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I brewed a batch of Pale (first solo brew) and after pitching yeast (6 hours), noticed yellow clusters of what I think was mold. I thought I had an infection, so I siphoned off, and collected supernatant in clean carboy (trying hard not to interrupt the surface where most of the visible yellow clusters were).

Fermentation ramped up and was vigorous all day yesterday (brewed on Sunday) but a day later, today, I noticed visibly that it slowed. Now, I know fermentation is still going because I see bubbles, but what could be gong on here? I've seen other beers rage for 2-3 days before slowing like this...

New to brewing so I haven't really seen anything like this. Good news is that I no longer see any yellow.

Not sure how I contaminated the beer, or if was even me... Probably was, but that's not really important: I will try even harder next time to be clean. What I'm getting at is can mold cause slowing of fermentation and if so, what should I do - wait? Anything else I should keep in mind? Will it taste funny when done, etc?

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what makes you think the initial stuff was mold and not just a krausen (yeast) ring starting to form? Different yeasts behave differently, especially with different conditions (temp, wort composition, etc.). My guess is you will be fine, but the risks probably outweighed the benefits of siphoning out of the original ferm vessel. –  Pietro Jan 8 '13 at 19:51
    
bright yellow clusters... looked artificial... you could be right, though. one reference: everything2.com/title/Homebrewing+105%253A+Troubleshooting –  nicorellius Jan 8 '13 at 20:24
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1 Answer

...and after pitching yeast (6 hours), noticed yellow clusters of what I think was mold

Mold isn't going to grow within 6 hours of chilling down the wort. What you saw was clumps of yeast from your pitch stuck to the sides of the fermentor or floating on top. Did you rehydrate your yeast (if using dry)?

What I'm getting at is can mold cause slowing of fermentation and if so, what should I do - wait? Anything else I should keep in mind? Will it taste funny when done, etc?

No, mold will not cause slowing of fermentation, however you didn't have mold to begin with. As for the fermentation times, those are highly dependent on a variety of factors like: temperature, yeast count, yeast health, gravity of wort, oxygen levels at pitch, and more. Sometimes, the beer bubbles for 2 days, sometimes for 7. Sometimes NONE if you've got a leaky bucket! (the C02 escapes silently). Bottom line: don't worry about it. So long as the beer starts fermenting & producing C02 within 24-36 hours, you will be OK. Use your nose to detect fermentation, it has a 'sharp' smell.

The best advice any newer brewer can get is this: leave the beer alone. Once you see it start going, keep the temp steady for a few days, then walk away from the beer for 3 weeks. Its so hard to do, but trust me, its worth it.

Back to your exact question, if you did get some kind of infection, here's what to expect: The beer will continue to dry out past its normal point, it will start getting flavors like cherry pie, pineapple, leather, burnt rubber or band-aid, and if you bottled it, the bottles might start gushing or even exploding. Those are the signs of a true infection. If you left the beer in the carboy long enough, you'd see a "spider web-like" mold growing across the top sometimes, or sometimes it would look like little steeples or structures of mold on the surface of the wort. Fortunately, this is pretty rare.

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I'm sorry I can only upvote this answer once! –  Denny Conn Jan 9 '13 at 18:31
    
Agreed with Denny and upvoted –  brewchez Jan 9 '13 at 23:35
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