Soak adzuki beans in plenty of water overnight until they double in size (they’re still firm to the touch). Drain off the water.
*Optionally, freeze rehydrated beans to shorten the cooking time in the next step (the water inside the beans will expand while freezing forcing the beans to break).
Cook
Put the rehydrated beans and orange zest into a pot/saucepan. Add water (5 times the volume of the dried beans).
Bring to a full boil then turn the heat to the lowest. Cover with a lid and leave to simmer for 1½ hours (for 1 hour if the beans have been frozen) until the beans can be easily mashed with fingertips.
*Alternatively, use an instant pot (or a stove-top pressure cooker) to speed up the process. Cook for 25 mins on high pressure. Then leave to release the pressure naturally.
Blend
Use an immersion blender or a regular blender to emulsify the beans and the cooking liquid. Stop when most of the beans turn into a runny paste while the rest remains lumpy.
*Optional step
Add tapioca pearls to the soup. Pour in 1 more cup (240ml) of hot water as the pearls will absorb quite a bit of liquid while cooking.
Leave to simmer over the lowest heat for about 12 minutes until the pearls become transparent. During this process, remember to stir from time to time and add more water if necessary to prevent burning at the bottom of the pot.
Sweeten
Gradually add brown sugar, along with a pinch of salt, to the soup. Taste to adjust the sweetness. Serve immediately, or leave to cool and enjoy at room temperature.
Store
Keep leftover red bean soup in an airtight container. Store in the fridge for up to 4 days or 3 months in the freezer.
Reheat
Reheat cold/defrosted red bean soup over low heat until piping hot. You may need to add a little water to thin it if it appears to be very thick.
Notes
1. You can use canned, ready-to-eat adzuki beans. In this case, heat the beans, along with the liquid in the can, to a boiling point. Then blend and follow other steps to finish (add some water if necessary).2. If using dried tangerine peels. soak them in water until they become soft. Then mince them finely.3. To sweeten the soup, here are alternative choices: white sugar, light brown sugar, rock sugar, honey, syrup, sweetened condensed milk, etc. Taste and adjust the quantity.