PDA

View Full Version : Attenuation & Final Gravity Q's



OldSoul
03-29-2015, 06:00 PM
I know there are many factors that affect what FG a mead will finish at, but in a hypothetical situation where the yeast is supported with the perfect environment throughout fermentation (appropriate nutrient additions, temperature control, pH balance, O2 levels, and never goes above the alcohol tolerance), then is it the "attenuation level" of the yeast that determines where FG will end up?

Basically I'm trying to understand why different yeasts end up finishing at slightly different FGs, if each yeast is optimized throughout fermentation. If they all finish dry, why would some yeasts consistently end up at 1.0 and others at .996 (again assuming optimal fermentation environment)? Why wouldn't all yeasts finish in that scenario just end at 1.0 when all the sugars are consumed?

And staying with the hypothetical "optimal fermentation environment" example, if a yeast never hit its alcohol tolerance, would the FG be different for two different OGs if they both finished dry? Meaning if a yeast had 18% alcohol tolerance, would the FG be different for a mead that had an OG of 1.085 compared to a mead that had an OG of 1.099?

For some reason I'm struggling to wrap my head around this, thanks in advance for any insight.

Cheers,

Billy

loveofrose
03-29-2015, 07:17 PM
Hi Billy,

Read the article in my signature in the yeast section. If it's still not clear, let me know. Always trying to improve!


Better brewing through science!

See my brewing site at www.denardbrewing.com

See my Current Mead Making Techniques article here:
http://www.homebrewtalk.com/current-mead-making-techniques.html

OldSoul
03-29-2015, 08:25 PM
Thanks for the reply Bray- I've actually read that post before at one point (as someone who also uses different ale yeasts for mead, I'm grateful for all the info. you've shared everywhere). I'm not sure it helps clarify the answer for me though; or at least I'm still struggling to wrap my head around it.

So in an optimal fermentation environment what causes different yeasts to reach different FG's, assuming in this hypothetical scenario they all ferment dry and don't ever reach their alcohol tolerance? Is it simply that certain yeast attenuate higher than others, and that's why some go below 1.0?

Cheers,

Billy

clone63
03-29-2015, 11:12 PM
Alcohol has a specific gravity of 0.79 or something... As alcohol can make up a fair amount of the solution (wine/mead) it starts bringing gravity below simple water (1.0). There are also maaaany other chemicals from everything that's been put into the fermented solution, and produced by the yeast, all affecting gravity a teensy bit.
As for why some yeasts may take a dry mead drier than others- though it doesn't happen much, especially when not near abv limits- I would guess because some yeasts ferment different types of sugars, like maltose.

Ironpapa40
03-30-2015, 08:22 AM
Aside from the obvious difference in fermentation kinetics from strain to strain, there is another slight problem here. You are trying to apply man made systems of measurement to an organism that doesn't know it exists. Yeast don't know what gravity measurements are, nor do they know how much alcohol or sugar percentage there is.
Differences from one to another can be due to an adaptation response or it could be as simple as a difference in the strength of the cell walls. While there are many variable to consider, you must realize it all comes back to the fact that our way of putting numbers and scales of measure to things doesn't mean a thing to the yeast.

Medsen Fey
03-30-2015, 08:31 AM
There are other factors as well. Some yeast produce more glycerol and more glycopeptides that can affect gravity. And the same yeast strain may produce different level with different fermentation conditions (such as in the presence of SO2, or temperature related). There are various unfermentable sugars in honey, but some yeast may be able to tackle a few more than others, but each batch of honey will be different. Meads rarely ferment as dry as wines which often finish at 0.990-0.995. The yeast will ferment up to their ABV tolerance, and the concept of attenuation is really a beer brewing phenomenon.

OldSoul
03-30-2015, 11:34 AM
Thanks guys- this all helps a lot.

Cheers,

Billy

bernardsmith
03-30-2015, 04:43 PM
Not a chemist or a biologist but in wine making I think a yeast's attenuation is 100 percent because all the sugars (for all intents and purposes) are simple and are fermentable. Not so with starch converted sugars where you can find long complex sugars that some yeasts can tackle with more or less success and hence their "attenuation"

OldSoul
03-30-2015, 07:54 PM
All makes sense, thank you! Have one more question somewhat related to all this: If there are two meads (identical ingredients used) but one had an OG of 1.080 and fermented down to a FG of 1.000, and another one that had an OG of 1.100 and also fermented down to 1.000, would you expect a significant difference in taste besides the latter one being more alcoholic?

Since both meads finished dry with no residual sugar, would the mead with the higher OG be likely to have any more "body" or honey flavor than the other one?

Cheers,

Billy

Medsen Fey
03-30-2015, 09:41 PM
Both meads still have residual sugar. 1.000 is not dry. As mentioned above the specific gravity of ethanol is 0.790. If you took water with no sugar and add alcohol, you get a gravity of <1.000. If you have two batches, and one starts with a higher gravity and both end up at 1.000, the one that started at the higher gravity has more ethanol and should have a lower gravity for the same amount of residual sugar. In you example, they both end at the same gravity, so the one that started higher has more residual sugar (and/or glycerol or other higher alcohols or yeast byproducts). It will have more body due to more alcohol and more residual sugar, and will probably taste sweeter, and since it has more honey, might develop more honey character with age.

Sent from my THINGAMAJIG with WHATCHAMACALLIT

OldSoul
03-30-2015, 10:17 PM
Great explanation as usual Medsen, appreciate it.

bernardsmith
03-31-2015, 03:55 PM
All makes sense, thank you! Have one more question somewhat related to all this: If there are two meads (identical ingredients used) but one had an OG of 1.080 and fermented down to a FG of 1.000, and another one that had an OG of 1.100 and also fermented down to 1.000, would you expect a significant difference in taste besides the latter one being more alcoholic?

Since both meads finished dry with no residual sugar, would the mead with the higher OG be likely to have any more "body" or honey flavor than the other one?

Cheers,

Billy

Since the higher gravity mead had about 1/2lb more honey per gallon I guess I would assume that all other things being equal, that mead ought to have more of the honey flavor and aroma in every glass. Now , of course, it is possible that the lower gravity mead was made with a honey with far greater floral notes and far more concentrated flavor than the higher gravity mead but all things being equal if the same honey was used and the same water and the same yeast etc etc I would expect more flavor and more aroma from the mead that started off at 1.100 than the mead that started at 1.080...
As to whether it might also have more body... Here I am more agnostic. Better mouthfeel, I would think (because richer ) but not necessarily more body (assuming "mouthfeel" covers a multitude of factors with "body" being but one)...