What Dictates Final Gravity Readings?

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vinny

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I understand that yeast are tolerant to different ABV's and no matter how much sugar you add a certain strain can only survive to a certain alcohol %. Some are going to be 13% other 16-18, etc.

That aside, what dictates where a wine will ferment out dry?

For example, I just made 2 kits, I chaptalized both to 13%. One finished At .996 the other .994. My Dragons Blood was at .994 and I lazily decided it was done. After adding clearing agents it continued to ferment out to .990.

For all of these I was aiming for ~13% ABV. All batches had the same yeast, tolerant to 18%. It wasn't alcohol % that stopped fermentation. What are the variables that account for the differences in FG?
 
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The final sg is determined by the phase of the moon, the time of the year, the outside temperature, the inside temperature, the last time you took a shower, the day of the month, the amount of non-fermentables in your wine, how well you can read that hydrometer, probably 50 other things I can't think of right now.

And then we get to the ABV calculation, all of the formulas are approximations. They depend on how well your yeast, in your must converted the sugar to alcohol. The only real way to know your ABV % is to send it off to a lab. Being a homewinemaker, I just go with close enough and does the SG appear not to change for a few days in a row.
 

vinny

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The final sg is determined by the phase of the moon, the time of the year, the outside temperature, the inside temperature, the last time you took a shower, the day of the month, the amount of non-fermentables in your wine, how well you can read that hydrometer, probably 50 other things I can't think of right now.

And then we get to the ABV calculation, all of the formulas are approximations. They depend on how well your yeast, in your must converted the sugar to alcohol. The only real way to know your ABV % is to send it off to a lab. Being a homewinemaker, I just go with close enough and does the SG appear not to change for a few days in a row.

You forgot barometric pressure, but point made.

Faaaggetaboutit?
 

Rice_Guy

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On a small ferment the phase of the moon is a reasonable guess. With industrial fermentations in a reactor conditions can be controlled hour by hour so that things are constantly added or waste product removed. In the scheme of things there probably are factors that are unknown, the technology is such that if we add a rich soup as yeast cells or coconut milk it works.
What are the variables that account for the differences in FG?
* nutrients in general, YAN is a biggie as well as vitamins, sterols, manganese
* the quality of sugar, honey is known to have some non fermentable sugars, or fructose will ferment early but not at the tail end.
* pH, yeast peter out below pH 2.8. This is complicated because yeast produce CO2 which lowers the pH and at lower temperatures more will stay dissolved
* temperature
* oxygen
* killer compounds as in some yeast are killer strains
* . . . the list is long, and smaller nutrients are hard to detect, zinc as an example is essential for mammals but the level of galvanizing on a cage is enough to provide the requirement.
 
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Craig and David are spot on. Also, the ABV itself affects the FG, as alcohol has a lower SG than water. I've also noted heavier reds may not have as low a FG, which I assume is related to the higher level of solids in heavy red wine.

My FWK Forte's from last fall are at 0.999 and 0.998, respectively. I expected 0.996, but accept that the SG is what it is. Neither tastes of sugar, so I'm sure they are done.
 

vinny

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Thanks gents. I do believe I get it. Dragons blood ferments out lower due to the fact that it is a light fast wine with little else to effect readings? Suspended solids, etc?

Deep 'thick' reds will actually read out as more substantial according to SG?
 

Raptor99

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Anything dissolved in the wine will affect the SG. Alcohol lowers it and dissolved solids raise it. If you added some salt to your wine (not recommended!) it would raise the SG. So it makes sense that a wine with more dissolved solids will have a slightly higher SG than one with an identical ABV but fewer solids. Of course this is also true with the pre-fermentation must. If a must has a lot of dissolved solids, the OG would be slightly higher than one with an identical amount of sugar but fewer solids.

The change in SG, from OG to FG, might gives some indication of ABV. But what if some of the solids settle out between those two measurements? Then the ABV might not be quite as high as you think it is. Anything that you add in secondary might also affect the SG.

Sometimes we forget that SG readings are not a direct measurement of sugar content. They only give us an approximate sugar level. In my case, if my estimate of ABV is within 1-2% I am happy with that.
 
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Thanks gents. I do believe I get it. Dragons blood ferments out lower due to the fact that it is a light fast wine with little else to effect readings? Suspended solids, etc?
That's my take on it.

Deep 'thick' reds will actually read out as more substantial according to SG?
That appears to be true -- most of my reds stop around 0.994 to 0.996. The Forte kits I've made have added fermentation oak and maceration enzymes, and the kits include skin packs, a seed pack, and aging oak. I was surprised at the FG, but after thinking about it, I'm not as surprised.
 

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