Finishing Gravity 3/4 Rule

[rehnton]

ive heard that FG should be roughly 3/4 of the OG; of course there are a number of qualifiers to this statement but my LHBS uses this as the rule of thumb. unfortunately ive stuggled to to get my beers down this far.

after considerable heartache to aerate, control fermentation & mashing temp, adding yeast nutrient & controlling pitching rate my brews are consistently still finishing above the 'rule of thumb'. For example my last brew = Mild Ale - mashed @ 65C; OG 1034, fermentation temp 20C using windsor finished at 1012!

I wondering is this common or is my fermentation still not up to scratch?

 

[Stuster]

Well, Windsor is known as a low attenuator so that might be all it is in this case. But if it's happening with other yeasts as well it could be a number of things. Have you calibrated your thermometer?

 

[Screwtop]

3/4 = 75/100 = 75%

A lot of ale yeast specs falls between 72% and 75% attenuation given optimal conditions and good fermentable wort.

 

[pint of lager]

That rule of thumb is fine for basic brewing except if you add a heap of malt and or maltodextrin.

For ag brewing, there are a heap of other variables.

Like Stuster said, how accurate is your thermometer at mash temps? If your setup is mashing at 68, this may be your answer.

Reading a glass thermometer is a skill. It must stabilise before you take a reading and must be immersed to the fill line to be accurate. Leaving the thermometer in the mash will also lead to inaccuracies as the whole stem heats up.

Stir your mash for 3 minutes, then immerse the thermometre to the immersion line, gently stir it around in the mash for a minute and then take your reading. Make sure your eye is level with the mercury/spirit.

If there is a mash day in your area, take your thermometer along and see how it stacks up with other thermometers.

Are you adding much crystal? Carapils is also a form of crystal and adding a heap of either will make for high fg.

 

[Trent]

as previously said, there are WAY too many variable for a rule of thumb to apply to all aspects of brewing. You should probably thank yourself lucky that it finished so high, there will be plenty of body left - I recently made a scottish 70/-, mashed at 72C with Maris Otter, fermenter at 16C, used scottish ale yeast (notoriously low attenuator) and even boiled the life out of the entire first runnings - the cheeky little bugger STILL fermented down to 1010 from 1038!
If you are worried about attenuation that much, mash at 63 or something (provided your thermometer is accurate) and ramp up the temp after 2 or 3 days. I never even check my attenuation %ages. Once it is done, it is done. Once you let the yeast go, there isnt too much you can do to meak em eat more or less. Just be happy you didnt get a stuck ferment at 1022!
All the best
Trent
EDIT - Ramp up the fermentation temp after 2 or 3 days. I am not advocating mashing for 72 hours :lol:

 

[rehnton]

thanks guys for the reponses

my thermo is ok but i must admit my temp measuring technique could be improved as there is limited stirring, waiting for temp to equalize then reading going on....i had thought that as my thermo was digital it would be pretty much a 'stick it in & read' type deal.

i do also brew largely british ales so crystal makes up a fair bit of my grain bill (often its 10% of total).......is this a heap?

brewing friday
will try the refined temp testing technique & see how the wort ferments out

 

[Trent]

10% is a fair slab of crystal, and could certainly account for having a higher than expected FG. I assume being a mild, there may have also been a dash of chocolate in there too? Some malts like that, and a low attenuating yeast, mean that 1012 is a fair finishing gravity. I would probably rack it to a secondary fermentor, if it is going to kickstart back to life (sometimes racking will do that) it is much better to do it in another fermenter rather than inside bottles! If you keg, you could fairly safely just keg it I would guess.
All the best
Trent

 

[cold...az]

Your finishing gravity is greatly affected by mash temp ( as well as several other variables) for example:
a 1.034 beer mashed at 69 C might finish at 1.011 or 1.012 whilst mashing at around 63 C might finish at 1.004 or 1.005.
If your mash thermometer is out a bit and you are using a good deal of crystal/dextrine malt etc plus, you use a high finishing yeast and ferment at fairly low temp etc. etc . . . you will certainly finish way higher than you might expect.

 

[bindi]

I thought I would post this here, did a search and found zip.
I have a Belgian going lower and lower [attenuation] using 3787 Trappist [it was once], 33 days fermenting 16 days prime and now 17 day in 2nd. Apparant attenuation 87.5%, real Attenuation 70.7%, Step Mashed 20min 52c, 30min at 62c and 30min at 69c, sparge 75c.
SG was 1.070 [brix 17.5] and now at 1.008 [brix 8] and still dropping, Beersmith says FG should be around 1.015 so I thought it's infected BUT it smells and tastes great , well to me it's outstanding the balance is just right.
I have reused this yeast a fair bit and it may have morphed into something special....Ideas anyone?

 

[mfdes]

Well, I'd say, especially if you used a domestic malt like Galaxy from B&B, it probably fully converted in your 63 degree step. Many of today's malts are so diastatic they convert almost as soon as they hit the water. There's even been comment on a thread of fully modified malts that convert substantially in the temperature range of a protein rest (50-55 C), becoming much more fermentable than expected, hence it not being recommended to do a step-temperature (infusion or decoction) with a fully modified malt.

 

[bindi]

I am aware of the fully modified malts converting at lower temps, some of the grain was not modified so I did a 52c rest, the three mashes I have done since [ all modified malts] I have left out the protein rest but still mashed low [62-63c] and raised it to 69-70c, but this one is so good I try to repeat it [with the rest]. B)