Organic Acids in Wine: Tartaric, Malic, Citric, and Lactic

The Role of Acids in Wine

Organic acids are fundamental to wine quality. They contribute to taste (providing the crisp, tart character that defines well-balanced wine), influence color (through pH-dependent anthocyanin equilibria), affect microbial stability (low pH inhibits spoilage organisms), and participate in aging reactions that develop complexity over time.

A typical wine contains 5-9 grams per liter of total organic acids, expressed as tartaric acid equivalents. This may sound like a small amount, but it is sufficient to create pH values of 3.0-3.8, placing wine among the more acidic foods and beverages we consume.

Understanding individual acids, their origins, their chemical properties, and how they interact with each other and with other wine components is essential for producing balanced, stable wines.

The Big Four: Tartaric, Malic, Citric, and Lactic

While wine contains numerous organic acids, four dominate the acid profile and deserve detailed understanding: tartaric acid (the backbone of wine acidity), malic acid (the primary grape acid that changes during winemaking), citric acid (a minor but useful acid), and lactic acid (produced during malolactic fermentation). Together, these four typically account for over 90% of the total organic acid content.

Tartaric Acid

Chemistry and Properties

Tartaric acid (C₄H₆O₆) is the most important acid in winemaking. It is a diprotic acid, meaning it can donate two hydrogen ions per molecule, with pKa values of 3.04 and 4.37. This means that at wine pH (3.0-3.8), tartaric acid exists as a mixture of fully protonated tartaric acid, the hydrogen tartrate (bitartrate) ion, and a small amount of the fully dissociated tartrate ion.

Tartaric acid is relatively rare in the plant kingdom but is the dominant acid in grapes. Concentrations in must typically range from 5-10 g/L. Its strong acid character (low pKa) means it contributes significantly to the low pH of wine, providing more acidifying power per gram than malic or citric acid.

Tartrate Stability

A unique and practically important property of tartaric acid is its tendency to form insoluble salts with potassium and calcium. Potassium bitartrate (KHC₄H₄O₆, commonly known as cream of tartar) has limited solubility in wine, especially at low temperatures and high alcohol levels. When the concentration of potassium and bitartrate ions exceeds the solubility product, crystals form and precipitate.

These harmless crystals, often called wine diamonds, can appear on the bottom of corks or in the sediment of wines that have not been cold-stabilized. While they are not a flaw, many consumers find them undesirable. Cold stabilization (chilling wine to near its freezing point for 1-2 weeks) forces crystallization before bottling.

When potassium bitartrate precipitates, it removes both tartaric acid and potassium from the wine, which raises pH and lowers TA. This is why winemakers should always re-measure pH and TA after cold stabilization.

Malic Acid

Chemistry and Properties

Malic acid (C₄H₆O₆) takes its name from the Latin malum (apple), reflecting its abundance in apples and many other fruits. It is also a diprotic acid with pKa values of 3.46 and 5.10. Because its first pKa is higher than that of tartaric acid, malic acid contributes less to low pH per gram compared to tartaric acid.

Malic acid concentrations in grape must vary widely, from 1-9 g/L, depending on grape variety, climate, and ripeness. Cool-climate grapes retain more malic acid, while warm-climate grapes metabolize it during ripening through a process called malate degradation. The concentration of malic acid in harvested grapes is one of the key factors determining whether a vintage will produce bright, crisp wines or softer, rounder ones.

Malolactic Fermentation

The most important transformation of malic acid in winemaking is malolactic fermentation (MLF), in which lactic acid bacteria (primarily Oenococcus oeni) convert malic acid to lactic acid and CO₂. The reaction is straightforward: one molecule of malic acid yields one molecule of lactic acid and one molecule of carbon dioxide.

MLF has several effects on wine. It reduces total acidity (because lactic acid is weaker than malic acid and CO₂ is lost), raises pH (typically by 0.1-0.3 units), and changes the flavor profile from sharp, green-apple tartness to softer, rounder, sometimes buttery character. The buttery note comes from diacetyl, a byproduct of MLF.

MLF is standard practice for virtually all red wines and for certain white wine styles (notably oaked Chardonnay). It is deliberately prevented in white wines where crisp acidity and fresh fruit character are desired, such as Riesling and Sauvignon Blanc, by adding SO₂ after primary fermentation, maintaining cool temperatures, and sometimes using lysozyme.

Citric Acid

Chemistry and Properties

Citric acid (C₆H₈O₇) is a triprotic acid (three ionizable protons) with pKa values of 3.13, 4.76, and 6.40. It is present in grapes at low concentrations, typically 0.1-0.7 g/L in must. Citric acid contributes a fresh, clean tartness that many people find pleasant.

In winemaking, citric acid may be added to finished wine to brighten acidity and enhance freshness. However, it has a critical vulnerability: lactic acid bacteria can metabolize citric acid to produce diacetyl and acetic acid. This means that adding citric acid to a wine that has not undergone MLF or that contains active bacteria risks generating off-flavors.

Using Citric Acid in Winemaking

When used appropriately, citric acid additions (typically 0.1-0.5 g/L) can enhance the perception of freshness in finished wines that taste flat or dull. It is best added after MLF is complete and the wine has been sulfited to prevent bacterial metabolism of the citric acid. Bench trials should always be conducted before committing to a full-batch addition.

