Thickening a Sauce: Starch, Roux, and Reduction

Three methods, three different results

• A starch slurry is the fastest and the clearest. It is the default for stir-fries.
• A roux (flour and butter) is opaque, heavier, and adds a toasted flavour. It is the basis of Western sauces.
• Reduction adds nothing and simply removes water. It gives the deepest flavour, but takes time and concentrates the salt along with everything else.

At the same thickness they feel completely different in the mouth. Roux in a stir-fry sauce reads as heavy; a slurry in a stew gives you a thick surface over thin liquid.

Mix starch cold, then boil it

Tip dry starch into hot sauce and the outside gelatinises first, sealing dry powder inside a lump. So it always goes into an equal volume of cold water first.

And once in, it must come to a boil at least once. Starch absorbs water and swells between 60 and 75C; short of that you get partial thickening and a raw, floury taste. Kill the heat immediately after adding and it thins again as it cools.

Starches are not interchangeable

• Potato starch thickens fast at lower temperatures and stays clear, but breaks down with prolonged cooking. This is the one Korean and Chinese stir-fries use.
• Cornstarch needs slightly more heat and is a little cloudy, but holds up better than potato.
• Tapioca starch is clear and springy, and survives freezing, which is why frozen foods use it.
• Flour is the most opaque and the most robust, but has to be cooked out or it tastes raw.

The glassy clarity of sweet and sour pork sauce is potato starch, and the gloss on mapo tofu and jjajangmyeon is the same family.

Why it thins when reheated

A starch sauce chilled and reheated is often looser than it started. Three causes:

• Acid. Vinegar or tomato cuts the starch chains, so acidic sauces need more starch — and the acid is better added after the thickening is set.
• Over-stirring, which physically bursts the swollen granules.
• Long cooking, to which potato starch is especially vulnerable.

So a sauce you intend to make ahead wants flour or cornstarch, and a stir-fry eaten immediately wants potato starch (reviving leftovers).

With a roux, cooking time sets both colour and power

A roux is equal weights of butter and flour, cooked together. The longer you cook it, the deeper the colour and aroma and the weaker its thickening power — the starch is breaking down.

• 2-3 minutes (white): strongest thickener, neutral flavour.
• 5-10 minutes: pale brown, nutty.
• 20 minutes or more (dark brown): there for aroma; expect almost no thickening.

Two minutes is the minimum to cook out the flour taste. Add liquid gradually, working each addition smooth before the next.

Collagen is the fourth method

Some sauces thicken with neither starch nor roux. That is collagen from bone and connective tissue turned to gelatin, and it feels different — starch thickness is slippery, gelatin thickness sticks to your lips.

It is why the sauce on galbijjim or hong shao rou has body with nothing added (braising collagen and time). Adding starch to a dish like that tends to mask the very quality you cooked for.

Working amounts

Per 250ml of liquid:

• 1 tsp potato starch or cornstarch: enough to give a broth a little body.
• 1 tbsp: a stir-fry sauce that clings to the ingredients.
• For a roux, 15g each of butter and flour is roughly equivalent to that tablespoon.

Too little is fixable by adding more. Too much is not. Always start short.