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.

That band is not a constant, and it is not a gate you cross once

Sixty to seventy-five is a property of starch in water — and the pot is rarely plain water, nor does it only cross the band once.

A concentrated solution raises it, because the dissolved thing is holding the water the starch needs to take up. Pozole writes it down on the salt side — "a concentrated salt solution holds water back from the starch and raises its gelatinisation temperature" — and is careful to separate that from soaking's finding that salted water softens a dried bean's skin: one is the starch inside the kernel and the other is pectin in the coat, and they run opposite ways. Gamja-jorim is the same move with sugar, which does it harder for being the larger molecule — syrup added early "does not merely fail to glaze; it holds the potato back." So seasoning something starchy while it is still cooking changes how long it takes, not only how it tastes, which is why the pozole's salt waits for the last thirty minutes and the corn syrup goes in at the end. The underlying fact — that sugar binds water so nothing else can use it — belongs to what sugar does; the starch cooking beside it is one of the things that cannot.

And you can drop back below the band mid-cook. Risotto keeps its stock at a simmer in a second pan for exactly this, and its page states what one cold ladle costs: "starch that has come off the grain needs to stay above its gelatinisation temperature to thicken anything. Cold stock drops the pan below it, so the rice goes on cooking while the body stops developing." The two halves of the dish come apart — grain and sauce are on separate clocks, and only one of them stops.

The same shape runs the other way as well. Cooking eggs records that swollen starch granules get between the egg proteins and raise the egg's setting temperature. Either direction, what else is in the pot moves the number on the chart.

Starch put on the outside instead of into the liquid

Everything above disperses starch through a liquid. Chinese stir-fries use the same starch the other way round, as a film on the surface of a solid, and used that way it thickens nothing at all.

Beef and Broccoli spells it out. A wet film of cornflour on a 2mm slice crosses the 60-75C band above in its first seconds in the wok, sets into a gel skin, and the meat's water is then behind something. Kung Pao Chicken does the same and says only that "a cornstarch marinade velvets the chicken so it stays moist."

So this is "mix starch cold, then boil it", above, deliberately broken. That rule exists because wet starch meeting heat sets on contact and seals in whatever is behind it — which is a fault when what is behind it is dry powder, and the whole point when what is behind it is meat.

Where the film goes is decided by where the water must not leave, and it need not be everywhere.

• All over, to keep water in. Velveted meat is sliced thin, so it is nearly all surface with almost no interior, and there is no water to spare.
• On the cut faces only, to stop it running out. Chilli Crab dusts cornflour "on the cut faces only. It stops the meat weeping out while it fries, and later that same starch gives the sauce a little help", and Poo pad pong curry does it for the same reason. Coat the shell as well and the sauce only clings where nobody eats.

The crab is also where the two uses meet: the same starch goes on as a barrier and ends the dish as a thickener, once it is in the pan with the sauce.

And a film meant to stay wet is handled in the opposite way to one meant to go crisp. It goes onto raw meat, is not allowed to colour, and finishes in liquid — which is why sauce ruins the other kind. Tangsuyuk is flat about it — "the sauce is water, and water softens a starch shell back down." Browning files the same gel film from the fault side: it holds water against the surface, and while water is there nothing browns.

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. (In one dish that bursting is the method — see below.)
• 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.

When nothing is added and it thickens anyway

All four above are things you add from outside. Some liquids thicken with nothing added at all, and what is doing it decides the result.

• The ingredient's own starch, drawn out. In Gratin Dauphinois the only thing thickening it is the starch out of the potatoes, so rinsing the slices washes that away — the most important step is a step you do not take. Jeonbok-juk goes further and bursts them on purpose, crushing the soaked grain first and then stirring. What is listed above as a cause of thinning is the method here.
• Starch that arrives already cooked, inside its cells. The sweet potato in Mee Rebus disperses as soft cell fragments rather than forming a network in the liquid. So it does not thin on reheating, does not slacken when the tamarind goes in, and cannot be over-stirred — at the price that body and sweetness cannot be set separately.
• Ground seeds breaking down. The walnuts in Fesenjan collapse over a long simmer and their oil and fine particles emulsify into the liquid. How finely you grind is the thickness of the dish, which makes grinding a step rather than a preference.
• Protein coagulating into curds. Egusi is close to 30% protein, so in Egusi Soup it sets instead of dissolving. That is why nothing is stirred for ten minutes after it goes in: stir in that window and the curds disperse into porridge and will not re-form.
• Egg that thickens without setting. The liaison in Blanquette de Veau is yolk and cream let into the broth and never taken past 70°C (cooking eggs). Set beside Carbonara and Chilli Crab, which use the same egg for an emulsion and for ribbons, egg in a hot liquid does three separate jobs.
• A gel set around a solid. Lo Bak Go gelatinises rice flour using only the water the daikon gives up, so the shreds are suspended in the gel rather than dissolved into it — which is what lets it be sliced cold and pan-fried.
• A soluble polysaccharide the ingredient weeps. The okra in Calulu gives the liquid body with neither starch nor collagen but mucilage. What separates it from everything above is that the knife sets the strength, not the quantity — whole pods thicken it smoothly, cut pods weep from the cut face and turn it ropy. One substance, and the cut surface decides between the finished dish and the failure. Nor does the class always do thickening: the fenugreek mucilage in Dosa is the same kind of thing holding air in a foam rather than body in a liquid.

When thickness is a question of gravity, not mouthfeel

Everything above is about how it feels in the mouth. One case wants something else entirely.

Churchkhela dips walnuts threaded on a string into grape must five to eight times. What the thickness has to do there is cling to something vertical and dry on it, and plain must is sweet water that runs off. The choice of thickener follows different rules too: must is acidic, so "acid cuts the starch chains" applies directly, and it uses flour rather than starch — the most opaque and the most robust — at the price of twelve minutes' cooking to lose the raw taste.

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.