Homemade blackcurrant jam Posted on August 28, 2019 by Old Nol Blackcurrants (Ribes nigrum) can be harvested from late June to early July Simmer 300g of blackcurrants with 250ml of water for about 20 minutes. Add 300g of sugar and the juice of one lemon. Heat vigorously until temperature reaches 105ºC. Pour the jam into warm sterilised jars close the lid and allow to cool. Tastes excellent on hot buttered toast with a nice cup of tea.