Pour the sudachi juice, mineral water and sugar into a saucepan and bring to a simmer over high heat. When all the sugar has dissolved, whisk in the agar and sodium citrate and bring to the boil. Boil for 30 seconds then strain into a container. Chill over iced water in the refrigerator for 3–4 hours until set.
Break up the sudachi gel with a hand-held blender. Pour into a vita-prep (or