How to thicken soup

The flavour is perfect, the ingredients are all cooked to perfection, but that sauce is just too thin. Sure, you could reduce it, but you’d be concentrating the flavour, overcooking all the other bits and throwing the sauce:contents ratio all off. The only solution is to use some kind of thickening agent, but what?

This post covers four basic techniques for thickening sauces using the three most common ingredients: flour, cornflour, arrowroot. I’m going to tell you the what, where and why. If you’ve come here for an easy reading recipe, you might want to take a look at my recipe file because this post is crammed full of information.

Welcome to the 101 guide to thickening sauces, soups and stews.

A 101 guide to thickening sauces, soups and stews. You might want to bookmark this kitchen tip sheet for reference |

Let’s begin with the most basic:

Thickening Sauces with Plain Flour

Plain flour, or all purpose flour is available everywhere and most people have some in their cupboards so it’s handy to know how to thicken sauces properly with this household staple.Everything you need to know about thickening sauces, soups and stews. You might want to bookmark this kitchen tip sheet for reference |

How it thickens:

Gluten flours mainly consist of starch, in fact 75%  of the average plain flour is starch. These starch molecules absorb water and under the right amount of heat swell up and burst, releasing a gel like substance which is what makes it such a good thickener. It’s an irreversible process, once the water has bonded with the flour and gelatinized, there’s no going back.

In the UK it’s against the law to use bleaching agents in flour, which is why the flour above (middle spoon) looks so cream compared to the cornflour and arrowroot on either side. I thought I’d point that out for my US readers who will need to use more flour if using a bleached variety due to the gluten that the process removes.

Note that if you intend on using wholewheat flour rather than plain, it has less starch per tablespoon than plain white flour so, you’ll need to add slightly more.

Texture, appearance, flavour:

Flour gives a velvety, creamy mouthfeel and adds more body to sauces so it’s ideal for rich or cream based sauces like my creamy chicken soup. Flour, if uncooked, can add an unpleasant raw flavour to sauces but once cooked (following the instructions below) it is nutty and rich.

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How to and how much?

The general rule is 2 tsp of flour to thicken 1 litre of liquid, but this of course varies depending on how thick you’d like the sauce to be and how thick it is already.

The easiest way to thicken a sauce with plain flour is to make a flour slurry. Simply mix equal parts of flour and cold water in a cup and when smooth, stir in to the sauce. Bring the contents to a simmer for 5 minutes to cook away the raw flour taste. By mixing the flour with cold water first, it ensures the starch granules are separated so they’re less likely to link together and form clumps when they meet the hot liquid.

The next option is a Beure marnie; equal parts butter and flour, kneaded in to a dough. It’s ideal for thickening small amounts of liquid, like a pan sauce. Add a small amount to a hot pan of sauce and whisk until combined. Simmer for 3 minutes to cook the flour and thicken.

If you’re making a recipe that you’ve previously found to be too thin, you can start it off with a roux or the dusting method to thicken the sauce.

A Roux is made of equal parts fat and flour, just like the beure marnie, apart from it is cooked first before the sauce is started. Simply add the chosen fat or oil to a saucepan until melted then add the flour, stirring to combine and allowing to turn a light golden colour. It’s important to remember that this colour will be transferred to the final sauce so it isn’t suitable for all recipes. You can of course cook a roux past that golden phase to achieve deeper colour in your sauce but the over cooked flour loses its thickening ability. Once the roux is made, add the liquid and continue with the recipe as normal, adding a 3 minute simmer to thicken.

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You’ll be familiar with the dusting method if you’ve made casseroles and stews. It involves tossing the meat, veggies or other ingredients in flour before cooking. Essentially this does exactly the same as a roux; the oil in the pan and fat from the meat (or what you’ve added) combine and the flour is cooked. I personally think this method is easier than making a roux. You’re less likely to burn the flour and it cuts the extra step out.

It’s worth noting that neither the slurry nor the dusting method involve added fats so may be your choice of thickening method if you’re watching your fat intake.

Most sauces and casseroles thickened with flour will freeze and reheat well. The sauce will become more opaque and solid as it cools but should return to the original consistency once reheated.

Thickening Sauces with Cornflour

Pin this for reference. A complete guide to thickening sauces soups and stews.

Cornflour is the ground up endosperm of the corn kernel.  It’s a starch, just like flour but is gluten free and as such has double the thickening pour of plain flour. That also means it’s the ideal thickener for those with Coeliac disease.

How it thickens:

As a starch, cornflour thickens in exactly the same way as flour, but the result is slightly different and there are some caveats.

Cornflour struggles with extreme temperatures, so although it will tolerates time in the fridge before reheating but the starch molecules won’t survive the freezer well or anything hotter than 96 degrees Celcius. Don’t boil it!

Texture and appearance?

Sauces thickened with cornflour are almost translucent, quite glossy but sometimes can be gelatinous. They’re prone to getting a ‘skin’ on top and may need straining to remove it.

How to and how much?

The starch content of cornflour is 92% so although you can follow either the dusting or the slurry method, as detailed above, you’ll need to use less cornflour. The guide line is 1 tsp per litre liquid, although it shouldn’t be used more for acidic sauces, such as those that contain a lot of tomatoes, vinegar or wine. In acidic environments, its thickening capacity is greatly reduced – try plain flour instead.

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Thickening Sauces with Arrow Root

You'll wish you knew this sooner. A complete guide to thickening sauces, soups and stews

Arrow root is a starch from a Carribbean plant and also gluten free. It gets its name as it was once thought to be the antidote to poison arrows and was widely cultivated by the Arawak natives of the Carribbean.

Why it thickens:

As a starch, arrowroot’s mechanism is exactly the same as flour and cornflour. It operates at the lowest temperature of the three starches we’re looking at, making it ideal for sauces that you’re worried about splitting.

It’s possible to heat arrowroot too much, to the point where the sauce returns to its thinner starting consistency.  It’s important to note that because of this, arrow root isn’t the ideal candidate for something you intend to reheat.

Texture, flavour and appearance:

Arrow root thickened sauces are completely translucent and super glossy. It’s exactly what gives fruit tarts their gorgeous sheen. It’s ideal for making mirror like glazes but can make more ‘rustic’ sauces like gravy appear artificial. Its bland flavour makes it easy to disguise in even the most light sauces.

How to and how much:

Because arrowroot can thicken at such low temperatures, it’s not ideal for thickening sauces in anyway other than as a slurry.  You’ll only need to use 1/4 tsp per litre of liquid but it’s extra important to whisk whilst you add it to the sauce to prevent clumping.  Adding arrowroot too early in the cooking process can be disastrous, so only add it when everything is cooked and all that is left to do is thicken.

Put it to the test with these saucy recipes:

Creamy Chicken Soup //  Winter Vegatable Cobler // West Indies Butternut & Chicken Curry // Minestrone // South Indian Style Fish Curry // Spiced duck with Asian Slaw and Sticky Glaze 

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