Protecting yeast starter while in keezer

[Homer]

I made a 3L yeast started this past Wednesday night for a 10 gallon batch I planned on brewing yesterday (sat). Looks like now I might not be able to brew until middle of the week or even later so, for the first time, I will need to store this in my keezer.

I have a foam stopper in the 5L flask now, should I lightly cover that with foil when I stick it in the keezer? I have had some mold in that keezer from time to time, but since only kegs have been in there, I haven’t had any issues. Should I be worried?

 

[IslandLizard]

Basically you'll be cold crashing it for a few days, which is good.

Leave the foam stopper in it. Wrap the opening and neck with sanitized plastic wrap or aluminum foil. Put in keezer.
Before use, sanitize and remove foil and stopper. Sanitize neck, pour off the clear starter beer, leaving enough to swirl up into a slurry. Let come to pitching temps before pitching.

 

[Homer]

Should I just cover it in foil just as people normally do, or should I really try and seal it, like with tape or something? Not sure what will happen with air suckling back into the flask when cooling

 

[IslandLizard]

Some air needs to get in as the cold will contract the volume inside, so your flask doesn't implode. You could drape a 2x folded-over wet Starsan soaked (but not dripping) washcloth over the foam stopper as an air filter for any bugs in the air. Once it's cold no more air will be sucked in, and you can cover it with foil or wrap and a rubber band.