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Why cook black beans for 8 hours and what does cooking them in sugar do?

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I live in Japan and I've started on Japanese cooking. I'm one of those people who likes to know why certain things are done and those answers aren't in any of the cooking blogs or books I've come across. For example, why cook black beans for eight or more hours in sugar?  


Around new years there's a very popular dish of sweetened black beans. I know the goal is to cook them so the skins don't burst, but up to half a day? What's more, most recipes add some of the sugar at the beginning. Were it rice, it would never cook. Does putting the sugar in the beans from  the beginning slow the cooking? Does a prolonged cooking partially candy the beans? I'd really like to know, because I made a dish with Azuki beans the other day (zen-zai) and to keep the skins from bursting I cooked the soaked beans for over two hours before adding the sugar and some of the beans did burst.  


For reference, the recipes I followed flowed like this:

1) Soak the beans over night, change the water.

2) Bring to a boil with a bit of salt or soy sauce and some amount of sugar.

3) Reduce the heat to barely a simmer with a piece of parchment over the surface and let it boil until they're just tender -- at least eight hours.

4) Add 1/3 the amount of the remaining sugar, cook for another half an hour. Repeat. Repeat.

5) Add a bit more soy sauce. Done. 

Edited by cteavin (log)
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No, it's not the precursor, it's not anko, which is made with azuki beans (and part of the zen-zai I mentioned). The black beans are popular as a side dish in a bento or as a part of the new years foods. And when I say a side dish, I mean like 10 beans, a splash of colour and sweets to the bento.  

Edited by cteavin
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  • 2 weeks later...

Are Japanese black beans, I wonder, the same as what we call black beans in the US?


I have a good stock of lovely Rancho Gordo midnight black beans and I am more than open to new and interesting ideas.  Though I admit I would have to think long and hard about the sugar.

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Hi Blue Dolphin. We make kuromame every year for New Year, but I must admit I don't always follow the same recipe. And yes, they are black soybeans.  I like the kind of recipe where you soak the beans with the seasonings - for example, one of the recipes on the NHK Kyou no Ryouri cooking site gives (for 300g dry beans) 2 liters of water brought to the boil, then 250g sugar and 50ml soy sauce dissolved in that, with 1/2 tsp salt and 1/2 tsp baking soda. Soak overnight, and cook rather gently for 8 hours. I think adding all the sugar at the beginning could make the beans harder, but the baking soda softens them.

I haven't found the perfect timing for doing it in a pressure cooker, but a slow cooker works quite well, especially if you are prepared to strain off the beans and boil the syrup to reduce it a bit. then combine beans and syrup again. I like to add ginger when I cook mine too.

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My big question is whether or not the sugar keeps them hard requiring the additional cooking time. I'm going to have to make them again without the sugar and see. 


Helenjp, you've tried making them in the pressure cooker? Did they burst? 

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To be honest, I can't be entirely sure, because I've never cooked black soybeans except in this dish. I think that trying to prevent the beans from bursting is one reason to cook them for so long (at a low temperature) but then, even ordinary soybeans cooked in plain, unseasoned water for miso are cooked for a very long time - I've cooked them for 5 or 6 hours. 

When I have made them in the pressure cooker, they have not always burst, but they tend not to plump as much in cooking - once they cool, they wrinkle. Of course, some people deliberately cook them so that they wrinkle!

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  • 8 months later...

Adding sugar does not significantly extend the cooking time. I was taught to add sugar little by little over the total cooking time. The reason is that adding all the sugar in the beginning or adding it all at the end will cause the beans to wrinkle because of a difference in concentration of sugar (osmotic pressure). You add the sugar little by little to let it absorb slowly through osmosis to keep the skins taut. The goal is to have the surrounding cooking liquid and the finished beans have the same flavor concentration. As this work is usually done in winter for osechi the typical workflow in a professional kitchen goes like this: day 1: Soak the beans before leaving work (at night), day 2: if there is space on the stove cook the beans the full amount of time, usually all day until you can smash a single bean in between you thumb and pinky finger easily. If at some point the stove needs to be used for other things you can stop the cooking and leave the beans at room temperature (which is very cold even indoors in Japan) and continue the cooking on day 3. day 3: finish cooking and then pack the beans and liquid into jars and process them like you are canning vegetables or jam. They will keep a long time in the refrigerator like this. If you want them to turn out dark black like the commercial kuromame you need to add iron oxide (rust) .

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