How to REDUCE ACID in JUICES—with Calcium Carbonate

By Dale Ims

Grapes grown in cool-climate regions such as the Finger Lakes frequently have high acid levels—with titratable acidity (TA) values significantly above the recommended 0.65% to 0.70% (which may also be specified in metric as 6.5 to 7.0 grams/liter). Winemakers faced with juices having acid contents above the recommended levels have several options: they may decide to finish their wines with some residual sugar to balance the taste, they can reduce the TA by adding water and sugar at some expense to flavor intensity, they can utilize one of the several chemical de-acidification processes described in books and pamphlets, or they can learn to appreciate crisp wines. The chemical de-acidification process utilizing calcium carbonate (CaCO₃) is the subject of this article.

Chemical de-acidification of grape juices has been done for a long time, and it seems likely that a number of different reagents have been used to remove some of the acids from grape juices. The requirements for the de-acidifying agent are fairly clear: the ideal agent would reduce the acidity of juices without adversely affecting the flavor of the grapes. Of course, the agent should also be inexpensive, non-toxic, and readily available. In the 50-odd years that the American Journal of Enology and Viticulture has been in existence, numerous papers have been published on the subject of de-acidifying grape juices, and after reading a number of those papers, it is clear to me that while no agent or process is perfect, within limits the process using calcium carbonate is a good one.

What doesn’t  work

The chemistry of the de-acidification of grape juices with CaCO₃ is—at first glance—pretty straightforward: when you add the powdered CaCO₃ to the acidic juice, the acids in the juice combine with the calcium portion of the reagent to form an insoluble salt, and the hydrogens from the acids react with the carbonate portion of the reagent (CO₃) to produce water and CO₂. The reaction is:

CaCO₃  +  H₂T →  CaT  +  H₂O  +  CO₂

where the H₂T indicates tartaric acid and CaT indicates calcium tartrate. This all looks very clean and tidy, but reality is somewhat different since the juice is a multi-component liquid.

 A major complication is due to the fact that there are two major acids (tartaric and malic) in grapes, and that these two acids have different strengths—with tartaric being the stronger of the two. If we simply add CaCO₃ to grape juices, the reaction indicated above is the dominant one, and calcium tartrate precipitates from the solution. (Calcium tartrate is pretty insoluble in wine-like mixtures of water and ethanol; it has a solubility of about 0.16 grams/liter.) Unfortunately, we may not wish to get rid of the tartaric acid in our juices because if we do, the pH of the resulting wine may rise above the level of 3.6 or 3.7 where bacterial toxicity is no longer assured.

There would be a smaller effect on the pH if we could make our CaCO₃ react with the malic acid in the juice rather than with the tartaric acid; however, there seems to be no way to do that, so the next-best thing is to cause the CaCO₃ to react with the two acids in the ratio at which they exist in the juice. That is done by adding the CaCO₃ to a measured portion of the juice so that all of the acid in that portion of juice is neutralized by the CaCO₃. The final step is then to re-mix the completely de-acidified portion of the juice with the untreated portion.

This seems like such a good idea that it’s hard to imagine it not working perfectly! Alas, it doesn’t, and there have been numerous process variations proposed over the years to improve the final result. A problem arises because the calcium salt of malic acid is actually relatively soluble in wine-like mixtures of water and ethanol (the solubility is about 4.0 grams/liter—some 25 times higher than that of CaT!) As a consequence, some fraction of the CaM may remain in solution and affect the flavor of the wine. The Double-Salt Technique was developed some time ago in an attempt to force the precipitation of the calcium salts of malic acid by causing the CaT and CaM molecules to link together in the form of a ring-shaped molecule which is very insoluble.

The Acidex treatment for high-acid juices (which is apparently no longer available) utilized CaCO₃ as the de-acidifying reagent, but there was also a small amount of the double-salt of tartaric and malic acids to act as a seed for the growth of that species during the process. The Acidex process utilized a portion of the juice in a manner similar to that described above (although the volume of the treated portion of the juice was a bit over the amount which would be completely neutralized by the reagent), and in this case it was recommended that the juice be added to the de-acidifying reagent.

The process for reducing acid with CaCO₃

David Gerling provided me with a one-page document from Presque Isle Wine Cellars entitled “Optimal Tartaric Acid Retention When Using Calcium Carbonate to Reduce Titratable (Total) Acidity” which describes the procedure they recommend. David or I could provide copies to anyone who is interested, but it is my impression that they (Presque Isle) were so intent on making the procedure clear and easy to understand that they made it hard to understand. Here is my (hopefully clear) version:

Step 1:

Figure out how much CaCO₃ is required to reduce the TA by the desired amount in the entire amount of juice:

Total CaCO₃ required = (# of gallons) * (% reduction in TA) * 25

The origin of  the “25” in the formula above is because 25 grams of CaCO₃ will completely neutralize 1 gallon of juice containing 1% tartaric acid.

Step 2:

Figure out how much of the juice to use for the de-acidification:

Volume = [(% reduction in TA) / (initial TA%)] * (total volume of juice)

Step 3:

Place the amount of CaCO₃ calculated in Step 1 in a container large enough to easily hold the volume calculated in Step 2.

Step 4:

While stirring continuously, slowly add the volume of juice calculated in Step 2 to the CaCO₃; there will be some foaming as the reaction proceeds.

Step 5:

Stir thoroughly every 15 minutes or so for a couple of hours, or until there is no foaming noted during stirring.

Step 6:

Filter or decant the juice away from the precipitate; I’ve found that a coffee filter works pretty well. Return the de-acidified juice to the untreated portion and dispose of the precipitate somewhere other than by flushing it down the drain—it is reported that it turns to concrete!

An example

That’s the process, but let’s do an example to be sure that it is clear. Let’s assume that we have 5 gallons of juice that has a TA of 1.1% and we want to de-acidify it to a TA of 0.8%. From Step 1 above, we find that we need:

(5 gallons) * (1.1% - 0.8%) * 25 = 37.5 grams of CaCO₃.

Now, from Step 2 above, we calculate the quantity of juice to use for the treatment:

[(0.3%) / (1.1%)] * (5 gallons) = 1.36 gallons of the juice

Now it’s just a matter of following Steps 3-6 above: we add the 37.5 grams of CaCO₃ powder to a container with a capacity sufficient to contain at least 1.5 gallons, and slowly add the 1.36 gallons of juice to the powdered CaCO₃ while stirring with a spoon. After all the juice has been added, continue stirring for a couple minutes. Then periodically (maybe every 15 minutes) stir the slurry over the next couple hours to make sure you give the reaction a chance to complete.

After a couple of hours, carefully decant the clear liquid away from the precipitate which settles to the bottom of the container. Don’t panic if a bit of the precipitate gets through or if you can’t quite get all the clear liquid; however, a coffee filter can be used to adequately separate the last of the liquid from the slurry. Finally, pour the treated clear liquid portion back into the container with the untreated portion of the juice and proceed with your winemaking process.

Caveats and limitations

The Presque Isle instructions suggest that TA reductions of no more than 0.2% be done via the above technique in order to avoid raising the pH of the juice too high or giving the wines a salty taste. However, an AJEV paper (Munyon, J.R. and Nagel, C.W. Comparison of Methods of Deacidification of Musts and Wines. Am. J. Enol. Vitic., Vol 28, No. 2, 1977) suggests that so long as the starting pH of the juice is 3.2 or lower, a TA reduction as large as 0.4% is acceptable. That gives us all a reason to record the pH values of the juices we purchase—or a reason to buy that pH meter that we’ve been thinking about!

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