Red bean soup (紅豆湯, hongdoutang) is a popular dessert throughout China, Taiwan, and Hong Kong. It’s made from red beans (紅豆, hongdou, also called adzuki beans) which are incorporated into many East Asian desserts. Red bean soup itself is versatile. It can be eaten hot or cold, plain or with toppings, liquid or frozen. When my mom first dictated her version to me, she cautioned, “糖最後加，不然紅豆煮不爛！Don’t add the sugar until the end, else the beans will never soften!” Today, I’m sharing my mom’s recipe, adapted for a typical Western kitchen and pantry, then discussing the science behind her words.
Red Bean Dessert Soup (紅豆湯)
Makes 6–8 servings
- 1 cup (226g) dried red beans, rinsed
- about 7–8 cups (1600–1900g) water
- about 1/4 cup (50g) granulated sugar, rock sugar (冰糖), or preferred sweetener to taste
- Place the beans in a pot and fill the pot with about 5-1/2–6 cups (1300–1400g) water. Soak the beans overnight.
- The next morning, transfer the pot to the stove top. Bring the water to a boil over high heat, then lower the heat and simmer until the beans split open, about 40 minutes. (Mom uses the steamer for this step.)
- If desired, crush the beans a couple times with the back of a ladle. Add about 2 cups (500g) water or enough to reach desired consistency.
- Add sugar or preferred sweetener to taste. I usually add about 45g granulated sugar. As the soup cools, it will taste even sweeter. Stir until the sugar is dissolved and remove from heat.
- To store, cool to room temperature, cover, and refrigerate. To reheat, bring soup to a simmer over medium-low heat. It also microwaves well.
- Enjoy hot, cold, or frozen into popsicles
- Add some milk for a touch of creaminess
- Eat as is, or add toppings like tapioca (珍珠), glutinous rice balls (湯圓), or grass jelly (仙草). Find these (and the red beans) at your local Asian grocery store!
- Use it to cook some rice for a sweet porridge
- I like to cook oatmeal in the soup, then add a dash of milk and a squeeze of honey for breakfast
Red bean science
So what was behind my mom’s advice? Why must the sugar be added at the end? To find out, I did exactly what my mother said not to: I added the sugar at the beginning. I cooked three batches of red bean soup, each with different amounts of sugar, and compared the results. If you read the section on fruits in the post “Sugar: Special Applications,” you might guess that the beans cooked with more sugar retained their structure and texture. But let’s follow these beans from the beginning.
Soaking the beans
In the first step of the recipe, we soak the dried beans in water. Because the beans contain no water to begin with, the water diffuses into the beans. As you can see, the beans became much larger, and the dark red color paled. Since they gained water, the beans got heavier, too. I added 226 grams of dried beans to the pot, but the next day, the rehydrated beans weighed 430 grams. They’d absorbed 90% of their weight in water! The soaked beans were crunchy and juicy. They were white inside, and they tasted raw.
Cooking the beans
Here, I diverged from the recipe. I cooked one batch of beans as written (control), adding 45 grams of sugar after the beans split. To a second batch of beans (low sugar), I added the 45 grams of sugar directly to the pot of soaked beans before cooking. And to a third batch (high sugar), I added 226 grams of sugar. All three batches were cooked for the same amount of time, 42 minutes.
As you can see below, after 42 minutes, the liquid in the control batch was red and murky, but in the high sugar batch, it was still quite clear. The low sugar batch was somewhere in between.
I also scooped out some beans to compare. While the control and low sugar beans had split open, the high sugar beans remained whole. I crushed the beans with the back of a spoon and found that, while the control and low sugar beans had taken on a grayish color, the high sugar ones were still white inside. They were also harder to break open. All in all, the high sugar beans just looked like they hadn’t been cooked as long.
This was reflected in the texture of the beans as well. The control and low sugar beans dissolved in my mouth, but the high sugar beans were hard and chalky. They’d lost their raw taste, but they were not cooked enough for red bean soup. I returned these beans to the stove for another 45 minutes, and although they continued to soften, they did not split. I gave up and took them off the heat. My mom was right: the sugar in the cooking water prevented these beans from softening.
Sugar preserves structure and texture.
As we’ve discussed previously, when we cook fruits or vegetables in a sugar syrup, sugar diffuses into the fruit or vegetable. Inside, the sugar preserves the structure and texture of the produce.
All plants, including the red beans in this soup, are made of cell walls. The cell walls provide the scaffolding that holds the bean together, and they’re glued together with pectin. If the pectin dissolves, the scaffolding falls apart and the bean loses both its firm structure and its crunch. One way to prevent pectin from dissolving is to add sugar. Sugar acts like a waterproof finish on the scaffolding. It holds onto water molecules and prevents them from dissolving the pectin.
In the control batch of beans, there was no sugar in the cooking water. As the beans were heated, the pectin dissolved and the cells began to fall apart. Ultimately, the bean split open and released some cells into the cooking liquid, making a red, opaque soup. The cells also separated easily in my mouth, so the beans seemed to melt away when I ate them.
In the high sugar batch, the cooking water contained a lot of sugar. As the beans cooked, the sugar diffused into the beans, where it protected the pectin glue and prevented it from dissolving. As a result, it was harder to dismantle the cell wall scaffolding, and the beans remained firm and whole. After 42 minutes of cooking, the cells still hadn’t quite separated, so the liquid stayed clear and the beans remained hard. When I chewed the beans, clumps of cells sat on my tongue without breaking apart, contributing to a chalky texture.
In the low sugar batch, the concentration of sugar was low, and I didn’t detect much of a difference between it and the control batch. However, I found that I had to add more sugar to the low sugar soup after it had finished cooking, presumably because the beans had absorbed some of the sugar and left the soup a little bland. The beans didn’t taste any sweeter than the control ones, though.
Sugar preserves the structure and texture of red beans, making it harder to cook them to a mushy consistency. We see this in other dishes, too—baked beans are cooked with plenty of brown sugar and molasses, so they retain shape and texture even after hours in the oven, while refried beans are cooked in water and ready to mash after just one hour. Although low concentrations of sugar in the cooking water don’t make much of a difference in cooking time for red bean soup, it’s fastest to cook the beans in water and then sweeten to taste after they’ve softened. Red bean soup is a light, refreshing dessert that can fit any season. I hope you give it a try!
BeMiller, J. N. An Introduction to Pectins: Structures and Properties. American Chemical Society, 1986.
Corriher, S. O. Bakewise; Scribner: New York, 2008.
Corriher, S. Getting the Texture You Want When Cooking with Fresh Fruit. Fine Cooking, 2001, 46.
Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.
Freeman, S.; Quillin, K.; Allison, L. Biological Science, 5th ed.; Pearson: New York, 2014.