In challenging vintages where grapes just don’t get ripe enough, or in areas where summer is cool and the growing season is short, acid reduction methods can really help. The most used acid reduction agents in winemaking are calcium carbonate (CaCO3), potassium carbonate (K2CO3), potassium bicarbonate (KHCO3), or commercial preparations like Acidex or Acidex-K. The carbonate anion reacts with the acids in your juice or wine to form the bicarbonate anion, which reacts with additional acid to form carbonic acid, which then becomes water and CO2 gas.
Because juice and wine generally have an excess of potassium bitartrate (KHT), precipitation of KHT (“wine diamonds” or “tartrate crystals”) will result if potassium carbonate or potassium bicarbonate are used, which is why it’s recommended to follow those additions up with a cold stabilization. This extra step can be a bit annoying and tedious . . . but it is important unless you don’t mind a lot of potassium bitartrate crystals coming out in your wine later. Calcium carbonate may not need this step, but the downside is that solid calcium salts, which can precipitate slowly over time, may also appear in your wine.
I personally tend to use one of the potassium products because the precipitation of any solids tends to be rapid and I always cold stabilize my whites anyway. With reds, I think it’s less of a problem because again, “time is your best fining agent.” With red wine’s long aging trajectory, most of the potassium bitartrate crystals will come out of solution during the aging process, as long as you don’t de-acidify too close to bottling.
If your malolactic fermentation (MLF) doesn’t get your acids into balance as you like, I recommend starting with small amounts of Acidex or one of the potassium-containing products as soon as possible after your MLF is complete. This will give any resulting KHT crystals produced plenty of time to precipitate out. You’ll need to do bench trials of course, starting with small additions first (around 0.25–0.5 g/L, for instance) and working your way up from there. Let the product react with your wine samples at least overnight before you taste and/or do any chemical analysis to assess changes. Always follow the manufacturer’s directions, of course. I also find it helpful to use online deacidification calculators like those found at winebusiness.com (link at the end). With any large wine adjustment, the earlier in a wine’s lifetime it’s made, the better. This will always give your wine its best chance to “bounce back” and integrate those changes more smoothly for long-term aging and character development.
For a wine de-acidification calculator, the following link gives recommended dose rate per desired TA reduction. It also provides answers for many possible different de-acidification agents: www.winebusiness.com/calculator/winemaking/calc/24/