I have heard that ascorbic acid alone can create hydrogen peroxide, which is bad in packaged beer, so it should be used in conjunction with SO2. I am wondering now about preventing oxidation at the end of a mash in a BIAB or AIO system, where the wort drains down out of the basket/bag into the kettle. What would be the best antioxidant to use at this point? Would any peroxide from AA be destroyed in the boil or would it still need to be combined with SO2?