Ya cai (芽菜, literally 'sprout vegetable') is the salted, slightly fermented preserved mustard green that is one of the most distinctive condiments and cooking ingredients of Sichuan cuisine. Made from the stem of a specific variety of Chinese mustard plant (zha cai's cousin), ya cai is finely shredded, salted, sun-dried, and then packed in sealed clay pots for fermentation — the best Yibin-style ya cai is aged for at least 3 years before sale. It has a salty, slightly sour, deeply savory flavor with a pleasantly crisp texture despite its preserved state. Ya cai is the essential garnish for dan dan noodles and is also used in dry-fried green beans (gan bian si ji dou) and stuffed meat preparations.
Yibin ya cai (宜宾芽菜) is the gold standard — Yibin city in southern Sichuan is the traditional production centre. The best ya cai has been aged 3-5 years and has a complex, almost wine-like fermented depth beyond the simply salty. In dan dan noodles: Ya cai is stir-fried briefly with minced pork (the meat sauce component of dan dan) — the moisture in the ya cai helps to prevent the pork from drying out during frying. As a condiment: Sprinkled directly onto congee, rice porridge, or plain steamed rice, where its saltiness and crunch provide contrast. Storage: Ya cai keeps virtually indefinitely if kept sealed. Once opened, store in a sealed glass jar in a cool place.
Substituting zha cai (榨菜, Sichuan preserved mustard root) for ya cai: They are related but different products. Zha cai is chunkier and more robustly spiced; ya cai is finely shredded and milder. They are not interchangeable in dan dan noodles.
Fuchsia Dunlop, Land of Plenty (2001); Fuchsia Dunlop, The Food of Sichuan (2019)