Combine the rest of the ingredients in a sauce pan, bring to a boil, then turn off the heat. Baking soda is added to soften the beans, but it is optional.
Put the kuromame into the sauce pan from Step 2, cover with a lid, and let sit overnight.
In the morning, heat over medium. Carefully skim the scum that rises to the top while it simmers. Set on the lowest heat right before it comes to a boil.
Set in a drop lid and keep on low heat to prevent the beans from moving around.
Cover the sauce pan with a lid, then simmer for about 2 hours. Check the beans from time to time and if the heat seems too high, slide the lid to create a gap.
With an IH induction cooker, I set the heat between 1 and 2, partially cover the pan with a lid, and it takes 2 and 1/2 hours to cook. I use large beans (Tanba kuromame variety).
Check the beans from time to time while they simmer. If the liquid boils down too much, add a little more water.
When they get to the right degree of tenderness, turn off the heat and let sit overnight in the liquid. The beans will wrinkle if they are not fully covered by the liquid, so make sure they are fully immersed as they cool.
Here they are, nice and plump.
The sweetness is quite mild. For those who prefer a sweeter taste, use 200 g sugar, 30 ml soy sauce, and 1/3 teaspoon salt.
Try these cupcakes made with sweetened kuromame. "Fluffy, Melt-in-your-Mouth Kuromame Cupcakes" (Recipe ID: 2376449).
Story Behind this Recipe
Everyone in my family loves these, but the store-bought ones are too sweet; so I created this recipe with just the right sweetness.
There are two key points to making these stewed beans nice and plump! 1 - Simmer on the lowest heat possible...if the heat is too strong, the skin of the beans will come off. 2 - Be sure to cool them in the stewed liquid. If the beans are not immersed, they will wrinkle. They should also be fully immersed while simmering.
The simmering time may vary depending on the size of the beans (between 1 and 1/2 hours to 2 and 1/2 hours).