When and ways to adjust acidity
Q. This is my third vintage growing Cabernet Sauvignon, Cabernet Franc, and Merlot on Mt. Veeder in Napa, California. At Harvest the grapes invariably come in at ~3.8–3.9 pH. I know I can add tartaric acid at the beginning to bring the pH down to a more common pH (3.4–3.5). Can you please explain the process — how and when to measure to determine how much tartaric to add, what the tartaric ppm should be, and any other question I don’t know to ask!
Cynthia Kerson,
Napa, California
A. Cabernet Sauvignon, Cabernet Franc, and Merlot grown on a cool, rugged site like Mt. Veeder can taste wonderful in the vineyard yet show up in the cellar with pH numbers high enough to make you nervous. Unfortunately, there’s no foolproof correlation between pH (measure of free hydrogen ions in solution) and a ppm titratable acidity (TA, total hydrogen ions in solution) addition but I’m glad you’re focusing on pH. It’s the most important number for microbial stability (spoilage bacteria love a higher pH/lower acidity), ageability, and balance.
Let’s start with timing. Most acid adjustments for red wine should be made at the must stage, before inoculation and fermentation, and ideally after the fruit has had a chance to sit overnight and be mixed thoroughly. I like to see the bins or tank get at least one good stir or pumpover before any measurements are taken. At this point in the wine’s life, the freshly crushed fruit is still sorting itself out. This “what did I end up with?” phase is when it’s critical to get a good pre-fermentation baseline for your Brix, TA, and pH. I’m an advocate of early must adjustment; now is the time to make the big swings in chemistry, if necessary. Big changes later on in a wine’s life can yield unbalanced wines that taste unintegrated.
Once the must has had time to soak up, pull your initial sample. If you do not have your own pH meter and TA lab setup at home, ETS Labs in St. Helena or My Enologist in the city of Napa are wonderful resources; I currently use both in my winemaking. They will provide free sample tubes and labels if you stop by and even have a courier service for sample pickup, especially during harvest. One basic juice panel at this stage covering Brix, pH, and TA may be all you need to make an informed decision for the overall balance of your wine lot.
Bench trials, however, can take your decision-making to the next level. You’ll need a graduated cylinder and small pipettes that can measure at least down to 0.1 mL. Micropipettes are even better (like those made by Eppendorf) and are workhorses in the labs of home and larger-scale wineries alike, allowing you to work with smaller samples.
Work with 100-mL aliquots (or even 50-mL if you have an Eppendorf to measure small amounts of your stock solution) to avoid wasting precious volume. Prepare a tartaric stock solution by dissolving 10 grams of tartaric acid into 100 mL of warm distilled water, giving you a 100 g/L solution. In a 100 mL juice sample, each 0.1 mL of that stock represents a 0.1 g/L, or 100 mg/L, tartaric increase in the must.
If you work with large batches (like you crushed over 1 ton of grapes) and have a pH meter at home, you could add 0.1 mL of the stock to your 100 mL sample, mix well, wait a few minutes for equilibration, then measure pH and taste your sample. Repeat this process, keeping detailed numerical and tasting notes as you go. At first the pH may barely move, which reflects the natural buffering of the must. Keep stepping up until you begin to see a more noticeable change. Realistically, you might not want to just go up in 0.1-mL increments, which could take a while (and a big ETS bill). Try adding 0.3 mL at a time to find where the pH really starts to change — but don’t go beyond your target. That’ll give you an idea of the narrower zone in which you want to be working.
pH almost always rises during fermentation and especially after malolactic fermentation (MLF). If your juice is already at 3.8 or 3.9, finishing well above 4.0 (and into spoilage microbe danger territory) isn’t unusual. That is why I recommend aiming to adjust red must down to around 3.45–3.50 before fermentation. If things behave as they typically do, you will often land somewhere around 3.65–3.70 after malolactic, which is a far more manageable place for long-term aging, color stability, and sulfur dioxide effectiveness.
Once your bench trial tells you how many grams per liter moved the pH of your 100-mL sample into that 3.45–3.50 zone, scaling up to your actual batch size is straightforward. One gram per liter equals about 3.8 grams per gallon. So, if your trial shows that 0.8 g/L, or 800 mg/L, is needed, that works out to roughly 3 grams per gallon. Multiply by your total batch gallons (estimate gallon yield from must at ~160 gallons/ton or 0.08 gallon of wine per lb. of grapes crushed), dissolve the tartaric completely in a small amount of warm water or juice, and add it during an early pumpover or punchdown right before you inoculate with yeast so it gets thoroughly mixed into the must.
It is also important to remember that acid chemistry does not stop once fermentation is underway. As malolactic bacteria eat malic acid and turn it into lactic acid (half as acidic) you’ll likely see your pH creep upward again after MLF. This is why I always suggest re-checking pH once malolactic is complete and before the first rack and SO2 addition. If the pH has drifted back above your comfort zone, you can repeat the same small bench trial process on the finished wine, again using 100-mL (or 50-mL) samples. Because you’ve presumably already made the “big corrections” earlier in your winemaking timeline, these should be small tweaks at this point.
Think of each harvest as building a small personal database about how your vineyard behaves. Keep good notes and over time those few bench trials and lab panels will tell you far more about your site than any guesswork ever could.
