If you went looking for ya cai preserved mustard for dan dan noodles, you may have instead picked up zha cai mustard tuber. It’s confusing, the names, the Chinese characters, the lack of consistency at Chinese markets. I had a hard time for years figuring out what was what and where to find them. Both are Sichuan seasonings – pickled and preserved vegetables that add big flavor boosts to food. They can be used in the cooking process or as a garnish.
Zha cai is pronounced “jah tsai” or “zsa tsai” like cross between actress Zsa Zsa Gabor and chef Ming Tsai. It’s crisp and firm in texture, with a very salty flavor. Zha cai is made out of the stem of Brassica juncea, subspecies tatsai. It's bulbous and awkward looking -- similar to super knobby knees. The stem is salted, dried, seasoned with chile and fermented. That's why it resembles a relic.
On the other hand, ya cai has a finer texture and less of a bite. If zha cai were Laverne, ya cai would be Shirley. If that show is too old for you, think of the two as the Thelma and Louise of Sichuan kitchens.