Japanese Kōjizuke: Koji-Marinated Preparations and the Sweet Fermentation

Japan — koji marination traditions documented from the Heian period; shio-koji as a home-cooking technique from the Edo period; contemporary revival of shio-koji as a professional kitchen tool from the early 2000s

Kōjizuke (麹漬け, 'koji marinated') refers to a family of preparations in which ingredients — fish, meat, vegetables, tofu — are marinated in or buried in a medium of rice koji (Aspergillus oryzae grown on rice), sometimes combined with salt, mirin, or sake, and allowed to undergo enzymatic transformation over periods ranging from one day to several months. The mechanism is the koji's secretion of proteases, lipases, and amylases: these enzymes break down the surface proteins and fats of the marinated ingredient, creating deeper flavour penetration, tenderising muscle fibres, producing amino acids (contributing umami depth), and converting starches to sugars (contributing sweetness). The most celebrated specific kōjizuke preparations include: sakekasu-zuke (marinated in sake lees — a related but distinct preparation); shio-koji (salt-koji marinade — rice koji combined with salt and water, used for light overnight marination of fish, chicken, and vegetables); misozuke (koji-formed miso as marination medium for fish, eggplant, and tofu); and the century-old tradition of Kyoto-style funazushi marination that uses koji alongside salt for specific preparations. Shio-koji in particular has experienced a significant revival in both Japanese home cooking and international professional kitchens, valued for its ability to transform a simple salt marinade into an enzymatic tenderiser and flavour builder over an overnight period.

Koji-marinated preparations have a distinctive depth — sweetness from amylase sugar conversion, umami from amino acid production, and tenderised texture; the flavour is not koji-flavoured but ingredient-enhanced through the enzymatic process