How to Read Grape Chemistry at Harvest (Brix, pH, TA)

The harvest decision is the most consequential judgement call in viticulture and winemaking. Pick too early and you lock in unripe, herbaceous, harsh character. Pick too late and you lose acidity, freshness, and in worst cases risk rot or shrivelling. Three chemical measurements guide this decision.

The Three Numbers

1. Brix (Sugar Content)

Brix (°Bx) measures the percentage of dissolved solids in the grape juice — primarily sugar. One degree Brix ≈ 10g sugar per 100mL (approximately). It's measured with a refractometer (on fresh juice in the field) or a hydrometer (in the winery on extracted juice).

Target Brix at harvest varies by style and climate:

Brix alone is not enough. A grape at 25 Brix with a pH of 3.2 is a completely different harvest prospect from the same Brix at pH 3.9. Always read all three parameters together.

2. pH

pH measures hydrogen ion concentration — the acid strength. A lower pH means more acidic (more hydrogen ions). Wine pH typically sits between 3.0 and 4.0, with most table wines between 3.2 and 3.8.

Why pH matters:

• Microbial stability: Lower pH (more acidic) inhibits bacteria and spoilage organisms. Wines below pH 3.5 are inherently more stable than those above.
• SO₂ effectiveness: Molecular SO₂ (the antimicrobial form) increases dramatically as pH drops. At pH 3.5, you need roughly twice as much free SO₂ to achieve the same molecular SO₂ as at pH 3.2. See the SO₂ article for the full table.
• Colour: In reds, lower pH preserves anthocyanin colour in its red-coloured ionised form. Higher pH causes colour to shift toward blue/purple and eventually brown.
• Taste: pH is closely correlated with how sour/fresh the wine tastes, but TA (see below) is the stronger predictor.

Target pH at harvest:

• White wines: 3.0–3.4 (higher acidity preferred)
• Red wines: 3.3–3.8 (some variation by variety and style)
• Sparkling base: 3.0–3.2 (lowest pH — maximum stability, highest acidity)

pH rises during ripening as potassium accumulates in berries and buffers the acids. A berry at 22 Brix might have pH 3.3; the same berry at 26 Brix could have pH 3.7. This pH rise is often faster in warm climates and hot vintages.

3. Titratable Acidity (TA)

TA measures the total concentration of acids in the juice — expressed as g/L of tartaric acid equivalent. It's measured by titration (adding a base until the acid is neutralised, measuring how much was needed).

Main acids in grapes:

• Tartaric acid: stable throughout ripening and winemaking; doesn't metabolise
• Malic acid: decreases with ripening (respired by the berry in warm temperatures); sharp, green-apple flavour
• Citric acid: small amounts; can be metabolised by LAB to acetic acid

Target TA at harvest:

TA vs pH: These are related but not the same. Potassium in the must can buffer pH (raise it) while TA remains constant — so pH can be high even when TA is adequate. Always measure both.

Sampling and Measuring in the Field

Refractometer (Brix only)

Drop 2–3 drops of juice on the prism, fold the daylight plate, and read the Brix scale at the boundary line. Fast, portable, no power needed. Accurate to ±0.5 Brix.

Important: Refractometers are calibrated for pure sugar solutions. Once fermentation begins, alcohol refracts light differently and the reading is no longer accurate. Use a hydrometer in the winery during and after fermentation.

pH meter

Calibrate the probe with fresh buffer solutions (pH 4.0 and 7.0 or 4.0 and 3.0 buffers). Measure a sample of fresh juice. Allow time for the reading to stabilise. Rinse with distilled water between samples.

TA titration

A basic titration kit: titrate 10mL of juice with 0.1 N NaOH (sodium hydroxide) to a phenolphthalein endpoint (colour change from colourless to pink). The volume of NaOH used (in mL) × 0.75 = TA in g/L as tartaric acid.

Ready-made TA test kits (e.g. Titrets, Vinoferm) simplify this for small-scale use.

Sampling Protocol

Chemistry varies across a vineyard. A representative sample requires:

• Sample randomly across the whole block — not just the easiest-to-reach rows
• Pick from multiple vine positions: inside the canopy, outside, top and bottom of bunch
• A minimum of 100 berries from at least 20 vines for a statistically meaningful result
• Squeeze and pool the juice; strain out seeds; measure immediately (juice oxidises rapidly)

At commercial scale, 100–200 berries per 5 acres is a reasonable sampling density.

Reading the Numbers Together

Grape chemistry rarely gives a clean single signal. Interpreting all three:

Scenario 1: Brix 24, pH 3.35, TA 7.2 g/L → Excellent harvest window. Ripe fruit, good acidity, manageable pH. Pick now or very soon.

Scenario 2: Brix 22, pH 3.65, TA 5.5 g/L → Worry. pH is high despite modest Brix. Fruit is losing acid faster than it's accumulating sugar. May need acidification at crush. Monitor daily.

Scenario 3: Brix 26, pH 3.25, TA 9.5 g/L → Cool climate/late season grape. High acidity — may need minimal deacidification. Could be excellent for sparkling or high-acid table wine.

Scenario 4: Brix 19, pH 3.20, TA 10 g/L → Underripe. Wait if weather permits. Will need significant sugar addition in the winery.

Taste: The Non-Negotiable Check

Chemistry tells you the numbers. Your palate tells you the story.

At harvest sampling, always taste the berries:

• Seeds: Brown and crunchy? Good ripeness. Green and watery? Underripe tannins.
• Skins: Do tannins feel velvety? Or do they pucker and grip harshly?
• Juice: Is the fruit flavour ripe and generous, or is there green or herbaceous character?

The numbers give you a framework; the taste makes the final call.