Bring a saucepan of water to the boil. Add the citrus zests and blanch for 1 minute, then drain. Repeat this process twice, changing the water each time.
Put the caster sugar and 100ml water into a saucepan over a low heat and stir to dissolve the sugar. Increase the heat, bring to the boil and let bubble for a minute or two. Add the citrus zests and simmer for