Some specific gravity questions

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Some specific gravity questions

Postby Kas » Wed May 23, 2012 6:39 am

Here are some of my observations regarding density and ethanol yield, which also lead me to some questions.

So say for example, I have a basic sugar wash with 215g of sugar in 1L of water (plus the regular nutrients and yeast etc).
According to charts, this will give me a SG of 1.08 and a brix of 19.2. Using the formula (0.6xBr) - 1 this gives a potential alcohol of 10.5% abv.
So I can expect around 0.488ml of alcohol per gram of sugar?

Also, when one ferments out until the SG goes down to 1.00, I see many of you referring to this as 'dry', i.e. no sugar left. So say the above wash ferments out to 1.00, then if there really is no sugar left, and I have an ABV of 10.5% then the density should definitely be lower than just 1.00, assuming we have no sugar left and just alcohol and water (according to charts it should be about 0.987) . So I've come to the conclusion that a sugar wash fermented to dry must be something else other than just water and alcohol.
I don't think that the nutrients would do much at all to the SG seeing as you only add around 1tsp per gallon. So this must mean that the yeast produce some other by product when fermenting that makes the SG higher than if it was just alcohol and water?
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Re: Some specific gravity questions

Postby Prairiepiss » Wed May 23, 2012 6:54 am

In an actual sugar wash you would more then likely get below 1.000 more like .998. But if anything else is in the wash and or a less fermentable sugar is used. You will be reading the unferementables. If left long enough it mite or mite not drop below 1.000. But for general purposes 1.000 is a good number to use when referring to a ferment being done. If another type of sugar is used it mite finish out at 1.020. Depending on how many unfermentables are in it. An AG would be a good example of that.

But really its done when the SG no longer changes. Depending on what sugar is used. Bla Bla Bla. Many things come into play. And for the hobby purpose how accurate do we really need to be? Approximations will work for us just fine.
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Re: Some specific gravity questions

Postby Kas » Wed May 23, 2012 7:03 am

So you're saying the only thing that's causing the difference is unfermented sugar? Do the yeast not produce some acids as by products that affect the SG? or is it only bacteria that do this if you get an infection?
I'm not worried about accuracy, just wondering out of curiosity :ewink:
Also what is the advantage of inverting sugar for use in brewing? I know that regular table sugar is sucrose, which the yeast can't ferment directly, but they will create invertase and invert it themselves if they need to.
So I'm assuming that inverting the sugar makes it easier for the yeast because they don't have to invert it themselves? So will inverting it result in a faster ferment?

Thanks.
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Re: Some specific gravity questions

Postby WV Shine » Wed May 23, 2012 7:18 am

You're right, plain sugar type washes often do ferment to less than 1.000 Kas... All grain, wine, beer, other fermentations, might not even get down to 1.00... It depends on the additional things in the liquid, suspended solids, unfermentable sugars, etc. 1.000 is just a general reference point, and is a good target to shoot for.
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Re: Some specific gravity questions

Postby WV Shine » Wed May 23, 2012 7:25 am

Mr. Piss beat me to it :)

Its 'unfermentable', not necessarily unfermented... There are some things the yeast just dont like to eat :) For instance, molasses has a lot of unfermentables in it... and honey has a lot of complex sugars that do eventually get broken down (they just take forevvvver, think 6 months :evil: ).

Inverting the sugar does save the yeast some work, and should theoretically make for a quicker, cleaner ferment. I think the strength of this effect has been debated, you can do a google search and find a lot of threads discussing it. My advice would be to test it out, as ymmv :thumbup:
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Re: Some specific gravity questions

Postby myles » Wed May 23, 2012 8:21 am

I ferment fancy treacle and raw cane sugar for my rum and it never gets to 1.000, if it gets as low as 1.02 I think it has done well.
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Re: Some specific gravity questions

Postby Prairiepiss » Wed May 23, 2012 8:41 am

Anything that is not water can and or will effect the SG reading. Yeast, extra CO2, unfermented sugars, grains, alcohol or what ever else you put in there. So to be truthful a hydrometer will only be completely accurate with a pure water and pure sugar solution. Once you add anything else it ferment starts accuracy is done. It gets you a good ballpark to work from.

