Not fully fermenting

What went wrong? Was this supposed to happen? Should I throw it out? What do I do now?

Moderator: slothrob

Not fully fermenting

Postby abaxter » Fri Nov 26, 2010 3:57 pm

I've been brewing bigger beers (imperials, doppels, baltic porters) lately w/ OG of around 1.1 (10-12pounds of light liquid malt extract) but still can't get alchol % greater than around 6 %. I am pitching 2 smack packs, shaking the heck out to the wort to oxygenate it and using yeast that should be able to handle higher alcohol (irish, scotch, etc) I read on the wyeast website that I should be pitching 3 packets of yeast for beers w/ such a high OG. Is this my problem? I also read that shaking the wort only imparts 8 ppm of oxygyen where as 12-15 is optimal. Should I be using pure oxygen on these bigger beers? Would the yeast need additional nutrients added to the wort? What am I missing here?
Also, can I fix the 2 batches I have going now; 1. an imperial stout, and 2. a baltic porter. The imperial is finishing up in the primary and the baltic porter is lagering in the secondary.
Thanks a ton
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high alcohol beers

Postby slothrob » Sat Nov 27, 2010 1:06 pm

So your beers are finishing above 1.028-1.043?

10-12# LME is around 1.074-1.089, which I'd expect to finish around 1.022-1.027, based on my extract days, or about 6.8-8.1% abv.

That makes it seem like the yeast is struggling more as you raise the gravity higher, so I think pitching rate, aeration and time are a good place to look.

For a 1.080-1.100 beer, if I was going to use 2 packs of yeast, I'd still make (at least) a 2-3 liter starter. I'd probably use a single pack of yeast and make a 3-5 liter starter. The thing to remember is that the more yeast you pitch, the less you need to worry about oxygenation.

A lot of people do use oxygen to get what they need for big beers, though. I just don't have much experience with oxygen. If you shake, you need to shake for close to 5 minutes to get the most oxygen in the beer as possible. You can't just shake it for a minute.

The other thing to consider is time. The yeast may struggle to eat up the last of the more complex sugars. Letting the fermenting beer sit for longer at fermentation temperature may allow the yeast to bring the FG a little lower. Be careful not to rush racking a big beer away from the yeast, or it may never finish fermenting.

Another thing to consider is recipe. Extract isn't the most fermentable sugar you can use. If you replace a pound of extract with about 0.8 pounds of table sugar you will drop the FG by about 2.2 points and increase the ABV by about 0.3%.
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