Managing High Acidity in Grape Must and Wine
By Jim Harbertson Associate Professor of Enology and Thomas Henick-Kling Professor of Enology
An Early Frost Might Force Harvest of Some Under-Ripe Fruit
Here are some notes on what can be done with fruit that has to be harvested with higher acidity than we typically see in WA. The comments here are based on talks we had with some of you over the last few days.
Monitor pH and Titratable Acidity
Mistakes with acidity can be made in cool years. High titratable acidity is more acceptable in white wines but it is very disrupting in red wines. Excessive acidity has been shown to accentuate astringency in red wines. Since there are several factors including acidity that affect astringency (tannin concentration, alcohol concentration) it important to keep these factors in mind to make a balanced wine. As a guideline, try to have must titratable acidity (TA) for red wines near 6 g/L in some cases, such as Pinot Noir, a TA of 7 g/L might be ok). Finished wine pH can be between 3.3 and 3.8, depending on the tannin content. Low tannin wines typically have lower pH. If the must TA is higher than the goal of 7 g/L then you should use some deacidification.
Potassium or calcium carbonate (K2CO3, CaCO3) can be used to remove wine acids. The addition is typically done prior to fermentation for a couple of reasons. One is because there is less danger of losing aroma compounds that are primarily in non-volatile precursor forms that are less susceptible to loss due to this type of addition. Two, wine yeast and lactic acid bacteria are sensitive to high acidity and low pH. Wine yeast can tolerate pH values below 3 but are stressed and fermentation of such very low pH musts should be done at moderate temperatures around 68°F.
Biological Deacidification – Malolactic Fermentation
Malolactic fermentation (MLF) is an excellent tool to lower the acidity of wine, improve mouthfeel, and remove some unripe, green flavor characteristics. It is used in (almost) all red wines and it works very well in most white wines. Consider especially for Chardonnay, Sauvignon Blanc, and Pinot Gris. It also can be used in Riesling. You might also want to use MLF only in part of the wine and later blend the two wine fractions together (sterile fiter the blended wine to avoid unwanted MLF in the bottle!). In order to avoid buttery ML odors it is best to use a co-inoculation of yeast and ML bacteria. Alternatively, inoculate the wine at the end of alcoholic fermentation and keep the wine on yeast lees. The yeast will remove excess diacetyl and other ML flavors and help enhance fruity flavors in the wine. Alternatively, a yeast fining can be done after completion of MLF to remove excess buttery flavors. A large number of ML starter cultures are available for direct inoculation. Though, all lactic acid bacteria are strongly inhibited at pH values below 3.2. To ensure malolactic fermentation, the must pH should not be lower than 3.2. Ideally, the starting pH for MLF is between 3.2 and 3.4, and in the presence of some alcohol (5% and more). In this condition, Oenococcus oeni will dominate all other lactic acid bacteria. But at pH of 3.1 and below it very much struggles. Special adaptation procedures will have to be used to induce MLF in such low pH wines. For wine with very low pH, it is best to take a small amount and deacidify it to raise the pH to about 3.4, inoculate it with a starter culture and once about 2/3 of the malic acid has been metabolized use it to inoculate another part of wine which can be of lower pH. When inoculating wine with a wine ML starter culture, the starter culture must be 10% by volume. Another possibility is to start with a wine/water/juice ML starter culture. If you need help with this adaptation process, please contact us. If you use a liquid starter cultures, check it under the microscope to be sure it is Oenococcus and does not contain spoilage yeast or unwanted bacteria.
Yeast and bacteria compete for many of the same nutrients. Special nutrient mixes for ML bacteria also can be helpful when inducing MLF soon after completion of alcoholic fermentation. Many of these nutrients are depleted during alcoholic fermentation. The autolyzing yeast relase some nutrients back into the wine.
Deacidification with K2CO3, CaCO3
Potassium and calcium will react with the grape acids (malic and tartaric) to form insoluble salts. Carbon dioxide will be given off as a result of the carbonate portion of the salt formation. Grape acids can have a negative charge and will react with the potassium or calcium that have positive charges. Potassium has a single positive charge and as a result will form several types of salts with tartartic acid and malic acid (K2TA, KHTA, K2MA, KHMA) that are naturally formed during winemaking. Calcium has two positive charges and will only form a couple of salts as a result (CaTA, CaMA). However calcium can form a salt with malic and tartaric acid simultaneously (MAHCaHTA) and the treatment has been dubbed the “double-salt” technique as a result. The formation of the double-salt is actually quite rare but the name persists.
