Put the sugar and 500 ml (17 fl oz) water in a saucepan over low heat. While the sugar is dissolving, peel the quinces, add to the pan and cook for 3–6 hours until the quince is soft and turns dark red-pink. Add the iota carrageenan, bring the heat back up to 84°C (183°F) and cook until the fruit starts to break down.
Pour into a shallow tray, allow to cool slightly and then leave in the fridge overnight. Purée until smooth and store in the fridge.