Water repellent-like residue on pot when cooking vegetables When I boil vegetables including broccoli the pot gets a water repellent-like residue at the water line that is very hard to remove. There is also residue similar to when cooking beans on the sides of the pots. This is in decent, newer stainless pots including the instant pot. My mother's older stainless pots don't seem to have this problem. 

Does anyone know why this is and what is the best way to clean them? I wonder with time if it will become easier to clean. Or if the manufacturer coats them with something.
I get a greasy residue in the water used to wash broccoli before cooking and in the blanching water before I freeze it. It literally only happens with broccoli so I make sure to wash it as many times as possible before freezing so it shouldn’t have any residue left when I want to cook it. I can only think that it is some sort of pesticide or preservative which is slightly worrying as I don’t want chemicals on my food! Organic is so expensive in comparison though.