Sake kasu is the leftover solids from the production of sake, which is a traditional Japanese rice wine. Sake kasu is a paste-like substance that is made up of fermented rice, yeast, and koji (a type of fungus). It is rich in nutrients and has a creamy texture, often used in Japanese cuisine as a seasoning or marinade for fish, meat, and vegetables. It is also used in beauty products for its moisturizing properties.