Prepare shiso leaves to boil. Wash and trim the shiso leaves.
Bring 2 cups of water to boil in a large sauce pan or pot and add the prepared shiso leaves.
Turn the heat down to medium heat and cook the shiso leaves for about a few minutes. The leaves colour will change from purple to a greenish colour.
Drain the shiso leaves with a strainer/sieve and squeeze the leaves to extract as much as you can. It is hot so be careful not to burn your fingers. I used a spoon to extract all the liquid.
Place the drained liquid back to the saucepan and bring it simmer.
Add the sugar, and when the sugar has dissolved, turn the heat off and add the apple cider vinegar.
Cool it down and keep it in a clean bottle in the fridge. If you sterilize the container in which you keep the syrup, it will last about 6 months in the fridge.
To make shiso juice, place 1/4 cup of the syrup into a glass and pour about 200 ml of mineral water or fizzy soda water over the top. Add ice cubes.
Notes
*1 You need a heaped colander full of shiso leaves which is about 18oz or 500g. It seems a lot but they shrink in hot water when they are wilted.*Nutrition fact is for whole juice. It will differ with how the juice will be diluted.