Which Carbonate Salt Should I Use?
If the TA must be lowered by only 2-3 g/L, simply use potassium bicarbonate or potassium carbonate (KHCO3 or K2CO3). If more acid needs to be removed, it is better to use the double-salt deacidification with calcium carbonate. It is important to note that the double-salt technique favors the removal of tartaric acid rather than malic acid, unless the initial concentrations of malic acid are double the concentration of tartaric acid. Thus this technique is carried out on a portion of the juice (~25%) because it would destabilize the wine otherwise by leaving primarily malic acid behind, which is easily metabolized by yeast and bacteria. This means that the amount of calcium carbonate for the entire lot is added to only a portion of the juice. Because such a large concentration of calcium carbonate is used, the treated juice will reach upwards of pH 4.5-6.5. You will need to add back the treated juice to the original lot and then eventually re-adjust the juice to the pH you are targeting with tartaric acid. Typically during the process the juice to be treated is agitated to help the precipitation process but requires only a short amount of time for reaction (30 min in a large tank) and settling (60 min). Two things to bear in mind when doing the calcium addition is that it is not necessary and actually counter productive to chill the juice. Calcium tartrate solubility is unaffected by cold temperatures and carbon dioxide is actually more soluble. Another simple indicator that the reaction is finished is that the bubbles have stopped evolving. Because the reaction generates carbon dioxide there is a danger of asphyxiation and it is better to do the treatment either in and out-door tank or in a very well ventilated room.
Small acid corrections (around 1 g/L) can be done in the wine after alcoholic or after malolactic fermentation, even just before bottling. When deacidifying the wine before malolactic fermentation is important to be careful that the pH does not increase too much. To avoid growth of spoilage bacteria, the pH before MLF should stay below 3.4. Use potassium bicarbonate to remove small amounts of excess acidity before bottling.
White wine TAs are typically higher than in red wines. At target value for white grape must is TA 8 to 11 g/L. Late harvest, botrytized, and ice wines of course have higher TAs since in these wines the acidity is balanced with the high residual sugar (in fact the high residual sugar left needs a high acidity for taste balance).
How much should I add?
Deacidification with CaCO3 and KHCO3
The limit to CaCO3 (calcium carbonate) and KHCO3 (potassium hydrogen carbonate) addition is the amount of tartaric acid available. First determine how much tartaric acid is in the must. Then plan your CaCO3 or KHCO3 addition to remove if necessary all tartaric acid but leave 0.5 g/L of tartaric acid otherwise the treated wine will oxidize rapidly at alkaline pH.
In the case of CaCO3: 0.67 g/L reduces TA by 1 g/L
With the double salt method in which you take 20 to 40% of the must to be treated and add all the CaCO3 calculated to be needed for the total volume, the goal is to also precipitate some of the malic acid. The high pH produced in this fraction of must is to facilitate this.
Add all this CaCO3 to about 1/3 of the total must. Agitate well, let settle and rack. Recombine the treated and non-treated fractions. Mix well. Wait several hours and check the TA and pH.
CaCO3: 0.67 g/L reduces TA by 1 g/L
(vol.) L x present TA (g/L) – a = desired TA
CaCO3 needed = a x 0.67
KHCO3 potassium bicarbonate: 0.673 g/L removes 1 g/L tartrate
K2CO3: 0.62 g/L removes 1 g/L
LEAVE O.5 g/L of tartrate!
230 g KHCO3/500 mL %TA x vol (mL) = mL of solution added
Be careful with CaCO3 additions. Excess calcium in the wine can cause ca-tartrate instabilities and unfortunately there is no way to test for Ca-tartrate stability.
With 0.67 g of CaCO3 you are adding 0.268 g of Ca because most of the molecules mass is carbonate but it is still a large amount of calcium during an addition. Wine typically contains 40 to 140 mg/L of Ca and the majority of the calcium added to the wine will be removed with the precipitated as tartrate and malate salts.
Calculating Wine Additions
Mathematics for calculating wine additions under duress can be stressful and tricky. We recommend the use of computers with spreadsheet applications to streamline the mathematics for several lots and for archiving information. There are several online wine addition websites available that can help calculate how much of a particular addition to make. Example URLs (http://wineadds.com/, http://vinoenology.com/, http://www.iwinemaker.com/)