I'm going to be making my first yeast starter, for a higher gravity beer.

While reading starter guides, and watching videos, I've noticed that there are several different techniques and opinions for the flask stopper. It seems that the common approach is to use a piece of sanitized aluminum foil, loosely wrapped around the top. I've seen others use airlocks, but have seen just as much on the side, recommending that they be avoided.

I bought a starter kit which included a semi-dense foam stopper (from NB). It appears that it will allow permeation of oxygen, which I understand to be important for the starter. I will soak it in sanitizer before I cap the flask.

Will this be an effective stopper? Should I skip this and go with the common aluminum foil approach?

I'm also worried about sanitization. How does the starter stay safe from infection if it's not airtight? I know it will only be exposed for a day or so, but I'm still curious about possible infection.

5 Answers 5


Those using airlocks are missing the point of the starter, as you mention: you need O₂ to be introduced to promote healthy yeast reproduction. (You also want to get CO₂ degassed out, as well, which is another benefit to a stirplate, as it will help achieve both goals.)

I wouldn't even worry about sanitizing the aluminum foil coming just off a roll, but if it makes you feel better, go for it. I use unsanitized foil, myself.

It's safe from infection because microorganisms can't crawl up; cf. Pasteur's swan-neck flask experiment. Unless you have significant wild yeast in the air and a serious pressure differential that would actively push stuff into the flask, nothing's going to get into the flask and outcompete the massive yeast population you've introduced.

Also, note that you should make a starter for any 5g batch above 1.040, which I'd consider a moderate gravity figure in most modern homebrewing. The recommended ale pitch rate of 0.75 million cells/ml/°Plato, with the commercially available liquid yeasts at around 150 billion cells, solves out to a gravity of only 1.040 at a 20L (~5gl) batch size. And depending on how your yeast are handled in transit, that 150 billion cell estimate is probably high. Plus, you'll have a good idea on the viability of the yeast with a starter. In short: always make a starter. :)

Also, wort oxygenation is very important, even with a good starter, so don't skimp there either.

  • Good answer. I didn't check your math, but WYeast smack packs are labeled as containing 100B cells, not 150B. White Labs states that their vials contain between 75 and 150 billion cells. These numbers are when the vial is produced. It's worth noting that viability goes down quickly over time. 6 weeks after production, only about 65% of those cells are viable.
    – JackSmith
    Commented Jan 27, 2012 at 13:31
  • Which would bring the gravity to something more like 100,000 (million cells) / 20,000 ml / 0.75 (ml cells/°P) = 6.66 °P * 4 °P/OG = 1.026. So, again, always make a starter! :)
    – jsled
    Commented Jan 28, 2012 at 2:35

You can put that foam stopper in your flask while you boil the starter wort. The steam from the boiling starter wort will sanitize the stopper for you. Or you could put the foam stopper in a covered saucepan with a bit of boiling water so that it's sanitized by the steam.

My understanding of foam stoppers is that, although not airtight, their structure makes it so that air-borne contaminants -- such as wild yeast -- can't make it through to your wort. I've used them for many of my starters and have never had a problem.

I've also used foil, which I sanitize by using it to cover the top of the flask while the starter wort is boiling. Just like the foam stopper, the steam sanitizes the foil.

  • Just be careful if covering the mouth of the flask with the stopper/foil while boiling. It can lead to a surprisingly fast and violent boilover. And the narrow mouth of a flask turns it into a dangerous high-velocity jet of boiling, sticky liquid. Ask me how I know :)
    – JoeFish
    Commented Jan 27, 2012 at 14:48
  • Oh, I learned that lesson the hard way as well. There's a certain amount of babying required when the wort first hits boil.
    – Hopwise
    Commented Jan 27, 2012 at 16:38

I always just use foil with a little starsan spray on it. I am pretty sure I heard that bacteria don't "fall up" so you just need to cover the top loosely so that you can have oxygen transfer. I've made 30+ beers with starters and have always just used loose tin foil. Don't forget to shake/swirl occasionally if you don't have a stirplate.


I've also bought those foam stoppers, and most times I've used them I've ended up with contaimation of the starter. It may be because I make my starters 5-7 days ahead of brewday - it seems that once CO2 production stops, then the microbes are free to drop in through the stopper.

Recently I switched out the foam stopper with an airlock when fermentation had started to slow, still got the same yield and no contamination that time.


I don't know why people recommend against using airlocks. When the wort is fermenting, there is a positive pressure of CO2 OUT of the vessel. Oxygen can't get in anyway because of the positive pressure in the flask, so tin foil and foam stoppers aren't doing what they are purported to do anyway.

  • You raise an interesting point.
    – GHP
    Commented Aug 18, 2014 at 12:33

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