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I made a yeast starter last night with 1500 ml of water, 150 g DME and a Wyeast 1056 smack pack. After boiling, total volume was just under 1500 ml. I put it in a 2 liter erlenmeyer flask on a stir plate and covered the top with foil. By morning it went Mt. Vesuvius and had bubbled out the top all over my stir plate. I've definitely lost some yeast.

I've made several of these before and never had this happen. Is this unusual? What is a reasonable volume starter for a 2 liter flask? Should I chill, decant, and do another starter to make up for the lost yeast or just pitch it and see what happens? It's for an IPA with an OG of 1065, so I don't want to underpitch.

  • Next time add a couple drops of Fermcap-S. I got tired of cleaning crusty krausen off of my stir-plate and haven't had a foam-over since starting to use Fermcap. Less mess, less chance of infection, less stress! – andrhamm Apr 16 '15 at 20:19
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It is not unusual. I'd easily believe that a 1.5L starter would overfill a 2L erlenmeyer at full krausen. I would suggest using an anti-foam agent in the future. I regularly do 1.6L starters in a 2L erlenmeyer w/o any hesitation or foam-over, due to the introductions of 1-2 drops of liquid simethicone (baby anti-gas drops), available in your local supermarket.

I don't have a good handle on how much yeast you might have lost, but I'd probably just plow ahead, maybe adding some new boiled/cooled wort to make up for any liquid volume lost.

If you're still worried about underpitching and don't have time to re-do the starter, see if you can grab a packet of US-05 to pitch along side.

But I think you'll be fine continuing the current starter.

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I don't think that's unusual. I've had a similar experience where the starter overflowed massively and constantly, creating a huge mess. It didn't happen every time I made a starter, but twice was too much. I don't actually use my Erlenmeyer flask anymore because of this. I may someday buy another 3 liter flask, but in the meantime I've reserved a growler for the purpose.

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I'd say a starter in an erlenmeyer flask really should be somewhat under filled. As you add more wort, you lose surface area to exchange CO2 and O2.

Back when I was doing actual microbiology, growing bacteria, we would only put about ~25% of the stated volume of medium in an erlenmeyer (somewhere around or below the lowest measuring line).

Yeast don't grow quite as fast as bacteria, so it's probably OK to fill them higher (White & Zainasheff show a picture of 2000ml flask filled right to 2000), but as stated that can overflow.

I don't have a stir plate, and when I stir manually all the foam comes up at once. I have keep the flasks below 50% filled, but I seem to get a good pitch from a couple 250ml flasks.

In general I'd say any starter that could be in one flask will be healthier in two. Or one larger flask. And it won't overflow.

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