I live in Japan and I've started on Japanese cooking. I'm one of those people who likes to know why certain things are done and those answers aren't in any of the cooking blogs or books I've come across. For example, why cook black beans for eight or more hours in sugar?
Around new years there's a very popular dish of sweetened black beans. I know the goal is to cook them so the skins don't burst, but up to half a day? What's more, most recipes add some of the sugar at the beginning. Were it rice, it would never cook. Does putting the sugar in the beans from the beginning slow the cooking? Does a prolonged cooking partially candy the beans? I'd really like to know, because I made a dish with Azuki beans the other day (zen-zai) and to keep the skins from bursting I cooked the soaked beans for over two hours before adding the sugar and some of the beans did burst.
For reference, the recipes I followed flowed like this:
1) Soak the beans over night, change the water.
2) Bring to a boil with a bit of salt or soy sauce and some amount of sugar.
3) Reduce the heat to barely a simmer with a piece of parchment over the surface and let it boil until they're just tender -- at least eight hours.
4) Add 1/3 the amount of the remaining sugar, cook for another half an hour. Repeat. Repeat.
5) Add a bit more soy sauce. Done.
