Lactic Acid vs Refractometer SG

[conradomioni]

Well, i'm doing a lactic fermentation on a wort with the OG=1043 (11 brix).
Now, by taking a measure with the refractometer I obtained 4 Brix.
I wanna know, to define the real FG, what kind of calc do I have to do?
Does the lactic acid has influence over the gravity?
To convert this measurement on SG, do i just have to consider the correction factor of alcohol calculated by BerrSmith and others convertion calculators or do I ignore the lactic acid formed and just convert the measurement as a unfermented wort? (EG: 4Brix = 15 SG - unfermented wort)

Thanks!!

 

[ajdelange]

Well, i'm doing a lactic fermentation on a wort with the OG=1043 (11 brix).
Now, by taking a measure with the refractometer I obtained 4 Brix.
I wanna know, to define the real FG, what kind of calc do I have to do?

You have to measure the specific gravity of the beer with an accurate hydrometer or better yet pycnometer or digital density meter and the refractometer and then repeat that for subsequent brewings of this same beer. When you have collected enough data pairs to define a regression line and if those pairs are reasonably close to the regression line you can, in future brews, rely on just the refractometer reading which you convert to SG using the regression law.

Does the lactic acid has influence over the gravity?

Of course, but more to the point it has influence over the refractive index of the beer though not very much. An approximately 1% solution of lactic acid has nD = 1.3339 which your refractometer thinks is 0.7% sucrose by weight. A 1% solution is pretty sour and you probably wouldn't encounter that much in the typical sour beer.

To convert this measurement on SG, do i just have to consider the correction factor of alcohol calculated by BerrSmith and others convertion calculators or do I ignore the lactic acid formed and just convert the measurement as a unfermented wort? (EG: 4Brix = 15 SG - unfermented wort)

I'd take the OG and reduce it by the average apparent attenuation of the yeast strain you are using, subtract that from the OG and multiply the difference by the Balling factor for that wort. For example if you have OG of 11 °P and the average ADF is 75% the estimated FG would be 2.75 and the difference 11 - 2.75 = 8.25. The Balling factor for 11 °P is
f = .39661 +0.0017091*11 + 1.0788E-5*11*11 = 0.4167
so your ABW is 8.25*.4167 = 3.44. Divide by the specific gravity of ethanol to get ABV = 3.44/0.791 = 4.3%

There are formulas out there which purport to 'correct' refractometer readings for the alcohol content of the beer but they are, IMO, pretty iffy. They work when they work but when they don't they don't and you have no way of knowing whether your application is one where they are working or not unless you check with a hydrometer.