Use the plums if they are yellow and ripe, but if they're still green, spread them out in an airy place and let them ripen for 3-4 days.
Place the plums in a bowl, fill it with plenty of water and soak for about 3 hours to remove any bitterness. If they plums are ripened, this step is unnecessary, but I generally prefer to do it for good measure.
Drain the water, and carefully wash the plums under running water. Next, use a toothpick (or a bamboo skewer) to remove stems and put them in a large pot.
Add plenty of water to the pot and heat over heat high. Once the small bubbles start to appear, reduce the heat to low.
Simmer the plums until they become soft (but with the skin still attached). This should take about 5 minutes.
Carefully drain in a colander, while preserving the boiled water for later use. Allow it to cool completely.
Remove the plum seeds from by mashing them with your fingers. Carefully remove the flesh around the seed as well. Mash the flesh with a masher. See Helpful Hint No. 1
Combine the plum and boiled water (100 ml) from Step 6, and turn on the heat. Simmer for 7-8 minutes while continuously stirring with a spatula. See Helpful Hint No. 2
Add the granulated sugar and honey, then simmer for another 7-8 minutes while continuing to stir. Once it becomes very silky and smooth, turn off the heat.
Store in a sterilized jar.
For a tasty plum juice, reheat the remaining boiled water and add as much sugar as you like and let it cool. Chill it in the fridge and enjoy!
Story Behind this Recipe
I was impressed when I had Nankou-bai Jam from the Japan agricultural cooperative in Kinan. I created this recipe so I could easily enjoy the jam at home.
1) Nanko-bai plums have soft flesh so they become softer as you cook them while mixing with a spatula. Use a food processor If you prefer a puréed texture. 2) I added this step to thoroughly cook the flesh, since it scalds easily when the sugar is added.