Zenzai — sweet red-bean soup with toasted mochi

i. Origin & history

Sweet bean soups go back at least to the Muromachi period (1336–1573), when azuki beans and mochi were already linked with Buddhist and Shinto ritual. The name zenzai is written 善哉, a Buddhist expression meaning 'excellent!' or 'well done', used to praise a disciple's answer. One popular story says the monk Ikkyū exclaimed it on first tasting the soup. Another theory traces it to Izumo in western Japan, where a soup of beans and mochi called jinzai mochi was offered to the gods gathered there in the tenth month, and the name shifted over time to zenzai. Both are often repeated; neither can be confirmed.

The naming varies by region, and it is a source of friendly debate. In the Kansai region around Osaka and Kyoto, zenzai means a soup with whole, chunky beans (tsubuan), and oshiruko (or shiruko) means a smoother soup made from sieved paste (koshian). In the Kantō region around Tokyo, both soups are usually called oshiruko, and zenzai means something different: thick, broth-less sweet bean paste spooned over mochi. In Okinawa, zenzai is a summer dessert of sweet beans under a mound of shaved ice.

Zenzai is part of the New Year cycle. On kagami-biraki, traditionally 11 January, the hard kagami mochi displayed as a New Year offering is broken up with a mallet — never cut with a knife, which is considered unlucky — and the pieces are cooked in zenzai or oshiruko and shared. The red of the beans is associated with warding off misfortune, making it a fitting start to the year.

It is also everyday comfort food: a standard in Japanese sweet cafés (kanmidokoro) in winter, sold in cans from vending machines, and made at home with leftover New Year mochi.

ii. How to keep the beans whole and the soup glossy

Good zenzai has beans that are completely tender but still whole, in a thick, glossy, deep-red liquid. Three points make the difference.

First, the beans are blanched. They are brought to the boil, cooked for a few minutes and drained, and the water discarded. This shibukiri step removes some of the bitter, astringent compounds in azuki skins and gives a cleaner, sweeter flavour. Some cooks do it twice.

Second, the beans are cooked gently until fully soft before any sugar is added. Sugar draws water out of the beans and strengthens their cell walls, so beans that are sweetened too early stay hard and chalky. A gentle simmer, rather than a rolling boil, keeps the skins from splitting and the beans from breaking into mush. Japanese cooks sometimes add cold water partway through (bikkuri mizu, 'surprise water') to even out cooking.

Third, sugar is added in stages, letting the beans absorb it gradually, and a pinch of salt at the end. The salt is essential: it sharpens the sweetness and brings out the beans' flavour. Simmering briefly after the sugar goes in thickens the liquid to a light syrup; the beans' own starch gives the soup body.

The mochi is cooked separately. Grilled until it puffs and browns, it adds a toasted flavour and a crisp edge that softens in the soup. Boiled mochi is softer and smoother, and both are traditional.

iii. Ingredients

Makes 4 servings · scroll the side panel to adjust

• 200 g dried azuki beans
• 1200 ml water, plus more as needed
• 160 g granulated sugar
• ¼ tsp salt
• 4 pieces kirimochi (block mochi), or 8 shiratama dumplings
• 4 pieces shio kombu or pickles, to serve (optional)

Azuki beans should be from a recent harvest if possible; older beans take far longer to soften. Soaking is not traditional for azuki — their skins are thin enough to cook without it — but an overnight soak shortens cooking time. Granulated sugar gives a clean flavour; some cooks use part brown sugar or Okinawan kokutō for a darker, richer soup. Kirimochi, the hard, shelf-stable blocks sold in Japanese grocers, are the standard; homemade mochi or small boiled shiratama dumplings work too. A few pieces of salty kombu or pickles are traditionally served on the side to refresh the palate between spoonfuls.

iv. Method

1. Rinse the beans. Put them in a pan with 600 ml of the water, bring to the boil and boil for 3 minutes. Drain and discard the water.
2. Return the beans to the pan with the remaining 600 ml water. Bring to the boil, then lower the heat and simmer very gently, partly covered, for 60–90 minutes, adding water as needed to keep the beans covered by about 2 cm.
3. Test a bean: it should crush easily between thumb and finger with no chalky centre. If not, keep simmering.
4. Add the sugar in three additions, 5 minutes apart, stirring gently so the beans stay whole. Simmer for 10 minutes more, until the liquid has thickened to a light syrup. Add the salt and adjust with a little hot water if too thick.
5. Meanwhile, toast the mochi under a hot grill, in a toaster oven or in a dry non-stick pan over medium heat for 6–8 minutes, turning, until puffed and golden with crisp spots.
6. Put a piece of mochi in each bowl and ladle the hot beans and syrup over it. Serve at once with green tea and pickles on the side.

v. Tips & common mistakes

• Discard the first boiling water. It removes much of the bitterness from the skins.
• Simmer, do not boil. A hard boil splits the skins and breaks the beans into mush.
• Add sugar only when the beans are fully soft. Beans sweetened too early stay hard.
• Add the sugar in stages. Gradual sweetening lets the beans absorb the sugar evenly and keeps them plump.
• Always add salt. A small pinch transforms the flavour from flat to rounded.
• Watch the mochi under the grill. It can puff and burst suddenly; turn it often.

vi. Variations

Oshiruko made with smooth koshian is the most common variation; thin ready-made paste with water to make it quickly. Kantō zenzai is thick bean paste served over mochi without broth. Okinawan zenzai serves sweet beans, often kintoki beans, cold under shaved ice with shiratama — see kakigori for the shaved ice. Other variations add chestnuts, sweet potatoes or shiratama dumplings instead of mochi.

Sweet bean soups are common across East Asia: Chinese red-bean soup (hong dou tang), often with tangyuan, and Korean patjuk, red-bean porridge with rice balls. The same beans set with agar make yōkan, and they fill daifuku and monaka.

vii. Serving & storage

Serve zenzai hot, in deep bowls with chopsticks and a spoon, as an afternoon snack or dessert in cold weather, and on kagami-biraki in January. Green tea and a little salty kombu or pickles are the classic accompaniments. In summer, serve the beans chilled over shaved ice in the Okinawan way.

The sweet beans keep for 4 days in the fridge and freeze well for up to 2 months; reheat with a little water. Cook the mochi fresh for each serving — mochi left in the soup turns very soft and starts to dissolve.

ix. More from Japanese

• Mochi — the pounded rice cake
• Daifuku — mochi with bean filling
• Yokan — firm red-bean jelly
• Anmitsu — agar jelly with bean paste
• Kakigori — Japanese shaved ice
• Tangyuan — Chinese rice balls in syrup