Why do frozen greens get fibrous and tough in the microwave?

Question

When I microwave frozen spinach or amaranth greens to cook them, they sometimes get tough and fibrous. There doesn't seem to be a consistent pattern in when this happens, so I can't figure out how to avoid it.

So I ask you: why do frozen greens get tough and fibrous when cooked in the microwave, and how can I avoid it?

Answer

Microwave ovens cook primarily by spinning water molecules. Frozen water is less susceptible to this affect. I would guess that with certain vegetables, as frozen water turns to a liquid, it is quickly heated to the point that it boils and evaporates. The heating is less even than cooking frozen vegetables in a pot and leads to areas that are somewhat dehydrated and fibrous.

Try cooking your frozen vegetables in a container with a lose cover and additional water. This will allow them to cook or steam from the the outside in much like in a pot.

http://en.wikipedia.org/wiki/Dielectric_heating

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