I have made a few batches of Cab from frozen grape must through Brehm. I want to try their frozen Chard juice. My question relates to primary fermentation. With the Cab, I fermented in a Brute garbage can and did not worry about oxygen since Co2 was being released. I thought I could do the same thing for the Chard, but according to Brehm's instructions I need to conduct primary fermentation in a carboy with airlock. I don't have a problem doing this. I just can't figure out why it's necessary. Won't there still be Co2 given off enough to keep it from oxidizing?