Plan to soak the beans in water for 1 night in the summer and for 2 nights in the winter, until the beans lose their wrinkles and become smooth. Change the water twice a day.
Put the beans in a pot, bring to a boil, and drain.
Peel off the thin outer skins.
The beans are all peeled.
Put the peeled beans back in the pot with water to cover, and simmer until tender with a small lid that fits right on top of the pot contents (drop lid or otoshibuta). It took about 40 minutes.
Put a sieve over a bowl filled with water, put the cooked beans and press through the sieve using the back of a ladle.
Pass the bean paste through a fine mesh sieve again into a bowl filled with water.
Strain the water from the bean paste using a clean cotton cloth (or cheesecloth).
This is nama-an (unsweetened fresh bean paste).
Weigh the nama-an, and measure 60% of its weight in granulated sugar. (I had 656 g of nama-an, and 393 g of granulated sugar.)
Put the nama-an and the granulated sugar in the pot and mix together over low heat.
When it's well blended, add remaining granulated sugar and keep on mixing and kneading with a wooden spatula over low heat. It will be very hot, so be careful not to burn yourself!
It will be creamy and loose to start, but keep on simmering and stirring until it reaches your desired consistency. It'll change texture in about 20 minutes.
The photo above shows "sticky" bean paste. It's the right consistency for firm foundation like a mochi ball or on a dorayaki pancake.
The photo above is a "moist" bean paste, or about midway. It is nice and shiny, and can be used for a lot of things, such as filling mochi dumplings.
The photo above is a 'floury' bean paste. It's quite stiff and potato-like in consistency. It's used for things like kashiwa-mochi (oak leaf-wrapped mochi cakes eaten in May) and sakura-mochi (cherry leaf-wrapped mochi cakes eaten in March to April). If you dry it out even further, it becomes the right consistency to use for nerikiri (a dough made with shiro-an and mochi).
Story Behind this Recipe
There was no recipe for making proper shiro koshi-an without cutting corners on the COOKPAD website yet!
If the beans poke out of the water a bit in step 3 they'll be OK, since you're just boiling them so that they peel easier. You can't make tsubu-an (chunky bean paste) with this method. Be sure to strain off the water properly in step 9. Peeling the beans makes them easier to strain and makes the paste whiter.