Acid Control in Winemaking

By Charles Plant

Types of Acid That Occur

Grapes naturally contain three primary organic acids: tartaric, malic, and citric. As grapes mature, malic acid diminishes while tartaric becomes predominant, though both remain present in ripe fruit. Minor acids like succinic contribute to aroma and flavor complexity.

Measuring Acid Concentration

Total acidity (TA) is expressed in grams per liter as if all acid were tartaric. Home winemakers should test every batch using affordable acid testing kits from winemaking suppliers, which include a syringe, indicator solution, and sodium hydroxide solution.

Testing White Wines: Add indicator to a 15mL sample, then titrate with sodium hydroxide drop-by-drop until a persistent pink color appears. The milliliters used equals g/L TA.

Testing Red Wines: The color change is harder to detect. Watch for the wine to shift from red to greyish-green, then slightly reddish when the endpoint approaches. Alternatively, use the “white tile method”—place indicator drops on marked circles and compare color changes as you add sodium hydroxide incrementally.

Appropriate Acid Levels

White wine musts should contain 7–9.5 g/L TA, while red musts should range from 6–8 g/L. Add tartaric acid to raise TA (approximately 1 gram per liter raises white must by 1.0 g/L). For excessively high TA, consider dilution, blending, or treatments like potassium carbonate.

pH Considerations

pH measures free hydrogen ions and differs fundamentally from TA. The relationship between these values varies unpredictably based on grape variety and conditions. Ideal white wine must pH ranges from 3.2–3.4; red musts from 3.3–3.5.

A pH pen ($50–$80) provides reliable home measurement. Calibrate using buffer solutions before each session, never letting the electrode dry out.

Malolactic Fermentation

MLF converts malic acid into lactic acid and CO2, producing “less aggressive acid and smaller quantity.” This process adds complexity, particularly benefiting red wines and some whites like Chardonnay. However, fruity wines like Riesling typically should avoid MLF to preserve their fresh character.

Track MLF progress using paper chromatography, which visually separates tartaric, malic, and lactic acids. Introduce ML bacteria cultures into warm conditions (around 25°C) with minimal SO2 interference.