'This syrup can be refrigerated for about a month, but must be used within two weeks after a bottle is opened.' Add several tbsps to glass and fill with cold water or sparkling water and ice. Mash the fruits together and let them stand in a warm place for a day. Strain the juice into a nonreactive pan, and add 4 cups (1 lite) of sugar to each 2 1/2 cups (625 ml) of liquid; place over low heat or inside a pan partly filled with simmering water and stir to dissolve the sugar. Cool the syrup, then skim it, pour it into bottles and cork them tightly.