sonvolt

Recipes 

What factors contribute to a particular brew's final gravity? Ok . . . let me see if I have this right . . .

Obviously potential gravity is a reading of the total amount of sugars that would "potentially" ferment. If every sugar in the brew does ferment, a final gravity reading would yield a 1.000 reading, right. Well, not all sugars are fermentable, so any gravity reading after fermentation will be a measurement of the sugars remaining in the beer which did not convert to alcohol, CO2, and other byproducts. Am I right so far?

Okay . . . now, non-fermentable sugars contribute to mouthfeel, body, etc. So, some unfermentable sugars are desireable as most beer drinkers are not looking to brew very "dry" ales & lagers. In fact BJCP identifies appropriate/expected final gravities in relation to specific styles of beer.

Here are my questions, and what I would like to talk more about . . . What factors contribute to final gravity? Obviously when brewers add unfermentable adjuncts like lactose, FGs will be higher, right? Are there factors in the mash that may contribute to creating a high yield of unfermentable sugars? What specialty grains may add unfermentable sugars?

In the last four brews I have made, I have become very frustrated in terms of FG readings. I did a Belgian Wit which finished at about 1.010 (perfect). Next, I did a berry wheat using virtually an identical grain bill and the exact same yeast - finished at 1.020 . More recently, I did a "Big" APA that finished at 1.010 and an ESB with a very similar grain bill - I think that it is finished at 1.019 which is high for the style.

What things can I do to have more control over my FG?

Lou

Recipes 

No. Pure ethanol has a specific gravity of 0.789. If you had a mixture of just ethanol and water (all sugars fermented, no proteins, starches, etc. present), you'd have a SG somewhere between 1.000 and 0.789 depending on the relative proportions.
that being said, excluding the intentional addition of unfermentable sugars, time and temperature of the mash has nearly everything to do with final gravities... i'm sure someone who is more knowledgable will share more on that.

also, everything in a solution contributes to the gravity of that solution -- not just sugars.

cweston

Recipes 

My understanding is that kilned malts like crystal, caramunich, etc. add a far higher percentage of unfermentable sugars than other grains. So one way to achieve a lower FG would be to scale those back from the grain bill. Ditto with carapils/carafoam.

Mashing at lower temperatures also tends to convert more of the starches into fermentable sugars and therefore lead to lower FGs.

Sucrose is virtually 100% fermentable--this is why so many Belgian styles use it: to have a high alcohol content with a fairly dry finish (lower FG).

Finally, some yeast strains are more attenuative than others. If you take the gravity from malts (not from sucrose, which ferments 100% more or less) and multiply it by the attenuation rate of the yeast, that should give a decent estimate of FG.

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Baron von BeeGee

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Perhaps the Kaiser will pop in...he's quite an expert on mash temps and fixing the fermentability of the wort. In any case, as cweston points out mash temps (which we don't always control as well as we'd like) will have a definite effect on the fermentability of the wort, as well as how long you mash.

Are you aerating your wort, and if so, how? Aeration will have an impact on how well your yeast performs.

Also, the physical condition of your yeast could have a bearing on how well they ferment the wort.

david_42

Recipes 

Final gravities can be under 1.000, I routinely see this in ciders, much less common in ales though.

The main contributors to FG for home brew are: proteins, unfermentable sugars, starches and unfermented sugars. When doing all grain, mash temperature and pH are the main variables. Mash temperature range is very narrow for a highly fermentable wort, 150-152F. Palmer has a good discussion in his Chapter 14. Just a few degrees, high or low, for the mash can raise the FG ten points or more.

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sonvolt

Recipes 

Ok, so for my last batch . . . I used the following grain bill:

12 lbs Pale Ale Malt
1/2 lbs. Crystal 40L
1/2 lbs. Crystal 10L
1/2 lbs. Carafoam

My OG was 1.071 and it is currently reading at 1.019. Mashed at 150 for just over an hour, fly sparged for just over an hour. Given my grain bill, is this about right for a final gravity reading? I feel like it is pretty high, but the OG is also higher than I anticipated as well.

Baron von BeeGee

Recipes 

Which yeast did you use? The attenuation for your yeast strain is just as important as any other factor for calculating the FG.

cweston

Recipes 

That's 73% attenuation. Not sure what yeast you used, but Wyeast claims 73-77% for 1056, for example. So you're in the ballpark, although I agree you might have expected a bit lower. 77% would be more like 1.016.

__________________

Primary: none
Secondary:
Bottle conditioning: Robust Porter
Drinking: Saison Dupont clone, tripel
Coming soon: Columbus APA, Rich Red ale

sonvolt

Recipes 

I usually use White Labs yeasts, but this was a "spur of the moment" brew. I didn't have a starter made, so I pitched two packets of Cooper's Dried Ale Yeast.

Kaiser

Recipes 

Yep, that's what I was about to answer as well.

One thing to keep in mind is that the FG we are measuring is the apparent FG of the beer. This means it has been skewed by the alcohol in the beer. If you want to measure the real true FG you will have to boil out the alcohol and replace the loss with water. Then you can measure the real FG, which can never drop below 1.000. One of the reasons why beers rarely have an apparent FG below 1.000 is due to the proteins and limit-dextrins. The latter are the branch points of the amylopectin which cannot be fermented by brewers yeast and cannot be split by the enzymes in the mash.

But there is the problem of hitting a particular FG with your mash schedule. I guess that's what you want to get at. Single infusion mashes have the inherent problem that the attenuation of the wort is very sensitive to the temperature since the mash is happening at a point where the beta amylase starts to break down and the alpha amylase starts to ramp up its activity.

Are any of you seeing consistent attenuations for your single infusion mashes for a given yeast strain and mash temp?

Since I see the hitting the target attenuation as the main challenge of mashing I also asked on a German brewing board, and the consensus there was that step mashing for saccrification is the answer. The common German step mash has 2 saccrification rests. One is the maltose rest at about 61-63*C (142-148F) which is held for time X. Then the mash is raised to the saccrification rest at about 71*C (160F) where it is held until the mash is converted (shouldn't take more than 20min).

Now why is this more reliable in terms of attenuation, because the rests are at the activity peaks of the amylases (beta a. for maltose rest and alpha a. for saccrification rest) and the amount of maltose produced is determined mainly by the length of the maltose rest, not its temperature. Both rests are fairly insensitive to temp changes of less than +/- 1K (+/- 2F). And time is more reliably measured than temperature anyway.

The careful reader may have already observed, that there is only so much maltose that can be produced by the beta a. until help from the alpha a. is necessary. If you need a more attenuative wort, you would add a rest at 66*C (150F) after the maltose rest. This rest is actually the only saccrification rest that is observed in single infusion mashes.

So far I have done only 3 mashes with stepped saccrification and I got fairly consistent results with regards of the achieved attenuation of the wort. Future step and single infusion mashed will tell me if my theory actually holds or not, or if it is only making things more complicated than necessary.

Regardless of your mashing schedule, you should keep good notes. These should include:
- grain bill
- actual starting mash temp
- actual finishing mash temp
- actual mashing time (make sure you mash-out and never let your run-off drop below 170 when it is in the kettle)
- yeast used
- OG
- amount of yeast pitched
- (forced ferment test when dealing with lagers)
- FG

Based on these notes you may or may not see a trend by which you can plan future mashes.

Kai

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