Matteo, I may be way off base on this but did you try any kind of acidulation when looking for a remedy? I don't know what the process is, but grapes are mainly water and introducing something like a measured quantity of, say, lemon juice could help. That is what is used when prepping fresh fruits and vegetables to keep them from browning with exposure to oxygen. If it is the same reaction, it could help.There’s an enzyme in the fruit matter that browns exactly the same way as fresh fruit. The concentration process does not deactivate it. Traditional kits don’t have this issue because they’re cooked to be stable at room temperature. The carbon bonds with those enzymes and pulls them out. Without the carbon, the wines are darker. All the residual carbon eventually drops out. This is why we recommend two additional polishing stages each a minimum of two weeks after the initial racking or filtering.