As you can tell from the recipes here, I’ve been cooking red azuki beans quite often. However, I haven’t tried to understand the principles behind cooking beans until now.
I’m thinking of making kuromame, or Japanese sweetened black soya beans in order to try this Kuromame Pound Cake. Many recipes on the internet include bicarbonate of soda and I was curious as to the function of this. (Some also recommend putting some rusty nails in the cooking water -!- this is to intensify the black colour of the beans; cooking them in an iron pot will have the same effect.) I was also interested to understand why Just Hungry says that twice-boiling the beans gives a better flavour.
Bicarbonate of soda & beans
The function of adding baking soda (or any alkaline) to the soaking and cooking water is to soften the bean skins and help them to cook faster. However, this is probably unnecessary unless you live in an area with unpleasantly hard water. The drawback of using soda bicarbonate is that it may destroy thiamine (Vitamin B1) in the beans, which contribute to their nutritive value. Bicarbonate of soda may also give the beans a soapy flavour.
If you do use soda bicarbonate to counter the problems of hard water, use no more than 1/8 tsp per cup of beans. This is a small enough amount to limit the loss of thiamine and hopefully avoid a soapy flavour.
Most obviously, soaking helps to shorten cooking time. Ratio of water to beans should be 3:1 or 4:1.
Here is a chart of soaking times from RecipeNet. For example, it recommends that azuki beans should be soaked for 4 hrs and then will cook with just 1hr on the stove. But others have different instructions for soaking times. Just Hungry says 24hrs in cold water, while Central Bean says 8 to 10hrs is optimum and that beans soaked for longer than 12 hours can lose their taste and texture.
Personally, I found that soaking red azuki beans overnight resulted in the water turning reddish and the bean skins splitting. This didn’t look too good to me. However, another time I followed a wagashi cookbook for an alternative method of boiling unsoaked beans for 2 mins then discarding the water and reboiling in fresh water. This initial short cooking produced a very similar result to overnight soaking with the water turning red and the beans swelling and splitting, so maybe this is what is supposed to happen?
As for the temperature of the soaking water, while warm water will speed up the soaking process, Central Bean points out that hot water may cause the beans to go sour, so room temperature is best. I’ve found beans also might go sour if you leave them to soak for too long, so personally I prefer not to soak for more than about 8 hours.
Perhaps the solution is to use warm water only if you intend to use a relatively short soaking time of say, 4 hours. If you want to leave the beans overnight or longer, then use room temperature water.
Alternative soaking methods
Quick soaking method involves bringing beans & soaking water to the boil for 2 minutes, remove from heat, cover, and leave for 1 hr.
Discarding soaking water
One of the problems of beans is that eating them can cause flatulence. Soaking reduces the complex sugars/ starches in beans that cause gas. So to make beans more digestible, one should throw away the soaking water and cook in fresh water, and once the beans are cooked, to rinse them again in fresh water.