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My second beer brew has pretty much finished its primary fermentation. It started at 1.044 and is currently at 1.014... I don't expect it will go much further.

Having brewed numerous wines I understand that yeasts are capable of getting as high as 15% alcohol by volume before starting to be killed by the very alcohol they are making.

By my reckoning this beer brew is currently sitting at approximately 4% but clearly there is quite a bit of sugar left in the brew for the FG to be over 1.014.

Reading around, it appears that this is perfectly normal - pretty much all beers finish well above 1.000.

So... Why does fermentation stop before all the sugar is used up?

Additionally, how do you determine a 'target' FG? Is this something I can do? At the moment I'm still doing extract brews but would like to move on to grains before too long.

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And just for the record - like wine, which has some residual sugar also, you don't want the beer to ferment out completely dry - too much attenuation is often not a good thing. homebrew.stackexchange.com/questions/7197/… –  mdma Nov 23 '13 at 15:12
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1 Answer

up vote 11 down vote accepted

About 80% of the sugars in the malt extract are fermentable, and about 20% are not. The main fermentables are maltose, maltriose, smaller amounts of sucrose, glucose and fructose. The remainder - about 20-25% are 20% unfermentable dextrins, the remaining 5% other less common sugars with variable fermentability by ale yeasts. Thus typical values are between 75-80% limit of fermentability for an All Grain malt.

With tins of extract, fermentability is usually much lower, since the producer is expecting you to add quantities of sucrose, which is 100% fermentable - so the overall fermentability ends up in the typical 70-80% range.

As well as wort fermentability, different yeast strains show differing amounts of attenuation - the percentage of total sugars fermented mainly influenced by their level of flocculation. So, the target FG is determined both by the fermentability of the wort and the attenuation of the yeast. The attenuation link goes into more detail about how you can find the limit of attenuation (wort fermentability) and the actual attenuation (governed by yeast strain, nutrients and environment). This can be used to determine the expected FG for the batch.

See

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Also, there's nothing "magic" about 1.000. If the yeast were somehow able to consume all available sugars and dextrins, we'd be left with a specific gravity below that of water (1.000) due to the presence of alcohol (at 0.787). In the real world of beer, this difference isn't nearly large enough to overcome the presence of sugars and dextrines you mention in your answer. –  Eric Miller Nov 24 '13 at 18:22
    
Great answer, @mdma, thanks! –  Doug Nov 26 '13 at 16:25
    
Absolutely, @EricMiller, indeed my original question subject did include the phrase "or below" but, for some reason, I edited it to remove that! –  Doug Nov 26 '13 at 16:25
    
So, @mdma, is it a simple calculation to estimate the FG from the OG, assuming that we'd be looking at 75-80%? Let's say my calculation for estimating FG went something like this... my brew starting at 1.044 (44 gravity points) would end up, after complete fermentation, with 20-25% of its original gravity points... i.e. approximately 8.8 to 11. In other words it's FG would be around 1.009 to 1.011. Would that be about right? –  Doug Nov 26 '13 at 16:36
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Yes, that's correct. –  mdma Nov 26 '13 at 16:49
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