Some countries and wine regions restrict or prohibit citric acid additions to wine. Check your local regulations if producing wine for sale.

Lactic Acid

Chemistry and Properties

Lactic acid (C₃H₆O₃) is a monoprotic acid with a pKa of 3.86. It is not naturally present in grape must in significant amounts but is produced during malolactic fermentation and, in smaller quantities, during alcoholic fermentation by yeast.

Lactic acid is perceived as softer and creamier than either tartaric or malic acid. Wines that have undergone MLF have their malic acid almost entirely replaced by lactic acid, which is a major factor in the softer mouthfeel of malolactic-converted wines.

Impact on Wine Character

The shift from malic to lactic acid during MLF changes both the chemical acidity (TA and pH) and the perceived acidity of wine. Because lactic acid is weaker (higher pKa) than malic acid, the same number of acid molecules produces fewer free hydrogen ions, resulting in higher pH. Additionally, lactic acid simply tastes less sour than malic acid at equivalent concentrations, so the wine tastes rounder and smoother.

This dual effect, chemical and sensory, is why MLF has such a transformative impact on wine style. A wine that tastes aggressively tart before MLF may become pleasantly balanced afterward.

Other Organic Acids

Succinic Acid

Succinic acid (C₄H₆O₄) is produced by yeast during fermentation, typically at concentrations of 0.5-1.5 g/L. It has a distinctive salty, bitter-sour taste that contributes to the savory complexity of wine. Succinic acid is relatively resistant to bacterial metabolism, making it a stable component of the finished wine.

Acetic Acid

Acetic acid (C₂H₄O₂, the acid in vinegar) is present in all wines at low concentrations, typically 0.2-0.6 g/L. It is produced in small amounts by yeast during fermentation and in larger amounts by acetic acid bacteria (Acetobacter) when wine is exposed to oxygen. Acetic acid is the primary component of volatile acidity (VA). At low levels, it can add complexity. At elevated levels (above 0.8-1.0 g/L), it produces a vinegar-like off-aroma that is a serious wine fault.

Gluconic Acid

Gluconic acid is produced by the oxidation of glucose by Botrytis cinerea and acetic acid bacteria. It is present at elevated levels in wines made from botrytized grapes (noble rot wines like Sauternes) and is a useful marker for Botrytis infection. In wines not intended to be botrytized, high gluconic acid indicates gray rot contamination.

Acid Management for Home Winemakers

The practical goal of acid management is achieving a wine that is balanced, where acidity is perceived as refreshing and integrated rather than harsh, flat, or flabby. This requires monitoring both pH (which measures acid strength) and TA (which measures total acid quantity) throughout the winemaking process.

If your must pH is too high (above 3.6 for reds, 3.4 for whites), add tartaric acid before fermentation. If pH is too low (below 3.0), consider potassium bicarbonate additions or blending with a higher-pH juice. Always make adjustments gradually, using bench trials to evaluate the sensory impact before treating the full batch.

Decide early whether you want malolactic fermentation for your wine, as this decision has major implications for acid management. If you plan MLF, anticipate the pH increase it will cause and start with slightly lower pH. If you plan to block MLF, ensure adequate SO₂ protection after primary fermentation.

Frequently Asked Questions

Which acid should I use to adjust wine acidity?

Tartaric acid is the preferred acid for winemaking adjustments because it is the natural dominant acid in grapes, is relatively stable in wine, and has a clean, neutral taste. Citric acid can brighten wine but risks being metabolized by bacteria to produce off-flavors. Malic acid adds a green, crisp character but will be converted to lactic acid if MLF occurs. For most situations, tartaric acid is the safest and most effective choice.

What is the difference between volatile and fixed acidity?

Fixed acidity includes all the non-volatile acids (tartaric, malic, lactic, succinic, citric) that do not evaporate from wine. Volatile acidity (VA) is primarily acetic acid, which can evaporate and be detected by smell. Excessive VA (above 0.8-1.0 g/L as acetic acid) produces a vinegar-like off-aroma. Total acidity equals fixed acidity plus volatile acidity, and titratable acidity measures the total acid concentration by titration.

Should I do malolactic fermentation?

MLF is recommended for nearly all red wines because it softens harsh malic acidity and improves biological stability. For white wines, the decision depends on style. Wines intended to be crisp and fruity (Riesling, Sauvignon Blanc) should skip MLF. Wines intended to be rich and creamy (oaked Chardonnay) benefit from MLF. If you are unsure, consider doing MLF on part of your batch and blending.

Why does my wine taste sharp and green?

Sharp, green acidity is usually caused by high concentrations of malic acid, which has a distinctly tart, green-apple character. This is common in wines made from cool-climate or underripe grapes. Solutions include malolactic fermentation (which converts malic to softer lactic acid), blending with a riper, less acidic wine, or small additions of potassium bicarbonate to raise pH. Time also helps, as the perception of acidity often softens with aging.

How much tartaric acid should I add to lower pH?

As a starting point, 1 gram per liter of tartaric acid typically lowers pH by approximately 0.1 units, though the exact change depends on the wine's buffer capacity. Always start with a bench trial: add measured amounts of tartaric acid to small samples (100-250 mL), measure the resulting pH and taste the samples, then scale up the best dose to your full batch. Make additions in increments and re-measure, as the response is not perfectly linear.