Same goes for a proof/tralles hydrometer. It is only accurate on a distilled alcohol. If you add anything to it accuracy goes out the window. Like sugar, fruit, glyserin, essences, even oak. The difference here is anything added this one won get you in the ballpark. That's why volume dilution calculations should be used from a known ABV distilled alcohol.
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Re: Some specific gravity questions

Postby Kas » Wed May 23, 2012 9:12 am

Very interesting stuff guys :)
So here is what I'm understanding from this:
Unfermentables affect the SG reading, but they were there when you took the inital reading, and they will still be there when you take the final reading. So effectively they cancel out when you subtract the final from the initial, and the difference you are measuring is only created by the fermentables turning into alcohol.

Also, since you mentioned distilled alcohol PP; in terms of measuring the density of 'white dog' distilled alcohol (with nothing added) to estimate the ABV, I'm assuming that fusels/heads and other natural flavourings that come over are in so small amounts that they make a negligible difference to the density, and we can assume it is basically water and alcohol.

Cheers,
K
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Re: Some specific gravity questions

Postby Dnderhead » Wed May 23, 2012 9:42 am

you about got it,,,the real way to figure alcohol in a wash is the OG (original gravaty) subtract the FG (final gravaty) then the alcohol if determined on the difference.
this is close,,OG-FG=?X 131=ABV
(or)
OG-FG=?<7.55=ABV
nither way is perfict,,
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Re: Some specific gravity questions

Postby rad14701 » Wed May 23, 2012 3:18 pm

The overall process of guesstimating the %ABV based on Specific Gravity is just that, a guesstimate... The final SG may be lower than 1.000 because of the alcohol content... 1.000 would be pure water... But if there is more alcohol in the wash than detectable, due to unfermentables... A final SG of 1.010 with unfermentables may in fact be 0.998 without them... Well, you get the idea...

Just ferment until done and then you get what you get regardless of SG readings... Sweating the details too much in this hobby will just make you crazy... Theory and reality are two very different things...
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Re: Some specific gravity questions

Postby Kas » Sun May 27, 2012 2:00 am

rad14701 wrote: Sweating the details too much in this hobby will just make you crazy... Theory and reality are two very different things...

I hear you well rad... But I find this stuff so interesting that I can't help my mind from churning it over :roll: Probably the case with most of us here, but without that, the good guys wouldn't turn into experts :wink:

I like to make mead every now and then, and some recipes call for adding more honey in stages, ie. adding another jar a week after pitching the yeast.
So I was thinking about how one would work out the alcohol content from SG readings in this case. My first thoughts were to take an OG before pitching, then a FG a week later before adding more honey, then another OG(2nd) after adding honey then another FG(2nd) once the ferment finishes, and then do ((OG-FG) x 131) + ((OG(2nd) - FG(2nd)) x 131) to get the ABV.
But after thinking for a while I realised you can't just do that.
At the end of the first week you need to regard the alcohol already produced as "seperate" to the "water and unfermented honey + new honey" mixture (both in the demijohn as one fluid).
Work out the density and volume of the "water & honey mixture" (from the current abv and SG and water/alcohol density charts) and then at the end of the ferment take the SG of the demijohn, and work out the SG of the "former water & honey mixture" and work out the amount of alcohol produced by this mixture, and add it on to the "first week" alcohol that you regarded as seperate earlier.
The maths are quite complicated but possible. And yeah, it's way easier to just scrap all that and use a vinometer for the ABV... :lol:
Anyway I'm probably not saying anything that is of much practical use to anyone here, just maybe something that is an interesting read for people who think like me.

Speaking of vinometers, I'm reckoning their accuracy is probably affected by sugar content... But probably way less than alcoholmeters...
What do you guys reckon on the accuracy of them?

