Carbonation and acidity titration

[Claude Jolicoeur]

It has often been mentioned here that when evaluating the acidity of a cider, it is necessary to decarbonate it prior to measurement, as the dissolved CO2 may affect the result. However this effect hasn't been quantified as far as I can remember.
So I made a test to see by how much the dissolved CO2 could change the TA.

I opened a bottle of a good well sparkling cider. I evaluate the carbonation of this cider to approximately 3 volumes of CO2, which would correspond to about 6 g/L of CO2 in solution.

1st test, I measured the TA as fast as I could after opening the bottle in order to have maximum possible CO2 in solution. Result was 6,6 g/L as malic acid equivalent.

2nd test, I heated the sample in the microwave oven. I didn't boil it, but heated it enough that it started smelling overheated (somehow like an overpasteurised cider, but worse). Result was 5,6 g/L.

3rd test, I used a large syringe to create a vacuum. I fill the syringe to half with cider, and I pull on the piston while putting my finger on the end to block the entrance of air, which causes a vacuum inside. Then I remove my finger, letting air in to equilibrate the pressures, and push on the piston to evacuate the gases. The procedure is repeated 4 to 5 times. Titration result gave 4,3 g/L.

Conclusions...
Yes the effect of dissolved CO2 may change the TA measure in a very significative way. In this case, the CO2 increased the TA by 2,3 g/L as malic.
Creating a vacuum in a syringe appears to be a more efficient way to decarbonate than heating. From the results obtained, heating removed less than half of the amount of CO2 that was removed by the vacuum method.

Claude

[Alexander Peckham]

Out of interest Claude why did you not boil the cider?  Surely Malic isn't volatile?  Alex

[Claude Jolicoeur]

Le vendredi 16 décembre 2016 13:01:49 UTC-5, Alexander Peckham a écrit :

Out of interest Claude why did you not boil the cider?  Surely Malic isn't volatile?  Alex

There is no particular reason...
I had always thought that a quick heating (like 15 seconds) in the microwave was sufficient to clear the CO2.
Now I know I was wrong...
More testing could be done to evaluate how much heating (or how long boiling) would be required to completely clear the CO2.
However for my part I have been using the vacuum method as my regular procedure for quite a while now, so I have no real incentive to investigate further in the heating/boiling method.
Claude

[Nicholas Bradstock]

A point occurs to me following Claude's very interesting report of the apparent contribution of CO2 to titrated acidity.

If acidity needs to be known to achieve a proper balance of sweetness and acid, then perhaps the TA should be established without interference from CO2.

If acidity and pH need to be known to ensure a final effective SO2 addition, then perhaps the contribution of acidity by the CO2 should be taken into account.

I would ask Claude's views on this and whether he also found significant variation in pH, as with acidity, in his 3 part test?

Best wishes

Nick

iPhone

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[Claude Jolicoeur]

Le vendredi 16 décembre 2016 17:59:55 UTC-5, Nicholas Bradstock a écrit :

A point occurs to me following Claude's very interesting report of the apparent contribution of CO2 to titrated acidity.

If acidity needs to be known to achieve a proper balance of sweetness and acid, then perhaps the TA should be established without interference from CO2.

If acidity and pH need to be known to ensure a final effective SO2 addition, then perhaps the contribution of acidity by the CO2 should be taken into account.

I would ask Claude's views on this and whether he also found significant variation in pH, as with acidity, in his 3 part test?

Your points are quite pertinent, Nick! And I don't have the answers...
In fact, when I decided to make this simple test, I thought the effect would be much smaller, and I didn't bother to measure the pH.
There is also another reason why one would wish to monitor TA during fermentation, and this is to evaluate malic acid loss due to MLF - and for this it is necessary to eliminate CO2 completely.
It seems to me we have some more tests to make on this!
Claude

[Nicholas Bradstock]

Always the way perhaps!

Research always brings a need for more research.....

My thoughts came from the very significant impact of CO2 on titrated acidity.  Since then Ive gone on to wonder whether CO2 acidity - carbonic acid- is inherently self-buffering and so the pH does not change easily.

All best wishes

Nick

iPhone

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[Andrew Lea]

Carbon dioxide dissolves in water to give carbonic acid. CO2 + H2O = H2CO3. Carbonic acid at the malic-buffered pH of cider (pH ca 3.5) is all in the molecular form H2CO3. It is not ionised hence contributes no free hydrogen ions hence it has no effect on cider pH. See the equilibrium diagram. When you boil off the CO2 you simply leave behind H2O and there is no change in hydrogen ion concentration. Hence the pH does not change. 

However, those hydrogens in the H2CO3 are 'replaceable'. That is, if you titrate the cider without degassing, they will react with the alkali to form a carbonate salt and hence the dissolved CO2 does affect the titratable acidity even though it has no effect on the pH. 

Andrew 

Wittenham Hill Cider Portal
www.cider.org.uk

[Martin campling]

On Friday, December 16, 2016 at 4:45:25 PM UTC, Claude Jolicoeur wrote:

 

the effect of dissolved CO2 may change the TA measure in a very significative way. 

By pure coincidence I had taken TA measurements of two of my batches of cider on the same day Claude posted the results of his experiment. So, I decided to re-test one of them today without boiling off the CO2. Unfortunately, I ran out of NaOH so could only do one test.

The gently boiled version came out at 8.0 g/L malic acid equivalent, the "raw" stuff at 9.1 so this does seem to concur with Claude's results. 

Cheers,

   Martin

[Nicholas Bradstock]

Thank you Andrew for answering and explaining so clearly!

Nick

iPhone

On 17 Dec 2016, at 22:09, Andrew Lea <ci...@cider.org.uk> wrote:

Carbon dioxide dissolves in water to give carbonic acid. CO2 + H2O = H2CO3. Carbonic acid at the malic-buffered pH of cider (pH ca 3.5) is all in the molecular form H2CO3. It is not ionised hence contributes no free hydrogen ions hence it has no effect on cider pH. See the equilibrium diagram. When you boil off the CO2 you simply leave behind H2O and there is no change in hydrogen ion concentration. Hence the pH does not change. 

[William Grote]

Claude

Kudos for discovering a better way to de-gas than boiling  (which always seemed an effort for such a small sample) and for quantifying your results!  I just ordered some larger syringes and thank you in advance!  

[Claude Jolicoeur]

Le lundi 19 décembre 2016 14:06:45 UTC-5, William Grote a écrit :

Claude

Kudos for discovering a better way to de-gas than boiling  (which always seemed an effort for such a small sample) and for quantifying your results!  I just ordered some larger syringes and thank you in advance!  

William, I don't like to take credit that I don't deserve... The vacuum/syringe idea was shown to me by Tim Jones, head cider maker at Willie Smith Cider in Tasmania, Australia, when I visited last year...
Claude