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Old 01-12-2009, 02:43 AM   #1
Petho
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Default Predict FG

Hi Guys,
I got another ale in the fermenter today however the recipe that I used does not specify the OG or the FG.
8# pale malt extract
1# 60L Crystal
1/2 # Carapils
1.5 oz Northern Brewer HOPS (7.0%) 60 min
1.5 oz Perle Hops (8.2%) 2 min
Ferment with W.L. Ale yeast WLP 001

I measured the OG at .054
Any predictions on what the FG should be?

How do you calculate the OG and FG, is there a calculation or is it just a prediction?

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Old 01-12-2009, 02:45 AM   #2
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Beersmith has the estimated FG of different styles of beer. I suggest using that. Free Trial plus it is only like $25.

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Old 01-12-2009, 03:08 AM   #3
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Beersmith has the estimated FG of different styles of beer. I suggest using that. Free Trial plus it is only like $25.
While Beersmith can be helpful for other tasks it cannot predict the FG. It tries to but this FG is based on the chosen yeast's attenuation numbers. And since these numbers may not take into account the actual composition of your wort they may not work. The only way to predict a FG before the beer is done fermeting is a fast ferment test (Fast Ferment Test - German Brewing Techniques) where you take a small sample of wort and ferment it with a lot of yeast to complete the fermentation in 2-3 days. This test is espcially helpful for AG brewers who can affect the fermentability of the wort through the mash.

This being said, if you just want to have a ballpark number for your FG I'd say that you should get 1.012 - 1.014 for your recipe. This assumes about 75% attenuation.

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Old 01-12-2009, 07:17 AM   #4
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You can take a quart of your wort after pitching yeast and mixing and ferment that (fast ferment) at a higher temperature and it will finish before the main brew. This gives you the FG ahead of time.

Here is a more accurate text fast ferment: Fast Ferment Test - German Brewing Techniques

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Old 01-12-2009, 02:45 PM   #5
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While Beersmith can be helpful for other tasks it cannot predict the FG. It tries to but this FG is based on the chosen yeast's attenuation numbers. And since these numbers may not take into account the actual composition of your wort they may not work.
I dont know about Beersmith, but I use QBrew and it does predict FG and OG based on what you put in the wort. It is pretty darn close on its prediction.
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Old 01-12-2009, 02:49 PM   #6
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You can take a quart of your wort after pitching yeast and mixing and ferment that (fast ferment) at a higher temperature and it will finish before the main brew. This gives you the FG ahead of time.
A quart is to much and might be a waste of wort. You only need enough to fill a hydrometer anyway. In my case this is 4-5 oz (allow for some buffer b/c of the yeast trub).

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Old 01-12-2009, 03:17 PM   #7
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I don't see why beersmith can't get very close, since yeast strains have a known attenuation value, and so you can judge pretty good using the OG number and maybe even grain bill?

I'm guessing here. Just seems like it ought to be able to factor the factors and come up with a value.

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Old 01-12-2009, 03:25 PM   #8
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I'm not sure why some folks have advocated going to some ridiculously elaborate double split batch to get an estimate of FG when for 99.99 % of the brewers out there it perfectly suffices to just read the frickin estimated FG that Beersmith gives you. Especially when the person asking the question is a new brewer seeking a tiny bit of assistance on one of the most basic topics out there.

Beersmith can't predict FG? Really? What's that number that appears everytime you plug in a recipe called "Estimated FG." It may not be the perfect down to the atomic weight exact final gravity of the beer but who needs that?

Besides, if you split out a cup of wort from your batch to test the smaller quantitiy, the different starting concentrations of yeast, the overall greater impact of the ambient temp on the smaller sample will effect the final outcome to the point where your margin of error is no smaller than the attentuation rates already listed on the yeast packets.

Read the OP's recipe. This is not rocket science. It is extract plus 1.5 lbs of fully malted steep grain. I think the OP will live with a margin of error of a few percent.

Beersmith tells me that your Expected OG is 1.067 and your Expected FG is 1.015 based on that yeast and assuming a 68 degree ferment. Your lower OG could be a result of not squeezing a percentage of the extract into the kettle, leaving some of the sugar in the grain or not mixing the wort thoroughly enough. Because of this your FG could end up a couple points lower.

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Old 01-12-2009, 04:35 PM   #9
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Besides, if you split out a cup of wort from your batch to test the smaller quantitiy, the different starting concentrations of yeast, the overall greater impact of the ambient temp on the smaller sample will effect the final outcome to the point where your margin of error is no smaller than the attentuation rates already listed on the yeast packets.


The idea of the fast ferment test is NOT to mimic the fermentation that is going on in the main batch, as you pointed out this would be really difficult, but to allow for the quick and (almost) complete fermentation of all fermentable sugars in the wort. A detail that is often misunderstood and overlooked. The resulting attenuation will be the attenuation potential and based on what you know about the ability of the used yeast to reach that attenuation you can predict the FG.

But yes, this is not the kind of involvement that the OP wants to go into and I just brought it up b/c I feel that BS has not enough information to make a reliable FG prediction. But I agree that its prediction might be good enough many times. Especially when brewing extract. But I must say that reaching this FG doesn’t mean that your beer is done fermenting.

Kai
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Old 01-12-2009, 04:53 PM   #10
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Originally Posted by Kaiser View Post

The idea of the fast ferment test is NOT to mimic the fermentation that is going on in the main batch, as you pointed out this would be really difficult, but to allow for the quick and (almost) complete fermentation of all fermentable sugars in the wort. A detail that is often misunderstood and overlooked. The resulting attenuation will be the attenuation potential and based on what you know about the ability of the used yeast to reach that attenuation you can predict the FG.

But yes, this is not the kind of involvement that the OP wants to go into and I just brought it up b/c I feel that BS has not enough information to make a reliable FG prediction. But I agree that its prediction might be good enough many times. Especially when brewing extract. But I must say that reaching this FG doesn’t mean that your beer is done fermenting.

Kai
Well now the topic is just an interesting discussion. (And I no longer have that bug in my posterior.) I think for beginners, or anyone working primarily with extract the formulaic calculations for OG and FG are fine. Heck, I can use a calculator to figure them out faster than putting them into beersmith.

I agree with you completely on BS falling short a little on calculating FG with AG. FG on AG is largely up to the brewer. One of the MANY reasons to do AG. I can see the value of doing a satellite ferment to gauge fermentability but with enough experience I think you just know what it will ferment out to.

When I do big Belgians, BS usually tells me FG will top 1.020 or thereabouts. In reality I am shooting for sub 1.010 on those and usually hit it. The thing is my guess is almost always dead on. I have a Golden Strong in secondary right now that I racked on Saturday. BS called for 1.019. I guessed 1.009, it came out 1.010.
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