Why do my mung beans turn from green to dark brown in a rice cooker?

Question

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?

Answer 1

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..

Answer 2

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.