Pickled with sake lees (kasu). 粕漬け。
Kasu are sake lees, the residue of rice mixed with yeast, koji and sake, which is pressed out of the fermented sake to obtain a clear and transparent liquid. On average, a tank of sake contains about 30% of lees, more in the case of ginjo sake. These lees are of course not thrown away, but they play an important role in the Japanese kitchen. They still contain about 8% alcohol and the faint but agreeable fragrance of sake. Sakekasu is used for cooking, for example for marinating fish, or to make soup from. And they are used for pickling.
When used for pickling, sake lees are mixed with salt and some sugar to make a pickling bed. This method of pickling takes a long time.
Sakekasu as a method of preserving vegetables has already been recorded in a 10th century source.
Best example: narazuke, the representative pickle of Nara City, mostly made with pickling melon (shirouri).
A sub-type of kasuzuke is karashizuke where karashi, Japanese mustard, is mixed through the kasu bed. Best example: karashi-nasu, using eggplant.
[Narazuke. PhotoAd Blankestijn]