if you put the cap on each one as you go (but don't crimp it on with a capper) you should be right.Whenever i've done this the yeast has already started to attack the sugar, making the caps lift off a bit. I don't think bacteria would be able to get in against the co2 flow. I wouldn't be leaving brews uncapped for half an hour, but i bottle in a filthy shed with mould on every surface.