pH in Wine — Acidity Measurement and Its Role in Winemaking

pH quantifies the concentration of free hydrogen ions (H+) in solution, calculated as the negative logarithm of that concentration (pH = -log10[H+]). Pure water is neutral at pH 7.0; all wines are acidic, occupying a range of roughly 2.9 to 3.9. Because the scale is logarithmic, each unit change represents a tenfold difference in hydrogen ion concentration. pH is measured most accurately with a calibrated glass electrode pH meter; strip tests exist but lack the precision needed for winemaking decisions.

• pH scale runs 0–14; wines occupy the acidic range of approximately 2.9–3.9
• Calculated as pH = -log10[H+]; measured electrochemically via calibrated glass electrode
• Tartaric acid (pKa 2.98 and 4.34) is the dominant acid in wine and strongest of the main wine acids
• Winemakers adjust pH via acidification (tartaric acid) or deacidification (potassium bicarbonate, calcium carbonate, or MLF)

pH and titratable acidity (TA) are complementary but fundamentally distinct. TA measures the total concentration of all acid molecules present, expressed in g/L as tartaric acid equivalents. pH measures the strength, or intensity, of those acids in solution. There is no direct or predictable relationship between the two: a wine with high TA from malic acid may have a higher pH than one with the same TA dominated by tartaric acid, because malic acid is a weaker acid that dissociates less readily. Buffering capacity, potassium content, and the ratio of tartaric to malic acid all decouple pH from TA.

• TA (titratable acidity): the total concentration of all acids, measured by titration to pH 8.2 with sodium hydroxide, expressed in g/L as tartaric acid
• pH: the strength or intensity of those acids; two wines with identical TA can have markedly different pH values
• Tartaric acid is a stronger weak acid than malic acid, so its presence lowers pH more per gram than malic acid does
• High potassium concentrations in must or wine increase buffering capacity and raise pH at a given TA, particularly relevant in warm climates

pH is one of the most important determinants of microbial stability in wine. Most spoilage bacteria and yeasts are inhibited in the pH range typical of wine, though the precise thresholds vary by organism. Critically, pH governs the efficacy of sulfur dioxide: only the molecular form of SO2 is antimicrobial, and its proportion within free SO2 rises sharply as pH falls. At pH 3.5, achieving a protective molecular SO2 level of 0.8 mg/L requires approximately 40 mg/L free SO2; at pH 3.9, the same protection demands roughly 99 mg/L. High-pH wines therefore pose compounding stability risks and require considerably more intervention.

• Molecular SO2 is the active antimicrobial form; its proportion within free SO2 increases dramatically at lower pH
• Brettanomyces is less active below pH 3.8, though low pH alone does not guarantee protection against Brett infection
• At wine pH (3–4), bisulfite is the predominant form of free SO2; molecular SO2 represents only 1–7% of free SO2 depending on pH
• High-pH wines (above 3.8) may require impractically high total SO2 additions to achieve adequate microbial protection

In red wine, pH directly controls the chemical equilibrium of anthocyanin pigments. At lower pH, anthocyanins favor the red flavylium cation form; as pH rises, they shift toward colorless hemiketal forms and eventually to violet or blue quinoidal bases. Research shows that roughly 20–25% of anthocyanins are in their colored (red) form at pH 3.4–3.6, compared to only 10% at pH 4.0. Higher-pH wines therefore appear lighter in color, with less vibrant hue, and higher pH also accelerates browning reactions in both red and white wines.

• Low pH favors the red flavylium cation form of anthocyanins; high pH shifts pigments toward colorless or blue-violet forms
• Approximately 20–25% of anthocyanins are in their red colored form at pH 3.4–3.6, versus only 10% at pH 4.0
• Higher pH in white wines accelerates phenolic darkening and browning reactions, shortening shelf life
• In red wines, higher pH (such as in Syrah-based wines) is associated with less stable, more blue-toned pigments that may shift to grey-brown with age

Malolactic fermentation (MLF) is the bacterial conversion of the diprotic malic acid to the monoprotic lactic acid. Because malic acid can donate two protons while lactic acid donates only one, MLF raises wine pH by approximately 0.3 units on average and reduces TA by 1–3 g/L. Winemakers use MLF strategically: it is almost universal in red wines to soften acidity and achieve microbial stability, routine in barrel-fermented Chardonnay, but deliberately blocked in aromatic whites like Riesling and Sauvignon Blanc where freshness is paramount. MLF bacteria (primarily Oenococcus oeni) require pH above 3.1 to thrive and are highly sensitive to molecular SO2.

• MLF raises wine pH by approximately 0.3 units and reduces TA by 1–3 g/L by replacing diprotic malic acid with monoprotic lactic acid
• Standard practice for virtually all red wines; common in Chardonnay; blocked in Riesling and Sauvignon Blanc to preserve acidity
• Malolactic bacteria require pH above approximately 3.1 and minimal free SO2 to initiate and complete MLF
• After MLF, winemakers sometimes re-acidify with tartaric acid if the resulting pH is too high for stability

Climate change presents a growing and well-documented challenge for managing wine pH. As temperatures rise, grapes ripen faster, losing acidity and gaining sugar, which pushes harvest pH upward. Research from Languedoc documents pH rising from 3.50 to 3.75 while total acidity fell from 6.0 to 4.5 g/L over recent decades as a direct result of warming. Harvests in most major wine regions now begin two to three weeks earlier than they did 40 years ago. Winemakers are responding by harvesting earlier, acidifying more frequently with tartaric acid, and selecting grape varieties or clones with greater acid retention. EU regulations permit additions of up to 2.5 g/L tartaric acid equivalent to wine, and the OIV sets a cumulative ceiling of 4 g/L for must and wine combined.

• Rising temperatures accelerate ripening, decrease acidity, and increase harvest pH across virtually all major wine regions
• Harvests now begin an average of two to three weeks earlier than 40 years ago, shifting ripening into hotter months and further reducing natural acidity
• Tartaric acid is the preferred acidification agent: it is naturally present in grapes, does not participate in MLF, and provides reliable pH reduction
• EU permits a maximum acidification addition of 2.5 g/L for wine and the OIV sets a cumulative ceiling of 4 g/L for must and wine; producers in Australia and California face fewer regulatory restrictions