In what way (if any) does active fermentation affect (a) pH and (b) TA?

[CaptainProg]

I've recently learned how to measure titratable acidity (TA) and have practiced on a white wine which is undergoing active (anaerobic) fermentation. The TA reading is a little high (7.6 g/l, whereas a 'good' range for a white wine is somewhere between 5.5 and 6.5g/l). However, I'm also finding that the pH seems lower than it was prior to fermentation (2.47 as opposed to 3.45 prior to fermentation).

I understand that the CO2 present during active fermentation can "throw off the pH reading", but I don't know in what direction; nor whether this would affect TA also.

Question: In what way (if any) does active fermentation affect (a) pH and (b) TA?

(Note that this question was originally posted on the 'Brew Science' forum as there doesn't appear to be a 'Wine Science' forum. I assume the same principles apply to fermentation of beer as they do to wine, but nobody seemed to know the answer there so wondering if this is something that will be known by the wine crowd...)

 

[toadie]

I am newish to TA measurement but I take measurement before fermentation and well after. When I take the first post ferm measurement I shake the hell out of the sample to remove as much CO2 as possible. Hope that helps.

 

[CaptainProg]

Thanks – I'm not asking what to do, my question is more specifically about the chemistry of fermentation, and in what way the process of active fermentation affects pH and TA. I know that CO2 may affect these things, but how?

Question: In what way (if any) does active fermentation affect (a) pH and (b) TA?

In other words:
(a) does pH go up or down during active fermentation or not change at all?
(b) does TA go up or down during active fermentation or not change at all?

I'd also like to know whether these changes (if any) are true reflections of actual changes in pH and TA (of the wine itself), or if the readings are 'temporarily messed around with' by the presence of active fermentation and 'go back to normal' when fermentation has calmed down.

 

[Raptor99]

Dissolved CO2 lowers the pH. But it can't lower it from 3.45 all the way to 2.47, so something else is going on as well.

 

[CaptainProg]

Thanks @Raptor99. That answers the question regarding pH.

Does anyone know about the effect on TA? Is TA changed by the presence of dissolved CO2 or is TA unaffected?

 

[Jacob_Marley]

Yes, it affects (raises) TA .

De-gas before TA measurement.

The simplest way is to put the wine sample in a small flask (etc) ... put a "stopper with hole" on it. Put your thumb on the hole and shake ... pull your thumb off the hole to let the gas escape ... do this 3 or 4 times.
This is for a couple tablespoonfuls of wine ... for larger samples do the degassing slightly more.
The "stopper with a hole" is used rather than just pulling out the cork because that exposure also puts air/CO2 back in contact with the sample. It's not much but, considering it takes the formation of the acid only about 15 minutes of standard contact of air time ... shaking it in, for lab purposes, can re-introduce to some extent.

(fwiw ... a couple other de-gassing methods involve heating, and also using a vacuum pump.)

btw... using old NaOH also scews your TA due to chemical changes in the reagent from dissolved C02 ... always try to use fresh sodium hydroxide ... and store it tightly capped and ideally in the refrigerator.

 

[Birhanen]

I've recently learned how to measure titratable acidity (TA) and have practiced on a white wine which is undergoing active (anaerobic) fermentation. The TA reading is a little high (7.6 g/l, whereas a 'good' range for a white wine is somewhere between 5.5 and 6.5g/l). However, I'm also finding that the pH seems lower than it was prior to fermentation (2.47 as opposed to 3.45 prior to fermentation).

I understand that the CO2 present during active fermentation can "throw off the pH reading", but I don't know in what direction; nor whether this would affect TA also.

Question: In what way (if any) does active fermentation affect (a) pH and (b) TA?

(Note that this question was originally posted on the 'Brew Science' forum as there doesn't appear to be a 'Wine Science' forum. I assume the same principles apply to fermentation of beer as they do to wine, but nobody seemed to know the answer there so wondering if this is something that will be known by the wine crowd...)

Different acids have different «dissociation» i.e they are different in the magnitude they produce ions in the solution. And it is the quantity of H+ ions that is the pH. When you titrate to find TA you measure the total amount of acid in the wine. Both the ions and the non-dissociated. During fermentation some acids may be produced by the yeast (like acetic acid) from sugars. And some acids may be converted to esters. If you do malolactic fermentation malic acid is converting to lactic acid which dissociate less H+ ions and the pH will increase (while TA will not be much affected). So the answer is that fermentation can affect both pH and even TA. And in both direction. But usually the pH will rise slightly while TA is not much different. This last statement is however more a «believe» than a confirmed by analysis on my part.