaque finish and requires longer cooking time to remove the raw taste.

Cornstarch, on the other hand, yields a glossier, more translucent finish with a neutral flavor that won’t alter your soup’s taste profile. It provides more thickening power per tablespoon — twice as strong as flour — and activates quickly, though it can thin if overheated or repeatedly reheated.

Making a Cornstarch Slurry

Adding any dry starch directly to hot soup will result in unwanted lumps, so proper technique is essential.

For a flour slurry, mix 1-2 tablespoons of all-purpose flour with 2-4 tablespoons of cold water, broth, or milk. The proper ratio is 1:1 or 1:2 (flour to liquid), not the common mistake of 1 tablespoon to a quart. For a cornstarch slurry, combine 1 tablespoon cornstarch with 2 tablespoons cold water or broth. For each cup of liquid you want to thicken, you’ll typically need about 1-2 teaspoons of cornstarch.

Whisk either mixture until completely smooth with no visible lumps. While your soup is simmering (not boiling), add the slurry in a thin, steady stream while continuously whisking to distribute it evenly. Allow your soup to simmer for the appropriate time: cornstarch needs to reach near-boiling to activate fully (1-2 minutes), while flour needs to cook longer (10-15 minutes) to eliminate the raw taste.

Incorporating a Roux

Professional kitchens often prefer using a roux over flour slurries. To make one, melt equal parts fat (butter, oil, or rendered fat) and flour in a separate pan, then cook the mixture over medium heat, stirring constantly.

A white roux (cooked 2-3 minutes) has the highest thickening power, a blond roux (5-10 minutes) offers nutty flavor with good thickening, and a brown roux (15-30 minutes) provides rich flavor with less thickening power.

Whisk the roux into your simmering soup gradually, then cook to desired consistency. This method produces superior flavor and more stable thickening than a simple slurry.

Temperature Matters

Temperature considerations also affect thickening.

Cornstarch thickens best as it approaches boiling point but can break down with prolonged boiling.

Flour needs to reach at least 180°F (82°C) and maintain that temperature for several minutes to activate fully and cook off the raw flavor. When soup cools, all starch thickeners will continue to set, becoming thicker. Reheating will thin it out again to the proper consistency.

Alternative Thickeners

Beyond flour and cornstarch, professional chefs employ other starch thickeners like arrowroot (creates a beautiful, clear sheen), potato starch (excellent for freezing and reheating), and tapioca starch (provides an appealing glossy finish).

Natural thickening methods include reduction (simply simmering soup uncovered to evaporate liquid), puréed vegetables (blending in cooked potatoes, cauliflower, or beans), starchy pasta water (adds silky texture), and egg liaison (tempered egg yolks for rich, velvety texture).

Special Diet Considerations

For special diets, particularly keto or low-carb, consider xanthan gum (⅛ teaspoon per cup of liquid), heavy cream reduction, puréed cauliflower, or ground chia or flax seeds. Avoid using mayonnaise as it can separate when heated and significantly alter flavor profiles.

Flour Types and Substitutions

When thickening with flour, always use plain (all-purpose) flour. Self-rising flour contains leavening agents that aren’t necessary for thickening and can impart unwanted flavors or bubbling. No professional kitchen would use self-rising flour as a thickener.

For substitutions, remember that 1 tablespoon cornstarch equals 2 tablespoons all-purpose flour, 1 tablespoon arrowroot equals 2½ tablespoons all-purpose flour, and 1 tablespoon potato starch equals 2 tablespoons all-purpose flour.

Mastering the art of thickening is about understanding each agent’s unique properties and choosing the right one for your specific culinary creation. Whether you prefer the neutral clarity of cornstarch or the hearty stability of flour, proper technique will ensure perfect results every time.