Drain the soaking water and place adzuki beans in a large pot. Cook it over high heat and bring it to boil.
Turn the heat down and simmer for about 15 minutes.
Remove the pot from the heat and drain the cooking water. Put the beans back into the pot and add clean water and repeat the same process two more times. *2
Remove the pot from heat and drain the cooking water completely using a strainer.
Put cooked beans back in the pot and add sugar.
Stir them all together with a wooden spatula. It will be a little runny and glossy when the sugar melts.
Keep kneading with the wooden spatula. It will become thicker as the liquid evaporates.
When the paste loses its glossiness, turn the heat off.
Scoop out the paste onto a stainless steel tray to cool.
Koshi-an (Smooth red bean paste)
First 5 steps are same.
Before adding the sugar, place the cooked beans in a food processor and blitz them.
Put them back into the pot and add sugar.
Keep stirring and kneading as it gets thicker with a wooden spatula.
When it loses the glossiness and becomes thicker, turn the heat off and cool on a stainless steel tray.
*1 This is for removing astringency off the beans and need to repeat this process three times.*2 To save time, you can use pressure cooker to cook beans at third time for about 15 minutes. *makes about 600g of Red bean paste (about 2 cups)