Few theoretical aspects
The pH is critical in winemaking. Starting from grape sampling, going trough crush-pad (juice), fermenting (wine) and prior to bottling after stabilization, pH is very important. Is showing you if the grapes, juice or wine are healthy or not.
A normal pH in white wines is between 2.9 – 3.5 , on the other hand in red wines pH is a little bit higher between 3.3 – 3.6 going over 3.6 in amarone style wines.
When pH is over 3.6 (sometimes in bad years pH can reach 3.9-4.0) it is a critical aspect in wine microbiology, in wine will be perfect conditions for microorganisms developing such mold, yeast, bacteria and the wine can get spoiled faster then you think. We say “wine doesn’t have a healthy pH” or “this is not a healthy wine”. In these situations is mandatory to do a pH correction or like most of the winemakers says to do an acidity correction (which is more correlated with balance of taste for them).
Other aspect regarding the pH in wine is the direct correlation between pH and molecular sulfur dioxide (SO2). We know, for example a Riesling wine need less sulfur additions than Chardonnay. Because, in general Riesling variety (no matter the growing area) has a pH around 3, most of the time 2.9. From this aspect the molecular sulfur (the active part of “Free Sulfur” ) stays more time in the “free form” then reversing/irreversible in to “bounded sulfur”. The direct correlation between pH and molecular sulfur can be found in such tables or formula, when you can find the right amount of sulfur needed, depending on pH of the specific wine.
How to prevent getting high pH in wines and how to fix the problem?
In time, after many vintages you will know how to prevent getting high pH in wines and how to deal with high pH in grapes or juices. You should be able to anticipate the final pH in the finished product. At one point would be like “riding a bicycle”, as winemaker you will know right away when to react and do the pH/acidity correction before is too late.
In practice, when I follow grapes maturity (from the fields) or lets say – when I do grapes sampling, I am not looking only for sugar (Brix level) , usually I am checking also pH, TA (total acidity) and VA (Volatile acidity) and like usual, tasting the juice, skins and checking seeds (specially for red varieties). From this perspective I know exactly when the grapes are ready for harvest, what is their health stage (if they can hold another week) depending on pH, TA, VA evolution. In some years you have to do compromises by giving up on brix level for a “healthy pH” or healthy grapes.
The right moment to do a pH correction is before starting fermentation, right after cold settling of juice. During cold settling the juice can have a total acidity dropping between 0.5 – 1 g/L tartaric acid and this can lead to a pH increasing by 0.1 . I know, you will say “well, 0.1 is nothing for pH!”, but you’ll be surprised down the road (after fermentation, aging, cold stabilization) how you’ll crave for that 0.1 dropping on pH. 🙂
After fermentation, the wine can lose more tartaric acid (the best example are the tartrates on tank’s walls). The amount of loses for tartaric acid during fermentation is correlated with temperature on fermentation, potassium content that came from grapes – usually when the farmer give lots of potassium-based fertilizers expect to lose TA during cold settling/fermentation, and depends also on grape variety. In this stage, wine can drop TA again between 0.30 – 0.5 g/L tartaric acid. In this case pH will increase little bit.
In case of malolactic fermentation, expect to have a pH increasing with 0.2 points.
So, for example if you crushing grapes with pH 3.4 expect the finish wine having a pH 3.6 -3.7 . If this is white wine, that’s not good. From this perspective, you have to do a acidity correction with at least 1.5 – 2 g/L TA and the final pH will remain around 3.5.
– Tartaric acid 🙂
Keep in mind, during cold settling/fermentation the wine is not losing only tartaric acid, is losing also malic acid. The ration in grapes/juice between tartaric and malic acid is around 2:1
So, if you add tartaric acid for TA increasing, don’t forget to add some malic acid too. This will help down the road in wine balance and pH.
But about total acidity in wine, we’ll talk in other post.