Effect of lactic acid on refractometer readings?

[kerklein2]

I made a lacto (Wyeast 5335) starter over the weekend for a Berliner brew and checked it's progress last night. pH had dropped from ~8 (tap water is ~9) to 3.6, but gravity as read by my refractometer had only dropped from 1.041 to 1.038.

I'm pretty sure the 1.038 reading is meaningless due to the high concentration of lactic acid at this point which I'm sure alters the refractive index, but wondered if there's any way to extract meaning from it. Not necessarily for measuring the starter, but for the actual brew. I'm doing a full lactic fermentation prior to a sacc fermentation and would love to know how much sugar is left for the sacc.

 

[kerklein2]

Anybody?

 

[mabrungard]

I don't think that the lactic acid concentration would be high enough to alter the refractive index appreciably. It might be at a few hundred ppm, but its still orders of magnitude less than the sugar or alcohol content (parts per hundred) that actually affect the refractive index. I can't say I'm an expert with the refractive index of lactic acid though! I wonder if putting a drop of 88% lactic on the stage would tell us anything?

 

[kerklein2]

Hmm. Hadn't considered that. I was just really skeptical it had only dropped 3 points when the pH had dropped so far.

I should do some experiments mixing lactic acid with a sugar solution and figure it out. I don't know why I didn't think of that...

 

[billl]

You aren't using your refractometer properly. It only coverts to OG like that if you have no alcohol present. Once fermentation starts, you need to use corrected values or use a hydrometer.

http://www.brewersfriend.com/refractometer-calculator/

According to this calculator, based on your starting gravity, your refractometer readings actually translate to 1.027

 

[Thedutchtouch]

A quick googling yielded these numbers for refractive index:
Water 1.33
80%sugar solution in water (guessing this is similar to wort?) 1.38
>85%Lactic acid in water 1.42


Since the lactic acid concentration in your starter is much much lower than 85% and the difference between strong lactic acid and sugar water is 0.04 I would guess it is not having a noticeable effect.


Sources
http://www.sigmaaldrich.com/catalog/product/sigma/199257?lang=en&region=US

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/indrf.html

 

[helibrewer]

what kind of starter did you make that had a starting pH of 8 ???? There doesn't seem to be a way for a wort-based starter to have that high of a pH.

 

[kerklein2]

You aren't using your refractometer properly. It only coverts to OG like that if you have no alcohol present. Once fermentation starts, you need to use corrected values or use a hydrometer.

http://www.brewersfriend.com/refractometer-calculator/

According to this calculator, based on your starting gravity, your refractometer readings actually translate to 1.027

I don't think the calculator is valid for a lactic fermentation.

 

[kerklein2]

what kind of starter did you make that had a starting pH of 8 ???? There doesn't seem to be a way for a wort-based starter to have that high of a pH.

I used 55% maltose and 45% glucose. Specifically, Austin Homebrew's alcohol boost. This was based on a recommendation from Mashweasel on the AHA forums. Granted, I forgot to take a pH reading of the actual starter, so I mixed up the same ratio in a measuring cup (~4oz or so) and measured that.

https://www.homebrewersassociation....6af49b5fbc54c&topic=10195.msg126144#msg126144