I use a rice cooker like the one shown below to cook dried mung beans and red beans (Adzuki beans) as well as rice of course.

After boiling and cooking, the rice cooker switches to a stable hot temperature to store the contents for a second meal, I suppose the idea being that bacteria don't grow readily when it's too hot.

That seems to work fine for all of them, except that while the rice is still white in the morning and the red beans still red, the mung beans turn dark brown in the morning. In fact they start changing color within a few hours. It doesn't matter if I've made them fairly dry or quite wet.

I cook them on different days, they are not mixed. Just the dried product and an equal or larger amount of tap water.

Is it possible to understand why the mung beans turn so dark brown if they stay hot? I don't notice any big change in taste, but the color looks quite discouraging. Is there anything I can add that would prohibit the color change?

rice cooker

  • 1
    Do you add any spices? or salt?
    – Ess Kay
    Oct 3, 2018 at 9:05
  • @EssKay nope, just the dried product and an equal or larger amount of tap water.
    – uhoh
    Oct 3, 2018 at 12:15
  • 1
    well, not relevant in this case then. I cook mung beans with spices and they do turn brown after cooking.
    – Ess Kay
    Oct 3, 2018 at 12:28
  • 1
    @EssKay I wonder if they would still turn brown without the spices!
    – uhoh
    Oct 3, 2018 at 12:30

2 Answers 2


My mung beans also always turn brown before it's fully cooked. I never tried blanching them though. It's the way of preserving colors in cooking. But I never even heard of blanching Legumes. I might give it a try though. Hot water/Cold water shock in the beginning, and then the rest of cooking. We'll see..

  • 1
    Interesting, I'm looking forward to hearing about your results.
    – uhoh
    Oct 27, 2020 at 21:49
  • Blanching and shocking is a good way to minimally cook vegetables and bring out their color without them softening, but it won't prevent discoloration during subsequent long cooking.
    – Sneftel
    May 18, 2022 at 9:42

I noticed this as well after cooking mung beans in a pressure cooker. I suspect it's oxidation in the same process as why potato starches turn brown. For example, the closer the beans to the top of the pressure cooker, the browner they are, because the beans near the bottom are not exposed to air.

I'll try solving this by adding some acid.

  • this isn't really an answer to the question, if you do add some acid and it works, feel free to come back and post that as an answer!
    – Esther
    May 17, 2022 at 21:37

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