Liu (熘) is the Chinese technique of pre-cooking a velveted protein in oil or water, then returning it to the wok with a pre-thickened sauce that coats and clings. The result is called slippery (hua 滑) because the cornstarch coating gives the protein a smooth, yielding texture quite different from the rough char of bao. Liu is central to many Cantonese restaurant preparations — sweet and sour pork, lemon chicken, and silk-textured prawn dishes all use the liu principle.
Liu dishes are about textural pleasure as much as flavour — the silken, yielding give of velveted protein in a sauce that coats without overwhelming.
The liu sequence: 1. Velvet the protein (egg white, cornstarch, sometimes baking soda). 2. Pre-cook: either oil blanch (guo you) at 120C for 30-60 seconds, or water blanch at 80C. 3. Drain. The protein is now 80% cooked. 4. Make the sauce separately: aromatics, liquids, and a cornstarch slurry that thickens to coating consistency. 5. Return protein to wok. Toss for 30 seconds — sauce coats every surface. Sauce consistency: The defining feature — a sauce that flows off a spoon in a thick, continuous stream and leaves a coating that remains visible for 10 seconds.
The oil blanching temperature is critical: 120C (not the 180C of deep-frying). Use a thermometer until you can identify this temperature by the gentle bubbling around the protein. Water blanching produces a lighter result and is preferred for delicate proteins like fish fillet.
Over-thickened sauce: Too much cornstarch produces a gelatinous, gummy coating. Cold protein returned to wok: The protein must be warm when it re-enters the sauce.
Irene Kuo, The Key to Chinese Cooking (1977); Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009)