How Starches Thicken Sauces: Flour, Cornstarch, Roux, Slurries and Why Gravies Go Lumpy

A thin, watery gravy becomes velvety. A fruit pie filling sets instead of flooding the plate. A stir-fry sauce turns glossy and clings to every piece of vegetable. Behind all of these small kitchen transformations is the same ingredient: starch. Flour, cornstarch, potato starch, arrowroot and tapioca are among the most useful tools in any kitchen, yet they behave quite differently, and using the wrong one, or using the right one the wrong way, leads to lumps, a pasty taste, a cloudy sauce or a filling that turns runny the next day.

This guide explains what starch is, what actually happens when it thickens a liquid, how the common starches compare, the main methods for adding them (roux, slurry and kneaded butter), how to avoid and rescue lumps, why some thickened sauces thin out or weep, and how to freeze starch-thickened dishes successfully.

What starch is

Starch is the way plants store energy. In grains such as wheat and corn and in roots and tubers such as potatoes and cassava, it is packed into tiny, tightly organized granules. Each granule is made of two kinds of molecules:

• amylose, long and mostly straight chains;
• amylopectin, large, highly branched molecules.

The balance between these two shapes, and the size of the granules, explains why different starches thicken differently.

What happens when starch thickens a liquid

When starch granules are stirred into cold water, very little happens; they simply sink. When the liquid is heated, the granules start absorbing water and swelling, sometimes to many times their original size. As they swell, some starch molecules leak out into the surrounding liquid. The swollen granules and the released molecules get in each other's way and trap water, and the liquid thickens. Food scientists call this process gelatinization.

A few practical consequences:

• Starch only thickens when heated. A sauce will not thicken properly until it reaches the right temperature, which for many starches means bringing it close to a simmer.
• Thickening happens over a range of temperatures, and each starch has its own.
• Too much heat and stirring can break the swollen granules apart, which can make some starch-thickened sauces thin out again.

The common starches compared

Flour contains protein and other components as well as starch, so it takes more flour than pure starch to thicken the same amount of liquid. The pure starches, such as cornstarch, potato starch, arrowroot and tapioca, have roughly twice the thickening power of flour, but the exact amount depends on the product and the recipe.

Methods for adding starch

The most common cause of lumps is adding dry starch straight into a hot liquid. The outside of each clump gelatinizes instantly, forming a sticky skin that keeps the dry starch inside from dispersing. The solution is to separate the granules first, either in fat or in cold liquid.

Roux: starch in fat

A roux is flour cooked in an equal weight of fat, usually butter or oil.

1. Melt the fat over medium heat.
2. Stir in the flour and cook, stirring, for a few minutes.
3. Add warm or cold liquid gradually, whisking constantly.
4. Bring to a simmer and cook for several minutes, stirring, until thickened and no longer tasting of raw flour.

The fat coats each flour particle so it disperses without lumping. The longer a roux cooks, the darker and nuttier it becomes, but also the less thickening power it has: a pale roux for a white sauce thickens more than a dark brown roux used in some stews.

A roux is the basis of white sauces, many gravies and cheese sauces, and dark roux is famously used in some Creole and Cajun dishes.

Slurry: starch in cold liquid

A slurry is starch stirred into a small amount of cold water, broth or wine until smooth.

1. Mix the starch with a little cold liquid.
2. Stir it into the simmering sauce gradually.
3. Bring back to a simmer for a minute or so until the sauce thickens and turns glossy.

Slurries are ideal for cornstarch, potato starch and arrowroot, especially in quick sauces such as stir-fries and pan sauces. Our guide to how to make a pan sauce shows where a slurry can rescue a thin sauce.

Kneaded butter

Kneaded butter, often known by its French name beurre manié, is equal parts soft butter and flour mashed together into a paste. Small pieces are whisked into a simmering sauce at the end of cooking to thicken it quickly. It is useful for adjusting a sauce at the last minute.

Avoiding and rescuing lumps

To avoid lumps:

• disperse starch in fat or cold liquid first;
• add liquid to a roux gradually, whisking constantly;
• whisk, do not just stir, when adding starch;
• keep the heat moderate while thickening.

If lumps form anyway:

• whisk vigorously off the heat;
• press the sauce through a fine sieve;
• blend briefly with a hand blender, if the sauce is suitable.

Why sauces thin out or weep

Starch-thickened sauces can change after they are made.

• Too much stirring or long boiling can break swollen granules, especially with cornstarch and arrowroot, and the sauce thins.
• Acid, such as lemon juice or vinegar, can weaken some starches during long cooking. Adding acidic ingredients towards the end helps.
• Enzymes in some foods can break down starch. For example, a starch-thickened custard thinned by saliva on a spoon that was dipped back into the pot is a well-known kitchen puzzle; keep tasting spoons separate.
• Setting and weeping on cooling. Starches rich in amylose, such as cornstarch, form a gel as they cool, which is why cornstarch puddings set firm. Over time, the starch molecules can draw closer together and squeeze out water, a process called retrogradation. You see it as liquid pooling around a day-old pudding or filling. The same process makes bread go stale, as we explain in our guide to cooling, storing and freezing baked goods.

Starches rich in amylopectin, such as tapioca and some specially processed food starches, are less prone to setting and weeping.

Freezing starch-thickened dishes

Freezing and thawing encourages retrogradation, so some thickened dishes become spongy, grainy or watery after freezing.

• Cornstarch-thickened sauces and fillings often weep after freezing.
• Flour-thickened sauces usually freeze reasonably well, though they may need whisking and a little extra liquid when reheated.
• Tapioca tends to hold up better in frozen pie fillings.
• A good strategy is to freeze sauces before thickening and add the starch when reheating.

Our freezing food guide and our guide to batch-cooking sauces and bases explain how to plan for this.

Thickening without starch

Starch is not the only way to thicken a sauce:

• reduction: simmering to evaporate water and concentrate flavor;
• puréed vegetables or beans, which add body to soups and stews;
• egg yolks, which thicken custards and some sauces, but must be heated gently;
• cream, reduced slightly;
• butter or oil, emulsified into a sauce; our guide to why sauces split explains emulsions;
• bread or ground nuts, traditional in some cuisines.

Quick reference: which thickener to use

Frequently asked questions

Can I replace flour with cornstarch?

Usually yes, using roughly half as much cornstarch, but the sauce will be glossier and clearer, and it may thin out if boiled for a long time. Add it as a slurry rather than in a roux.

Why does my gravy taste floury?

The flour was not cooked long enough. Simmer the gravy, stirring, for a few more minutes, or cook the roux a little longer before adding liquid next time.

Why did my pudding turn watery the next day?

That is retrogradation: as the starch cools and ages, it releases water. Starches such as tapioca are less prone to it.

You will find more of the science behind good cooking in the Kitchen Science section.

The bottom line

Starch thickens liquids when its granules absorb water and swell during heating, a process called gelatinization. Flour is flavorful but needs cooking and more of it; cornstarch, potato starch, arrowroot and tapioca are stronger and clearer, each with its own quirks. Disperse starch in fat as a roux or in cold liquid as a slurry to avoid lumps, bring the sauce to the right temperature, and avoid boiling and stirring some starches for too long. Expect cornstarch dishes to set and possibly weep as they age, choose tapioca for fillings you plan to freeze, and remember that reduction, purées, eggs and emulsions can thicken too.