K.
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Re: Some specific gravity questions

Postby Dnderhead » Sun May 27, 2012 6:08 am

The vinometer is calculated on the basis of pure alcohol and water solutions, whereas wine is not a pure water/alcohol solution and the accuracy of a vinometer can not be relied upon.

Sugar interferes with the interfacial effects that a vinometer's action relies on and hence the method can only be applied to dry wines.

Similarly other components in wine can also interfere with accurate measurements.

there is nothing available that is accurate..that most can do.
if the tax man cometh they whould take a sample,send it to a lab where it be weighed and then distilled then weighed again .the diferance of corse being alcohol.
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Re: Some specific gravity questions

Postby flyingdutchman » Wed Jun 06, 2012 2:59 am

Hi Kas,
I had a thought on your mead which may or may not work.
I like to make mead every now and then, and some recipes call for adding more honey in stages, ie. adding another jar a week after pitching the yeast.

If I remember right Charlie Papazian had a table in his book that gave a rough number of gravity points /fermentable added. If this isnt close enough, what if you added your original pounds of honey (say 10#) to your water (say 5 gal) take a reading with your brewing hydrometer and get a number of S.G. points (P) (minus what the water originally was b-4 the honey like say 1.00) . From there you should be able to quickly derive your S.G. points per pound of honey added to 5 gal water, (P/10)=(1# honey) /(5 gallon water) for the exact type of honey you are using Then when you do your second addition you already have a very good approximation of how many S.G. points you would gain w/out chancing contaminating the wort/mead?
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Re: Some specific gravity questions

Postby Kas » Sat Jun 09, 2012 3:32 am

Very interesting, I never thought of doing it that way.
That would probably be way more accurate, you'd just have to take into account the extra volume that the honey was adding too, but that wouldn't be difficult at all.
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Re: Some specific gravity questions

Postby Bushman » Sat Jun 09, 2012 4:15 am

The beauty of this hobby is we can get as scientific as we want or if it is just too mind boggling we can do as rad suggests and come up with a close estimate. The more times to duplicate a mash and run the more you accurate our guess becomes.
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Re: Some specific gravity questions

Postby Bushman » Sun Jun 10, 2012 2:52 am

Kas wrote:
rad14701 wrote: Sweating the details too much in this hobby will just make you crazy... Theory and reality are two very different things...

I hear you well rad... But I find this stuff so interesting that I can't help my mind from churning it over :roll: Probably the case with most of us here, but without that, the good guys wouldn't turn into experts :wink:

I like to make mead every now and then, and some recipes call for adding more honey in stages, ie. adding another jar a week after pitching the yeast.
So I was thinking about how one would work out the alcohol content from SG readings in this case. My first thoughts were to take an OG before pitching, then a FG a week later before adding more honey, then another OG(2nd) after adding honey then another FG(2nd) once the ferment finishes, and then do ((OG-FG) x 131) + ((OG(2nd) - FG(2nd)) x 131) to get the ABV.
But after thinking for a while I realised you can't just do that.
At the end of the first week you need to regard the alcohol already produced as "seperate" to the "water and unfermented honey + new honey" mixture (both in the demijohn as one fluid).
Work out the density and volume of the "water & honey mixture" (from the current abv and SG and water/alcohol density charts) and then at the end of the ferment take the SG of the demijohn, and work out the SG of the "former water & honey mixture" and work out the amount of alcohol produced by this mixture, and add it on to the "first week" alcohol that you regarded as seperate earlier.
The maths are quite complicated but possible. And yeah, it's way easier to just scrap all that and use a vinometer for the ABV... :lol:
Anyway I'm probably not saying anything that is of much practical use to anyone here, just maybe something that is an interesting read for people who think like me.

Speaking of vinometers, I'm reckoning their accuracy is probably affected by sugar content... But probably way less than alcoholmeters...
What do you guys reckon on the accuracy of them?

K.


Here is a calculator that should help with amounts of honey vs sugar:

http://www.gotmead.com/index.php?option ... &Itemid=16
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