Kudzu root, known in Japan as kuzu, is the massive starchy taproot of the vigorous climbing legume Pueraria montana, and the source of one of the most prized thickeners and setting agents in Japanese and Korean cooking. The starch is washed from the root in a long, water-intensive process and sold as hard, chalk-white, irregular lumps that crumble to a fine powder. Dissolved into liquid and heated, it sets to a smooth, clear, silky gel with a clean mouthfeel and a faint natural sweetness — the basis of translucent kuzu-kiri noodles and quivering kuzu-mochi — and as a thickener it gives sauces and soups a glossy, delicately clinging body that arrowroot and tapioca cannot quite match.