Sweet mashed beans fill many Japanese confections from mochi to pan (sweet bread). The most popular is an or anko which is made from small red azuki beans while another common type is shiro an which is made from either lima or navy beans. An is available in small cans but it is so easy to make at home and far more economical.
The beans soak and cook in the same method used to cook any other dry bean, except that the cooking time is a little longer to achieve a very soft, mashable texture. Then, the beans are drained, mashed with sugar and cooked until the sugar is dissolved. It is that simple.
To clarify some terminology: Tsubuan is whole red azuki beans boiled with sugar. Tsubushi-an is azuki beans cooked and mashed with sugar. Koshi-an is either azuki or other type of cooked beans mashed with sugar and then sieved to remove the bean skins and achieve a smooth consistency. Different recipes call for different types of an. But now you know how to tell them apart and here is the recipe so you can make all of them yourself.
Tsubu-an or Anko
1 (12 oz.) package dried azuki beans (small red beans)*
water to cover
1-1/2 to 2 cups white sugar**
Wash the azuki beans, picking out any small stones or withered beans. Place them in a heavy bottomed pot and cover with clean water. Bring them to a boil over medium-high heat. Cover and turn off the heat. Let this soak for at least one hour.
Drain off the soaking water. Fill the pot with fresh water. Bring this to a boil once again. Skim off any foam might accumulate (the white beans tend to foam). Lower the heat and simmer until the beans are soft enough to mash, about one hour. Drain off water.
Add sugar and mash the beans. Adjust amount of sugar to taste. I think that 1-1/2 cups is more than sweet enough but some like it sweeter; others less so. Continue mashing until you achieve the desired consistency for your recipe. Stir over low heat until the sugar is completely dissolved into the mashed beans and the liquid is evaporated. Some recipes require the an to be thick enough to roll into a ball while others require a softer, spreadable consistency. It will thicken slightly as it cools.
*Sometimes, they are spelled adzuki beans.
** You can adjust the amount of sugar according to your personal taste.
Shiro-an or Sweet White Bean Paste Recipe
Follow the same recipe as above except use lima or navy beans.
Once the cooked beans have been mashed with sugar, press the mixture through a sieve or a food mill to remove the bean skins.