This being harvest season, I thought I'd share a typical consulting conversation. I taught for 24 years a class at the UC Davis Extension called "Fundamentals of Wine Chemistry," and this is a former student's question which encapsulates many nuances of must correction principles which may be of interest to commercial winemakers and home enthusiasts, and may offer for general readership a glimpse into real life winemaking.
We took your “Fundamentals of Wine Chemistry” class last fall, and have a question. We just picked up some Viognier from Paso Robles with 3.5 pH and .5 TA; we have not started fermentation yet.
We’re thinking we want to raise the acid a little. In looking over my notes, I see that you gave an example of a Yolo Semillon that had the same specs, and you said the WRONG thing to do would be to add tartaric. You recommended adding 2g/l of malic acid instead, since adding tartaric would result in too low of a pH.
Unfortunately, our notes from the class did not list the malic component of your example.
Our goal is to capture the wonderful aromas of a traditional Viognier (and hopefully achieve good balance) with a cold fermentation at 50 F.
(the complete specs at harvest are brix=26.5, TA=0.5, pH=3.5, NH3=38ppm, AH=76ppm, YAN=114ppm, malic=2.19g/L, tartaric=4.64g/L, K=1789ppm, VA=0.013g/100mL)
We’d GREATLY appreciate any advice you have!! Thanks!
Go to the head of the class. You are exactly right, and you have avoided the disaster of an extremely low pH, perhaps 2.85 as in my example in 1986. The problem, which is common for German wines, is that at this pH, sulfur dioxide is very volatile, even at very low levels, so it interferes with the nose unless you use so little that you risk having none at all, in which case the wine may brown as did my Semillon.
Be sure your numbers are correct, and then adjust the TA with maybe 1.5 g/L using malic and see where you end up after fermentation. Because it’s lighter, you adjust with less; the ratio of the molecular weights is 0.893, so you really add 1.34 gm/L malic acid to achieve a boost of 1.5 gm/L in the TA.
The combination of this acid add and the precipitation of KHT should lower your pH to a good place, maybe 3.35 and 6 gm/L. That’s probably still too low a TA for you, but will give you a nice healthy fermentation and you can see how you like the result. Depending on how crisp you want the wine, you’ll probably want to finish with 6 – 7 g/L and a pH around 3.2 – 3.4. (I'm assuming no malolactic.) Depending on where you are, adjust further with malic or tartaric, bearing in mind that cold stabilization will further lower the pH and also the TA, and more so if you use tartaric.
Before you taste, it’s best to wait a few weeks for the hotness of freshly added acid to abate before you decide on further adjustments.
Congratulations on your excellent recall.