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I bought two packs of the American Ale Propogator yesterday, but then took a bit longer than planned to get home and the packs got warm and were swelled a bit.

I put them in the fridge as soon as I got home, wondering if they are still OK to use or if I should get more.

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The Wyeast liquid yeast can survive warmer temperatures for some time and still be viable. Although the longer they are kept above 40F the less viable they will become. The only way to know for sure is to activate the pack and see if it expands due to activity. I would advise you do this before you start brewing to confirm that you have active yeast to pitch when you are finished. If the yeast is not active I would postpone brewing until replacement yeast is available.

There are a few yeast strains that Wyeast produces that have a tendency to inflate partially prior to activation and 1056 American Ale is the prime suspect. The slight inflation is due to residual CO2 in solution left over from propagation being released. Wyeast has a process to remove most of the CO2 prior to packaging but for some reason 1056 wants to hold on to some. The CO2 is released when the temp is raised and the package is agitated, this happens quite often when shipping the yeast.

The swelling can also be attributed to minimal amounts of propagation media making it into the package. In this case the swelling is actually a sign of healthy yeast that is more active due the increase in temperature.

In either case it is best to refrigerate as soon as possible. You should also try and use any yeast exposed to extreme temperatures as soon as possible or to make a yeast starter to build up the yeast cell count. A starter can be made immediately and then stored in the refrigerator for a few weeks before pitching.

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Awesome thanks, this makes sense. I'll probably pick up another pack or two just in case though as I'll be brewing on a Sunday. I'm not sure if I'll have time to make a starter as I'm out of town until then, I'd rather have extra yeast around. Thanks for the detailed explanation! –  applecran Jul 11 '11 at 22:12
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They probably used a bit of their glycogen reserves when they warmed up, but as long as they were cooled down again fairly quickly they should be fine. I am guessing you will be using a starter, since you are using a Propagator, so if they seem to have any problem in the starter, they may have used up too much of their stored reserves and you'll need to buy more, but I'm guessing they are ok.

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