Author Topic: Can't understand what's the calculation for FG/Attenuation  (Read 2012 times)

Offline the dragonlord

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Can't understand what's the calculation for FG/Attenuation
« on: October 21, 2019, 12:35:28 AM »
Good Morning, could someone help me to understand what calculation BS make for FG/Attenuation?

I was expected that if in my recipe I use a Yeast with known attenuation of about X%, the calculation of FG should be OG "with applied" this X%...

I've just brewed a Belgian Tripel using M31 yeast and starting from an OG of 1.083, BS calculates an FG of 1.005 wich will lead to an attenuation of almost 94% wich is ABSOLUTELY unreachable by M31 wich have a given attenuation in the range 82-88%....

I can't understand why in the calculation of FG the given attenuation of the used yeast is completely ignored and what calculations lay behind the calculation of FG....

I'm not telling BS is wrong, of course, it's just me that don't understand....

Could someone help me please?

Thanks and regards

Offline Oginme

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Re: Can't understand what's the calculation for FG/Attenuation
« Reply #1 on: October 21, 2019, 02:46:55 AM »
Apparent attenuation of a yeast strain is determined based upon testing using a nearly all maltose wort.  As soon as you introduce simple sugars, such as dextrose or fructose, into the recipe the straight calculation for apparent attenuation no longer applies.  Simple sugars attenuate completely.  So the program will calculate the apparent attenuation of=n the percentage of the gravity which comes from the grains only and assumes that the simple sugars (table sugar, dextrose, candi sugar, fructose, honey, etc.) will be completely consumed by the yeast.

Not being able to see your recipe, I would guess that you have a fair amount of simple sugars in it which leads to a lower final gravity.  This is very common for higher gravity Belgian styles where the use of such simple sugars is common to achieve a high gravity and yet a very light, dry finish.
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Online GigaFemto

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Re: Can't understand what's the calculation for FG/Attenuation
« Reply #2 on: October 21, 2019, 08:35:37 AM »
If you go to Options->Advanced you will see a section called "Adjust Final Gravity for Mash Temperature". There is a center temperature (153.5 by default) and a slope (default is -1.25%/deg F). My understanding is that if you mash at the center temperature BeerSmith will use an attenuation that is the middle of the range for the yeast. If you change your mash temperature then BeerSmith adjusts the attenuation correspondingly. As Oginme said, that applies to the wort from grains. If you have simple sugars that attenuate 100% then you need to factor that in with a weighted average.