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Yeast Starter w/ Stir Plate Question

Discussion in 'Homebrewing' started by jbakajust1, Nov 27, 2012.

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  1. jbakajust1

    jbakajust1 Moderator (1,092) Aug 25, 2009 Oregon
    Subscriber Beer Trader

    So I have often wondered this, and a friend who is much more knowledgeable than I actually pointed this out to me. When you make a starter with a Stir Plate the whole point is to exchange CO2 for oxygen, right? So once the active growth phase is done and there is no longer a high amount of CO2 being released, the starter is now pulling in outside air, correct? So now the starter is functionally being infected with wild yeast and bacteria from the air that is being sucked into the starter under the foil cap thus rendering it a possible source of contamination instead of a guard against it, correct?

    To clarify, stir plate releases CO2 from the starter during the active growth stage keeping unwanted contaminants out of the culture, but once the active growth phase is finished the vortex is now sucking in outside air full of unwanted yeast and bacteria into the culture. If this is true, why would we use them? What is the best way to protect against this and still get maximum growth?

  2. OddNotion

    OddNotion Devotee (478) Nov 1, 2009 New Jersey
    Beer Trader

    The amount of unwanted yeast/bacteria is negligible. There is no way any other part of brewing at a homebrew level is completely sterile other than the wort during the boil. Once you open that yeast tube/pack there is always the chance of unwanted yeast/bacteria intermingling with the pure culture. It just becomes a numbers game where the dominant yeast that you want has a way higher cell count as will act much faster and stronger than any contaminant.
    mountsnow1010 likes this.
  3. mountsnow1010

    mountsnow1010 Initiate (0) Jan 23, 2009 Vermont

    I disagree that there is a large "sucking motion" into the flask. I expect that it's more of a diffusion process of air under the small gaps in the foil lid, and into the flask, where it is there stirred into the liquid. As such, you don't have to worry about contamination because you aren't sucking air filled with yeast into the flask, and yeast aren't going to "diffuse" into the flask - they are much too heavy for that. Instead, yeast tend to fall downwards in the air, which is why you put the foil cap on.

    On another note, I also don't think that there is a large "exchange" process - I doubt adding O2 is going to shift the equilibrium of dissolved CO2 much. I think you are really just adding dissolved O2 by stirring it into solution.

    For what it's worth, I'm a novice homebrewer but I have a lot of experience working in yeast and mammalian tissue culture labs. Most of sterility concerns lie with trying to prevent crap (yeast and bacteria) from falling into whatever we are working with.

    edit: if there is a true "sucking" effect that would pull stuff under the lid, I think that OddNotion's point would come into play
    warchez likes this.
  4. VikeMan

    VikeMan Meyvn (1,345) Jul 12, 2009 Pennsylvania
    Beer Trader

    Some thoughts.
    - Gas exchange isn't the only reason. The constant stirring also helps keep the yeast in suspension, which accelerates the whole process. (Sort of like rousing the yeast in a fermentation.)
    - Once the CO2 production ceases, you should be pitching the starter or crashing it.
    - You're actually exchanging gases the whole time. Infection risk is miniscule, given the empirical evidence of all of our infected starters divided by total starters.
  5. AlCaponeJunior

    AlCaponeJunior Poo-Bah (2,077) May 21, 2010 Texas

    Yes, the small gaps between the foil and the flask are large as far as molecules go, but will still likely be very difficult for bacteria and yeast to navigate (upwards). There will be exchange of O2 and CO2 by diffusion though, enough to continually add oxygen as the yeast need it while your stir plate is on. If you don't have a stir plate, you'll need to shake, but obviously there will still be new oxygen diffusing in (and CO2 diffusing out) while it's sitting on the counter.

    Note that I agree with what was said above that complete sterilization and absolute prevention of wild yeast/bacteria getting into your beer is impossible. It's about competition, and your healthy culture of lots of good yeasties will out-compete any odd bugs that make it into your beer.
  6. barfdiggs

    barfdiggs Initiate (0) Mar 22, 2011 California

  7. GreenKrusty101

    GreenKrusty101 Crusader (718) Dec 4, 2008 Nevada

    What's a stir plate? : )

    For normal gravity beers, I select for yeast couch potatos that work with minimal stimulation (bag of chips and a good movie).
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