While looking around at different ways of cooking nopales and okra that dealt with the mucilage, I found a few references to cooking nopales with a copper coin. Some people seem uncertain whether it's an old wives tale or not, and I haven't found an explanation of why it would work yet, but it seems pretty wide-spread. Does it actually work? And if so, why?
I just came across to this website looking for an answer related to sushi rice. Then I started to look around to find out what is all this about, so I found this question about nopal cooking. It happens that I am mexican so I wanted to give some advice about it. I have never seen a person here in Mexico cooking nopal with a copper coin. It seems to me like a way to do things on those days where people had only copper and "barro" pans and the so to cook. I am not saying there are no people out there who are not using them nowdays, of course, but they might be the least. As I have not done it nor seen it, I can not say if it works. Maybe I can help by telling you the way I cook them, which is very simple actually. After the nopal has been peeled and washed, I put it directly in a hot pan just like that, let it cook for a moment at medium temperature, then turn it over to allow the other side to cook as well. That is it. No oil, salt, water, and nopal turns just mucilage free. I hope it helps.