White Labs Yeast, may have insects in it.

[Dog House Brew]

I use the coffee filter because of the bugs. I get them routinely in the House. Foil only lets them in. I would think the foam stoppers work great. After losing a few starters, gotta have an addition to the foil. If I hadn’t had the trouble I’d never use anything but the foil.

 

[MaxStout]

Hah!
Intuitively I don't think the gas exchange through a foam stopper is as good as it is with a foil hat. Those openings around the hat are indeed huge (on molecular scale) and offer NO resistance. Turbulence around that top opening allows for gas exchange. I see the foam as a more restrictive barrier, especially at 1.5" thickness, with far less turbulence at the gas exchange site.

Would be a nice exbeeriment to do 2 starters side by side, one with foam, the other with foil.

Or have two chambers, one containing air, the other containing a different gas. No pressure differential between the two. Connect the two chambers via a tube that is plugged with a foam stopper. Wait 24 hours and analyze the gas in each. Repeat the experiment using foil over the rim of a tube.

 

[Grossy]

Ok the results are in.

This might be the best beer I have ever made.

Conclusions:

1. The infection that I feared never happened.
2. I really doubt that the flies came from the yeast, or the DME. they had to have gotten in somehow. I will re-evaluate my process.
   
3. And finally... flies make great beer, I highly recommend them.