Thickening With a Slurry.
A Chinese cook pours a thin stream of starch water into the wok, and three seconds later the sauce clings to every piece. Thirty seconds too early, and it would be thin again.
勾芡 is the finishing step of countless Chinese dishes: starch (cornstarch, potato starch 生粉, or sweet-potato starch) stirred into cold water, then poured in a thin stream into a boiling sauce while the wok is tossed. The starch must reach about 90–95 °C to gelatinise fully (VII.1), which in a boiling sauce takes seconds; the swollen granules then crowd the liquid into a translucent, glossy coating that clings to food and carries flavour onto it. Pure starches thicken more cleanly than flour (VII.2): no cereal taste, no cloudiness, and two or three times the power, gram for gram.
It goes in last for two reasons. Starch thickening is fragile: swollen granules break apart with continued heating and stirring, and the sauce thins again — potato and tapioca starch, the clearest and strongest, break down fastest; cornstarch holds longer. And starch added early catches on the wok and makes the vegetables slimy. Cooks name the consistency: 玻璃芡, a thin glaze; 流芡, a sauce that flows; 包芡, one that wraps each piece. The workshop thickens and then keeps boiling.
Below: the thickness of a sauce from the starch you choose per 250 mL (course starch-thickening index of VII.2), and how much it keeps as it goes on boiling and being stirred for the minutes you choose (teaching breakdown rates), against the bands for a glaze, a flowing sauce and a coating sauce.
- Make and add a starch slurry.
- Choose a starch for clarity and stability.
- Name and hit the consistencies of 芡.
- Explain why the slurry goes in last.
Stir it in at the boil, at the end
On the course index, about 5.8 g of potato starch or 7.5 g of cornstarch per 250 mL makes a sauce that coats a spoon — roughly half the flour a roux needs (XXII.5). Keep boiling and stirring for five minutes after adding it, and potato starch keeps only about 0.67 of its thickness while cornstarch keeps 0.90. Hence the rule: add the slurry last, to a boiling sauce, toss for a few seconds until glossy, and serve.
Method: mix 1 part starch with 2–3 parts cold water (never hot, which makes lumps) and stir again just before using, because starch settles fast; have the sauce at a boil; pour the slurry in a thin stream around the wok while stirring or tossing; stop as soon as the consistency is right — you can add more, never take it out. For a 羹 or 酸辣汤, stir the soup in one direction while pouring. Acid and long holding both thin starch sauces (VII.2), so sweet-and-sour sauces are thickened at the last moment, and leftovers often need a little fresh slurry when reheated.
Thicken two cups of boiling stock, one with cornstarch slurry and one with potato starch, to the same coating consistency. Keep both boiling and stirring for five minutes and compare.
Terms
- 勾芡
- Thickening a sauce at the end with a starch slurry.
- Slurry
- Starch stirred into cold water.
- 流芡 / 包芡
- A flowing sauce / a sauce that wraps each piece.
- Breakdown
- Swollen starch granules breaking apart and thinning a sauce.
Sources
- Starch gelatinisation ranges measured by DSC: wheat about 52–64 °C, potato 58–66 °C, maize 62–72 °C, rice 68–78 °C, tapioca 59–70 °C (S. Damodaran and K. L. Parkin (eds.), Fennema's Food Chemistry, 5th ed., 2017, ch. 3). The course uses the midpoints.
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
- H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.