I'm having an issue where recently, I've been making sweetcorn puree and adding (7ml) 0.04M of copper sulphate (taken from 0.1M solution). Initially the puree is green after the copper sulphate is added. I then transfer some of the food onto a wet cotton pad with larvae on it. Why is it that the copper sulphate treated food turns red after a week or so. Is there a way to stop this?