Interestingly, according to traditional Chinese medicine, red beans are considered 'warming' and mung beans 'cooling'. The weather turned really cold in the last few days so I made some red bean soup for some extra warmth. Red beans are also known as azuki beans in Japan and they're as common in Asian desserts as chocolate in western baking. They are rich in iron and Chinese medicine claims that they are good for replenishing the blood and getting rid of excess liquid in the body (利水消腫).
Red bean soups can be found all over China, but the Cantonese call their version 'red bean sand' (紅豆沙) rather than 'red bean soup' (紅豆湯). With normal red bean soup you simply boil the red beans with water until it becomes a a watery mixture, with the Cantonese version it is imperative that the starch inside the beans be released into the soup so that the soup is not watery, but has a hint of sandiness - 起沙. Cantonese people also like to add aged tangerine peel (陳皮), an obligatory addition to a genuine red bean soup for any 老廣東.
Here's how I go about it, inspired by the various methods I've read about:
1. Rinse the red beans and soak in a big bowl with plenty of water to cover. Chill in the fridge for at least 5 hours, preferably overnight.
2. Soak a small piece of tangerine peel with enough water to cover in a small bowl for at least 30 minutes. Use a paring knife to scrape off the white pith - the bitter part.
3. When ready to cook, drain the red beans and add them to a big pan with water and the tangerine peel. The bean to water ratio should be about 1:6. You can use a bowl for measuring.
4. Bring to a boil and cook on high heat, covered, for 15 minutes. Turn the heat down to medium low and cook for another hour and a half. During this time the beans get thoroughly hydrated and cooked.
5. Turn to a high heat for 5 minutes, uncovered. The agitation caused by high heat gives the starch a last chance to break through the skin. Season with brown sugar and a pinch of salt - remember it will taste sweeter when cold, so be cautious!
6. Cover with lid again and wait for at least 4 hours. During this time more starch will flow from the beans to the soup.
7. Serve chilled or hot. Unlike mung beans, red beans have a great affinity with dairy. Some people add a splash of evaporated milk or coconut milk to the soup before eating - the choice is yours.
I'm not sure if it's conventional, but I actually thicken the soup with some cornflour solution at the very end so that the starch binds homogeneously with the water. The result is a much more voluptuous consistency. Traditionally some people soak a few tablespoonfuls of rice and throw them in with the beans. The rice acts as a binder much in the same way as my cheating method.
Try and see if this method works for you!