Shiruko is a sweet soup made from adzuki beans boiled in a mixture of water and sugar, with grilled pieces of mochi rice cake floating in the bowl. The soup is served hot, and is one of Japan's best-known traditional sweet dishes. It is eaten as a snack, generally during some hungry moment between lunch and supper.

There are two variations—the beans can be left whole, or they can be strained when soft, to mash them and remove the skins. In western Japan, the former variety is often called zenzai. Shiruko is eaten throughout the year, but because it is served piping hot, it is considered a special treat for the cold winter months.

Adzuki beans were introduced to Japan from China. Archaeologists have discovered traces of them in sites in Japan dating back about 2,000 years, so we can assume the Japanese were growing them at least that long ago. An old document written in the 6th or 7th century, called Keiso Saijiki, tells us that the Chinese ate adzuki beans on the day of the winter solstice, the shortest day of the year, to ward off evil. Soon after this, the custom spread to Japan, and before long it was common to eat this type of bean on special occasions at other times of the year as well, such as annual events and ceremonies.

It was not until much later that it became common to use adzuki beans in recipes for some sweet foods. This was after Japanese farmers began producing sugar in large quantities, starting around the end of the 1700s. In a book on customs called Morisada Manko, completed in 1852, we read, "In Edo [present-day Tokyo] the skins of adzuki beans are removed, and the beans are boiled with white or unrefined sugar and a cut slice of mochi rice cake. The soup is called shiruko. In the Kyoto and Osaka areas, soup with the bean skins removed is called shiruko or koshian-no-zenzai." This indicates that shiruko was being eaten all over Japan at least as far back as the mid-1800s, and that even at that time there were different variations with different names.

Adzuki beans and sugar are readily available and fairly cheap, and this may explain why shiruko became a popular, traditional homemade snack. But making it is somewhat complicated and time-consuming, so since around the mid-1800s it has also been sold as a ready-to-eat soup to take home. In the past, people were sure to find it served in tearooms called kanmi-dokoro, which were like the coffee shops of today. And peddlers used to carry it in wooden boxes on their backs, doing a brisk trade in the evening.

We see, then, that shiruko has been popular for a long time. These days, it is even sold in a number of instant varieties—with some, you just put it in a bowl and pour on boiling water; with others you open the container and heat up the soup.

If you cook shiruko you have to be willing to follow a number of time-consuming steps. Adzuki beans contain a bitter substance called tannin, and if you do not remove it you will find the soup rather unpleasant. After the beans have boiled long enough, drain and rinse them well. Then place them in a fresh batch of water and remove the scum that floats to the top with bubbles. Next, boil them again and remove any more scum. This takes time, but the result is a delicious soup.

Before serving, we add some grilled mochi rice cake. The fragrance of the grilled mochi goes well with the taste of the beans, and the mochi gives a soft yet solid texture to the liquid soup, making for an interesting culinary experience. Adding mochi gives substance to the soup, making it quite filling—an ideal snack! Although the preparation takes time, I highly recommend this traditional dish.