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This is my first shot at homebrewing, using Northern Brewer's American Wheat Beer kit which is based on liquid malt extract, Willamette and Cascade hops, and a packet of dry yeast. I brewed and put the hopped wort into fermentation on Friday. By Sunday morning (~40 hours later), fermentation seemed to be moving along at a good clip -- I could see bubbles traveling through the airlock about once every 2 seconds.

However, by Monday morning, bubbling had completely stopped. The weather (and my house) had gotten considerably cooler overnight, so thinking the cold was to blame, I moved the fermenter into the warmest room in the house (which still probably never goes over 70 Fahrenheit). Still, no bubbling as of Tuesday evening.

I was under the impression that fermentation should be vigorous for at least a few days.

I've been doing some research and realize now that when I was brewing I didn't really understand the importance of aerating the wort. I shook it around for a minute or so in the fermenter on brewing day, but maybe that wasn't enough? Also, I didn't realize until brewing day that I should have been keeping the dry yeast packet in the fridge/freezer -- it had been sitting out in my basement (60-75 Fahrenheit) for at least a month. Oops.

Any thoughts on if I screwed something up or if there's a way to fix it (if it really needs fixed)?

I'd appreciate any help. Thanks!

Update: At the suggestion of Denny (and some other homebrewers I talked to in person), I decided to just let the fermentation run its course and I'm happy to report that fermentation seems to have worked -- my original gravity was 1.041 (possibly slightly higher as the reading was taken warm) and after two weeks on bottling day I was at 1.016 = 60% attenuation (right?). Although the Safale US-05 Ale Yeast I used doesn't give attenuation specs, some folks on a forum said they usually get at least 70% from that strain, so it's possible that fermentation might have been a little stunted from the yeast not being refrigerated.

The beer is conditioning in the bottles now, so hopefully it will taste good when I crack one open in a couple weeks. Thanks everyone for all the input -- if nothing else I've learned the importance of taking good care of the yeast.

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Can you take a hydrometer sample? With that reading we will know for sure where your beer is in the fermentation process. –  Matt Darby Jan 4 '12 at 4:21
    
Also, dry yeast doesn't need to be refrigerated, so no worries. A 40 hour lag time between pitching and fermentation is very long. Do you know what temperature the wort was when you pitched your yeast? –  Matt Darby Jan 4 '12 at 4:28
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Sorry, dry yeast DOES need to be refrigerated. –  Denny Conn Jan 4 '12 at 16:53
    
It's preferable to refrigerate, but it is not necessary: lalvinyeast.com/faq.asp –  Matt Darby Jan 4 '12 at 20:57
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Then let me rephrase..."Dry yeast DOES need to be refrigerated to maintain the highest viability". –  Denny Conn Jan 4 '12 at 22:07
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2 Answers

up vote 5 down vote accepted

Bubbling really tells you nothing other than CO2 is being released. It's not a reliable indicator of fermentation. In addition, if you ferment in a bucket and have a loose seal you won't see bubbles. Aeration is needed so that the yeast can use the O2 to synthesize sterols to build new cells. Because the cell count in dry yeast is so much higher than liquid yeast, aeration is much less important. Your yeast should have been kept refrigerated, though, so the cell count may be down a bit. It probbaly wasn't a fatal mistake, though. In short, I wouldn't worry. Give it a week or so, then take a gravity reading to see where you're at.

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Previously I was mistaken. Yeast do not abide by the same rules as mammalian cells which I am familiar, who prefer aerobic respiration when there is oxygen. Instead when yeast are introduced into the wort (which is over 1% by weight sugar), regardless of the presence of oxygen, they immediately enter fermentation and begin producing ethanol. This is called the Crabtree effect and is probably induced because they yeast want to use the O2 create sterols that are required to make more yeast. Apparently the yeast trade off a more efficient respiratory process for growth.

The sugar from the wort is thus fermented and during this process CO2 is released in the final step where lactic acid is converted into ethanol and CO2. For each of the glucose molecules produces 2 molecules of ethanol and CO2. Thus the bubbling observed in beer fermentation comes from fermentation which is initiated at pitching of the yeast. The yeast are using primarily anaerobic respiration and also dividing simultaneously.

In answer to your question your beer is likely going to be fine. I typically wait till the yeast has dropped out of suspension (flocculated) to make a layer on the bottom of the fermenter. That usually means the yeast are done and it is time to bottle. Just make sure to take hydrometer readings after a week or so and bottle or keg when it reaches the final gravity you want.

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I'm no biochemist, but I'm pretty sure that anaerobic respiration in yeast produces CO2. –  Tobias Patton Jan 9 '12 at 19:44
    
You are right that anaerobic respiration itself converts glucose into 2 molecules of lactic acid, which subsequently during fermentation proper is converted into two molecules of ethanol and two molecules of CO<sub>2</sub>. During aerobic respiration a molecule of glucose is broken down into six molecules of CO<sub>2</sub> and six molecules of water. So it would be more accurate to say each glucose molecule produces three times as much CO<sub>2</sub> during anaerobic respiration as compared to aerobic respiration. –  Chris Plaisier Jan 17 '12 at 22:25
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