Author Topic: Rate of specific gravity change / Need to pitch more yeast?  (Read 6100 times)

Offline BrisBrewer

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I'm brewing an imperial porter.  The original gravity was 1.076, and FG is supposed to be around 1.017.  It was my first time using liquid yeast, and I neglected to make a starter (a mistake I'll be sure not to make again.)  It was Wyeast British Ale yeast in the smack-pack.  I think I did everything else right: pitched the yeast into well-aerated wort at 73F.  After a lag-time of more 6-14 hours (i.e. when i was asleep) the airlock started bubbling pretty rapidly and a nice 2-inch krausen formed, but by 36 hours later airlock action was slowing way down and by 48 hours later it had stopped.  I gave it another day or two and then measured the gravity, which had fallen to 1.044.  I was concerned that I had a stuck fermentation (knowing that I should have made a yeast starter) but decided to give it some more time.  I measured the gravity again today, which is 13 days after brew day, and the gravity has dropped to 1.027.   So to summarize, in the first ~60 hours, gravity went from 1.076 to 1.044, and in the next 8 or 9 days it went from 1.044 to 1.027 (but without any visible/ audible airlock activity.)  My fermenter temperature control isn't perfect, but it's been between 64 and 70F for the whole fermentation.  My questions:  1) Is this a normal / acceptable rate of gravity decrease?  In my previous beers (using rehydrated dry yeast) it seems like the peak rate of airlock-bubbling was slower, but that it continued for four or 5 days and beer was closer to its FG by the time bubbling stopped.  2) Should I pitch more yeast?  Should I keep waiting and checking the gravity every few days?  Would some yeast nutrient or something help it attenuate those last 10 points?  3) Is this a fairly normal fermentation trajectory, or would it have attenuated more quickly if I had made a starter and pitched more yeast cells?   Any suggestions appreciated!

Offline brewfun

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Re: Rate of specific gravity change / Need to pitch more yeast?
« Reply #1 on: July 10, 2015, 11:03:29 AM »
I thought someone would have answered this by now.

I'm brewing an imperial porter.  The original gravity was 1.076, and FG is supposed to be around 1.017. <edit> but by 36 hours later airlock action was slowing way down and by 48 hours later it had stopped. <edit> I measured the gravity again today, which is 13 days after brew day, and the gravity has dropped to 1.027.
<edit>
My questions: 
1) Is this a normal / acceptable rate of gravity decrease? 
2) Should I pitch more yeast? 
3) Is this a fairly normal fermentation trajectory, or would it have attenuated more quickly if I had made a starter and pitched more yeast cells?   Any suggestions appreciated!

Pitching more yeast after primary fermentation rarely shows rapid results. The sugars and nutrients are depleted enough that the new yeast has a big handicap. So, unless you're going to introduce more sugars or give it a long time, attenuation will be very slow.

Answers:
1) Sometimes. If you've underpitched and under aerated, yes, this can be a normal curve. The symptom of underattenuation is usually accompanied by higher esters than you'd expect.
2) No.
3) A starter would've helped. With an active starter and proper aeration, faster, cleaner fermentations are the typical result.

Starters help, but aren't mandatory, though. Buying and extra package of yeast and fully aerating the freshly chilled wort will give you a vigorous fermentation, too. Perhaps a little longer lag time, but the results will be solid. Proper sanitation is assumed, here.
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