Put all ingredients in a 1-litre preserved food jar, cover tightly with microwave plastic wrap, microwave at High (600-700 Watts) for 5 minutes. Remove and stir until all castor sugar is dissolved. Keep for 1 week, remove the lemon peels and all the coffee beans sink to the bottom of the jar. Serve with crushed ice. Actually the coffee liquor is kept for one year in a dark, cool place to produce the same result as by the microwave oven.