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To make the rhubarb gel, transfer any remaining rhubarb juice to a bowl and whisk in agar agar using a 100:1 ratio (so if you are left with 100ml of rhubarb juice, for example, you would use 1g of agar agar). Leave to stand for 10 minutes, then transfer to a pan and gently bring to a boil while whisking continuously. As soon as the liquid comes to the boil, remove from the heat and transfer to a container. Leave to set in the fridge