View Full Version : mid stage back-sweetening, feedin' the beasties

r¡g¡dRºº$t€r

05-18-2011, 04:19 AM

right, so i started a batch at the moment because i could wait no longer. having not quite as much honey as i would have liked to start with, i added a solution of more honey=nutrient to the batch on the 4th day of fermentation. i measured the sg at each stage. the original SG was 1.103, 1.04before adding the additional honey, and 1.066 after adding the additional honey. to determine the potential alcohol of the entire batch, should i add the difference of .026 to the OSG, to determine potential alcohol?

wayneb

05-18-2011, 01:31 PM

I guess it is time to re-state the formula that one should use in calculating the net specific gravity of the mixture of two liquids of different initial SGs. Here it is:

S1*V1 + S2*V2 = St*Vt

where S1 and V1 are the initial specific gravity and volume of the first liquid,

S2 and V2 are gravity and volume of the second, and

St is the net specific gravity of the mixture of the two, and Vt is the resulting volume.

If you're adept at algebra you can solve for St, once you know that V1 + V2 = Vt

Did you add any water to your honey, or was it just honey plus some dry nutrient? Did you measure the amount (volume) of honey that was added? These will make your problem solving pretty straightforward. You can use the SG of your honey (or a reasonable approximation for it), and the volume of your honey addition as S2 and V2 in the above equation, along with your initial SG and must volume, to calculate an equivalent SG for the batch, had all the ingredients been mixed at the beginning.

The problem as you posed it, is a little less straightforward. Since some fermentation has taken place in between the initial mix and the time when you added the honey, you can't just add the differential SG to get an estimate of the new potential ethanol by volume, since the relationship between SG and ethanol potential is not linear. It is close, but not exactly linear. So changing the SG by an amount midway through the fermentation will change the potential ethanol at the end by a slightly different amount than if the same SG adjustment were made before fermentation started. Is it a big enough difference to matter? Depends on how accurate you want to be.

Chevette Girl

05-18-2011, 01:34 PM

Hey Rooster. What you did is usually referred to as "Step Feeding".

For a rough idea of the change in potential alcohol, I calculate it in steps.

1.103 to 1.040 gives you 8.56% alcohol

do the same using your new SG and the reading when you get your final gravity measurement when everything's done (let's say it finishes at 1.000, it might not get that far or it might go past)

1.066 to 1.000 gives you another 8.97%

Add those together and you get 17.53%.

Alternately, the way you suggested is equally valid because the scale is close to linear... If you take your difference and add it to your original gravity, you get 1.129 to 1.000 which also gives 17.53%.

However, this calculation is off by the volume of your honey, since you added more water (contained in the honey) to your must. Medsen explained to me once how to calculate it but since I never have accurate volume measurements I have not gone through the math myself yet and will not attempt to explain it here... But I see WayneB did!

I did a search on {step feeding alcohol content} and came up with this one, http://www.gotmead.com/forum/showthread.php?t=17312, where we describe how to do spirit indication testing to determine a final alcohol percent. I did this on a finished maple mead that I'd step-fed because it was easier than estimating volumes. You can probably find more info if you play with the search tool for a while.

TheAlchemist

05-18-2011, 01:38 PM

I guess it is time to re-state the formula that one should use in calculating the net specific gravity of the mixture of two liquids of different initial SGs. Here it is:

S1*V1 + S2*V2 = St*Vt

where S1 and V1 are the initial specific gravity and volume of the first liquid,

S2 and V2 are gravity and volume of the second, and

St is the net specific gravity of the mixture of the two, and Vt is the resulting volume.

If you're adept at algebra you can solve for St, once you know that V1 + V2 = Vt

Sweet! Thanks Wayne. I could have used this when I was constructing Buckwheat Pancakes!

r¡g¡dRºº$t€r

05-18-2011, 05:05 PM

brilliant! those are some truly genius answers! and step feeding makes more sense than what i said

no on the pseudo-linearity of the potential alcohol curve i have this to say: my batch is just 10 litres, could that lend to a negligible discrepancy?

as i am well aware of all my volumes and gravities, doing the math now...

edit: result= ST 1.1196 potential alcohol 16.3% sounds about right! thanks guys!

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