I even used to keep mine in a cooler bag, but gave up and just started eating them plain. They will not last long without browning, even waiting 30 minutes makes a big difference. Oxidation is always happening with food left out whether it is blended or not.
Yeh when I've been drinking them I've thought to myself these can't be good anymore... I suppose plain yellow bananas last a lot longer but these bad boys are ripe and prime for blending. I think I'll stick with the banana smoothies for when I'm at home, glug datorade at work (though my shifts can be up to 13 hours...) may be take some less ripe bananas with me, I could mash them up with some dates.
This is what I do;
Blend bananas on a slower Vitamix setting till smooth (try to get no air bubbles),
pour in glass jar all the way to the very top (completely inline with the glass rim),
close tight with it's screw on lid,
keep it cool till you leave the house for work, if your work has a fridge, even better!
-This works well for me. It does not turn brown for a long time. Looks fresh for hours & hours... :)