Simmer rhubarb, ginger, star anise and zest in water for 5 – 10 mins until the rhubarb collapses. Then allow to cool in the pan. Strain through a jelly bag or muslin, then add 600g sugar and 10g of citric acid per litre of strained liquid. Reheat gently to about 85°C to dissolve the sugar. Bottle hot to preserve. To use, dilute to taste.