Add all ingredients to a large thick-bottomed pot. Stir well and slowly bring to a boil. Using a wooden spoon (a metal one gets too hot), crush a portion of the fruit against the side of the pot, in order to release the pectin. This is necessary for the jam to set. Allow the jam to simmer for approx. 15 minutes. Scoop a spoonful of jam onto a saucer and briefly place in the refrigerator to test whether it sets. If not, continue to cook briefly. Using a skimmer, scoop the foam from the jam in the pan. In the meantime, boil the jam jars and lids in another pot for 10 minutes. Remove them from the water using tongs and drain on a clean towel. Scoop the jam into the jars, allow to set briefly then stir the mixture in each jar using a clean spoon in order to spread the fruit. Tighten the lids and store the jars upside-down to cool.