Starters and foam stoppers

I've seen several places - and I can't find one right now - where they say to not use an air lock when making a starter but rather to use a foam stopper.
I think someone said that was so air could get into the vessel but if the yeast is making CO2 it looks like the air couldn’t get in anyway because of the CO2 escaping through the foam stopper.

Does anyone have a real argument for using a foam stopper – other than you can get away with it and they are cheaper.

Handsaw said:

I've seen several places - and I can't find one right now - where they say to not use an air lock when making a starter but rather to use a foam stopper.
I think someone said that was so air could get into the vessel but if the yeast is making CO2 it looks like the air couldn’t get in anyway because of the CO2 escaping through the foam stopper.

Does anyone have a real argument for using a foam stopper – other than you can get away with it and they are cheaper.

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Yes, you want gas exchange. Yes, you get oxygen exchange through a foam stopper. Yes, it will improve the yield of yeast in your starter. Much of the need for oxygen is before the yeast are generating tons of CO2 (i.e. lag phase).

Just use aluminum foil to cover it. As long as you dont tie it on it will be able to exchange gas and honestly its easier to work with than the foam stopper. I started with a foam stopper but i quickly moved to just using foil for all my starters now.

Thanks. So mostly it is in the lag phase that the foam stopper is useful?

Sort of the same topic, but not exactly.
One of the yeast calculators showed the growth rate for using a stir plate vs using continuous aeration vs shaking it every time you walked by vs doing nothing.
The stir plate was the winner with continuous aeration close behind. Is there a reason to not continuously aerate while using the stir plate? I tried it earlier today and the swirl of the wort was significantly slower than without the air stone in the jug, but it still kept the yeast mixed up. Oh another thing that happened was that there was a buildup of junk (probably yeast) on the upstream side of the air stone. Of course I don’t know the results of this yet, actually, I won’t know the results tomorrow either because I pulled the air stone out after half an hour.

I believe that too much O2 will end up inhibiting the yeast to some degree because they are having to chemically deal with the O2/ nothing likes too much O2. Really just atmospheric air being constantly stirred into it will be fine. Direct aeration is typically used to get a high amount dissolved O2 into wort when before you close it off to the outside world that way the yeast have plenty to grow with, here the yeast will just continue to get new O2 as they grow because the starter is being exposed to air.

Handsaw said:

Thanks. So mostly it is in the lag phase that the foam stopper is useful?

Sort of the same topic, but not exactly.
One of the yeast calculators showed the growth rate for using a stir plate vs using continuous aeration vs shaking it every time you walked by vs doing nothing.
The stir plate was the winner with continuous aeration close behind. Is there a reason to not continuously aerate while using the stir plate? I tried it earlier today and the swirl of the wort was significantly slower than without the air stone in the jug, but it still kept the yeast mixed up. Oh another thing that happened was that there was a buildup of junk (probably yeast) on the upstream side of the air stone. Of course I don’t know the results of this yet, actually, I won’t know the results tomorrow either because I pulled the air stone out after half an hour.

Click to expand...

Yep, same as with a regular fermentation. Oxygen at the beginning is the critical thing. I wouldn't waste my time with continuous aeration while on a stir plate. Mostly I wouldn't want to keep pumping outside air into the starter. The stir plate is going to give you way more than enough O2 to get good yeast. If you're using one already, you are ahead of the game.

the stir plate/shake method is more effective because it both encourages gas exchange but also keeps the yeast in suspension

i like to use sanitized coffee filters with a rubber band over the top of my flask - i had a fruit fly get under the foil once and this keeps any bugs etc out without inhibiting gas exchange

The coffee filters are a good idea... may have to use that next time.

I've switched from aluminum foil to a foam stopper on a 2L flask and like the foam stopper better because of the fruit fly protection. Visually, there's been no difference in the yeast cake after crashing the starter. The coffee filter idea is great and will do that with gallon jug sized starters where the stopper doesn't fit.