Help With Measuring Acidity Of Sauergut

[BeerMakesPeopleHappy]

Hello,

I've been reading a lot about using sauergut for biological acidification lately. Exciting topic! I plan to use it for the first time in an upcoming Munich Helles.

Over the past week I soured my first batch, but am a little confused about measuring the acidity in order to know how much to add to the mash. I don't have a deep understanding of chemistry, so I have no clue if I'm doing it correctly. Here's my process:

1. Make a starter from DME, same as a yeast starter.
2. Add lactic acid to drop PH to 4.5
3. Inoculate with raw pilsner malt (Weyermann), 1/4 cup per 500ml
4. Hold at 48C for about 5 days.

The final result was a brown-ish, sweet, grainy liquid that tasted mildly tart; it reminded me of kombucha a little. The measured PH was 3.8. That seems about right.

I used a wine making acid test kit in order to get a rough idea of the acid %. I drew a 15ml sample according to the instructions, but unlike wine samples I've measured in the past, there was no color change when I added the sodium hydroxide. So I tried another approach for measuring acid using a PH meter. I neutralized another 15ml sample to PH 8.2, which required 3.5cc of Sodium Hydroxide(1/5N). My understanding is that's about an acid content of .35%. It seems kind of low. Can anyone tell me if I'm testing this properly? I'm not quite sure why 8.2PH, it's just what I read on line.

Thank you!

 

[rabeb25]

http://www.********************/ingredients/a-sauergut-reactor/

Bottom of the page.


Cheers

 

[ajdelange]

In doing this test the hydroxyl ions from the sodium hydroxide (NaOH) combine with hydrogen ions from the lactic acid (HLac) to form water:
HLac + NaOH ---> H2O + Lac- + Na+
As hydrogen ions are consumed the pH rises and eventually all the hydrogens have been stripped from the HLac molecules and addition of more NaOH just increases the number of free (OH)- in the solution since there are no more hydrogen ions to neutralize them. At pH 8 there are but 10^-6 mols of (OH)- in the solutions (as opposed to 10^-7 at pH 7 and that is considered to indicate that you have gone far enough with the NaOH addition to get all the lactate associated hydrogen ions. More to the point, it is near the pH where phenolpthalein indicator changes color.

Since the colors associated with phenolpthalein are red and clear I don't see how vintners are able to use it with red wines and I expect the color of the sauergut may have obscured the indicator's color in this case as well. That's where the pH meter comes in. Titrate near pH 8 and read the amount of base required and then convert that to acid % per the usual formulas.

It is, in terms of the way we calculate mash pH, more meaningful to determine the number of mEq (OH)- required to neutralize a given amount of sauergut but that is not the traditional way.

 

[BeerMakesPeopleHappy]

Thanks for the great responses! You've helped me understand the process a lot more.

One last question:

Rabeb25's link mentions using a .1N NaOH solution. The solution I have is 1/5N. Are these the same?

Thanks again!

 

[ajdelange]

No. As 1/5 = 0.2 the 1/5 N base is twice as strong as the 0.1 N = 1/10 N base. Thus multiply your base utilization by 2 before using formulas for calculation based on 0.1 N NaOH.

The Normality (N) is the number of mEq of OH- contained in 1 mL of solution. Thus if you use 2 mL of 1/5 N NAOH to neutralized 100 mL of sauergut the strength of the sauergut is 2 mL * .2 mEq/L= .4 mEq/0.1L = 4 mEq/L. If you used 3.5 mL to neutralize 15 mL of sauergut the strength of the sauergut would be 3.5 mL * .2 mEq/mL /0.15L = 4.667 mEq/L. If the proton deficit of your mash is 10 mEq you would need 10/4.667 = 2.14 L of this sauergut.

 

[BeerMakesPeopleHappy]

Great! Thank you for all your help!!

 

[Tomabrew]

Ah, I am getting a headache or maybe I already have one, it's New Year's Day. I am new to the whole idea of Sauer Gut. I am getting it slowly though. I have read that "you will generally reach about 0.8% acid after a 24h incubation. At that concentration it takes 60 ml sauergut for every kg of malt in your mash to drop by 0.1 pH, and 30 ml to drop by 0.1 in the boil". Not sure how the formulas outlined above get to the .8% acid. using the 60 ml per kg of sauer gut for the mash seems easy. If the formula outlined above says that, I need help. I can follow the math, just missing somethings to see it. Any help would be great.

Tomabrew

 

[ajdelange]

You may have a worse headache in a minute but here goes. Lactic acid is a weak acid meaning that it does not release all its protons at moderate pH the way a strong acid (like hydrochloric) does. The fraction of the protons released increases with pH. By the time pH 8.3 is reached in a titration nearly all the protons have been released. The tricky bit is that at the typical pH of sauergut a fair chunk of the protons have already been released. This fraction is:

f = 1/(1 + 10^(3.68 - pH))

Thus the fraction of the protons already free in a lactic acid solution at pH 3.8 is 56.9%. This means that in titrating to pH 8.2 you will absorb the remaining 100 - 56.9 = 43.1%. This will take 0.431 mEq of base for each millimole of acid. OP used 3.5 cc of 0.2 N base to reach pH 8.2 meaning that 3.5*0.2 = 0.7 mEq of protons were absorbed. Since this is 43.1% of the number of millimoles of lactic acid in the 15 mL they must contain 0.7/0.431 = 1.624 mmol. The molecular weight of lactic acid is 90.08 so that means the 15 mL contain 90.08*0.7/0.431 = 146.3 mg lactic acid. Assuming this sauergut has a density of 1 gram per cc the percentage by weight of the lactic acid is 100*146.3/15000 = 0.975%

What is important to you, however, is not the strength of the solution in terms of the weight of lactic acid (though it seems to be so to vintners) but the mEq of protons released in transitioning to mash pH from original sauergut pH. Assuming mash pH to be 5.5 all but 1.5% of the protons will be released ( 1/(1 + 10^(3.68 - 5.5)) = 98.5% ) so if you determine, in the titration, that it takes 0.7 mEq of base to get all the protons out of 15 mL you can correct that to 0.69 (if you like or just stay with 0.7 - should be close enough) meaning that the normality of the sauergut WRT mash pH 5.5 is 0.69/15 = 0.046 N meaning that each mL of it delivers 0.046 mEq protons and each liter 46 mEq. From this alone you can calculate how much you need. If your mash proton deficit is 92 mEq you would need 2 L.

3.68 is the pK of lactic acid: a measure of the energy required to get a proton off it. If the pH is equal to the pK half the protons will be released at that point. If the pH is 2 or more higher than the pK then you can assume that all the protons have been released.

 

[Tomabrew]

Thank you AJ. I will let that soak in for awhile and check back with you if I still have questions. Thanks again, your help is always appreciated.