Dun (炖, literally to stew or simmer) is the Chinese long, slow braising technique for preparations that require extended cooking time to transform tough collagen-rich ingredients (pork trotters, oxtail, beef tendon, whole chicken) into tender, gelatinous, deeply flavoured preparations. Unlike hong shao (which involves caramelization and reduction), dun is gentler — a barely simmering liquid over a very long time, producing results that are tender rather than sticky-glazed. The dun preparations of Chinese cooking often use a sealed vessel (clay pot or sealed metal pot) to prevent moisture loss during the long cooking.
Temperatures and timing: Dun should be maintained at 85-90C — the water should be trembling with small bubbles but not boiling. A boiling dun toughens the protein fibres and produces a cloudy, greasy result. The clay pot's thermal mass helps maintain the low temperature by buffering heat fluctuations. Sealing technique: The traditional Chinese method for completely sealed braising (called mi feng dun, 密封炖) uses a flour-water paste to seal the lid of the clay pot to the pot — preventing any steam from escaping and producing a dish that cooks in its own condensation. This technique is used for the most elaborate dun preparations. Time ranges: Whole chicken — 1.5-2 hours. Pork trotters — 2.5-3 hours. Beef tendon — 4-5 hours. Pork belly — 1.5-2 hours.
Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009); Fuchsia Dunlop, Every Grain of Rice (2012)