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Old 05-30-2009, 04:07 AM   #1
jigidyjim
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Default understanding attenuation, beer being done, bottle bombs, etc.

I'm still pretty confused about some stuff that happens when the beer is done fermenting. I guess my basic question is "how do I know I won't get bottle bombs"? But I'm looking for some more technical detail.

Yeast has an attenuation rating. I saw it described pretty well in this thread: http://www.homebrewtalk.com/f39/final-gravity-question-118851/

Yeast does its thing, and fermentation stops. Then I bottle, and add sugar again. For some reason, the yeast starts up again and I get carbonation. So I guess the first question is: Why didn't the yeast continue fermenting the original stuff before I added the carbonation sugar? What is the additional sugar doing that the beer couldn't?

The second question is: does adding the carbonation sugar kick the yeast active again such that it goes back to doing its thing on the original stuff that is still left to ferment in the beer? Does my FG drop again when the yeast is eating the sugar?

Third question: I've read about fermentation getting stuck, or people adding additional yeast to "dry out" their fermentation. How do you calculate an FG when you add this 2nd yeast? Is it based on the OG, or is it based on multiplying the attenuation percent against the gravity of the beer when you add the 2nd yeast?

Finally - is there anything else I need to know about to do my best to avoid bottle bombs, other than don't bottle until the FG is within the attenuation range? (and of course the other advice which is use kegs instead).

I think that's it.



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Old 05-30-2009, 04:37 AM   #2
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i believe the yeast reach a point where it's easier to ingest sugars like dextrose. once the yeast has stopped feeding on what you provide it (e.g. after hydrometer readings are consistent), it ain't gonna feed anymore unless it gets something it can eat. plus the fact that the amount of yeast that ends up in the bottles is a small fraction of what was produced/settled during fermentation.

i think the ABV can be somewhat affected by the addition of priming sugar, but at the volumes most use, it will not make a difference... the danger with bombs is when you bottle before readings are the same, somehow restart a stuck fermentation in the bottles, have an infection, or use too high a volume for the bottles you're using.

i THINK you add more simple sugars (e.g. sucrose/dextrose) to dry out a beer and lower the FG, but maybe additional yeast has something to do with that. i think a stuck fermentation is a whole different subject. there, you're just trying to lower your FG to what it "should" be for the style... with drying out a beer, you're aiming to lower the FG further than what it would be with grain alone.

i wouldn't be too concerned about bottle bombs... not to say they don't happen, but if you're good about sanitizing (the dishwasher has worked well for me, no soap, heat dry), and mix the priming sugar well (put in the bottling bucket and rack the beer onto it), and use the right amount (in weight, get a cheap digital scale), you should be just fine. you can put the bottles in a plastic bin if you're paranoid.

sorry, i'm rambling... good luck!

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Old 05-31-2009, 02:01 AM   #3
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Quote:
Originally Posted by jigidyjim View Post
I'm still pretty confused about some stuff that happens when the beer is done fermenting. I guess my basic question is "how do I know I won't get bottle bombs"? But I'm looking for some more technical detail.

Yeast has an attenuation rating. I saw it described pretty well in this thread: http://www.homebrewtalk.com/f39/final-gravity-question-118851/

Yeast does its thing, and fermentation stops. Then I bottle, and add sugar again. For some reason, the yeast starts up again and I get carbonation. So I guess the first question is: Why didn't the yeast continue fermenting the original stuff before I added the carbonation sugar? What is the additional sugar doing that the beer couldn't?

The second question is: does adding the carbonation sugar kick the yeast active again such that it goes back to doing its thing on the original stuff that is still left to ferment in the beer? Does my FG drop again when the yeast is eating the sugar?

Third question: I've read about fermentation getting stuck, or people adding additional yeast to "dry out" their fermentation. How do you calculate an FG when you add this 2nd yeast? Is it based on the OG, or is it based on multiplying the attenuation percent against the gravity of the beer when you add the 2nd yeast?

Finally - is there anything else I need to know about to do my best to avoid bottle bombs, other than don't bottle until the FG is within the attenuation range? (and of course the other advice which is use kegs instead).

I think that's it.
fermentation stops because the yeast have converted all the fermentable sugars into alcohol. at the same time, its releasing TONS of CO2. when you add bottling sugar (more fermentables) the yeast that are still left in suspension convert it, producing a very very small amount of alcohol and co2. think of it as a very small fermentation going. but all the co2 is trapped in the headspace of the bottle, creating pressure which eventually equalizes throughout the beer = carbonation.


if you were to pitch a second time, you would still use the same OG to measure the gravity. its like if you tried to burn down a house with a lighter, it didnt work, and then you used a flamethrower to finish the job i dont know why i thought up that analogy.

my .02 cents.

hope that helped.
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