Have you ever noticed tiny, glass-like crystals at the bottom of a wine bottle or clinging to the cork after taking it out of the fridge? If not, look closer next time. Though they might look strange, these are simply crystallized forms of tartaric acid — one of wine’s natural acids. Acids can be complex in structure, but they play a central role in shaping how a wine looks, tastes, and evolves.
It’s not hard to see how our mouths react instantly to acidity. Take a sip of wine and keep your mouth slightly open for a moment. If your mouth starts to water, that’s the acid at work. The more saliva, the higher the acidity.
The pH Scale and Total Acidity
Wine contains many different acids, but two of them — tartaric and malic acid — occur naturally in grapes. When we talk about “acidity” in everyday language, we usually mean total measurable acidity. This is closely linked to how our mouth perceives sourness: the more saliva we need to balance a sip of wine, the sharper it feels.
The pH value, on the other hand, measures the strength of the acid. The pH scale runs from 0 to 14 — with the extremes representing the strongest acidity and alkalinity. It’s worth remembering that this scale is logarithmic, meaning each number represents a tenfold change in acidity. So, a liquid with a pH of 5 is ten times more acidic than one with a pH of 6.
High-acid wines have low pH values, and these wines are naturally more resistant to bacterial or microbial spoilage — making them more stable.
Most wines fall within a pH range of 3 to 4.
How Acidity and pH Shape Wine
Acidity defines both the appearance and the taste of wine. As pH decreases, acidity increases.
For comparison:
- Vinegar ≈ pH 2.5
- Milk ≈ pH 6.5
- Water (neutral) = pH 7
- Wine = usually pH 3–4
There’s also a strong relationship between acidity and color. High-acid red wines appear bright ruby, while lower-acid, high-pH wines tend toward purplish or bluish hues. Low-acid wines are more prone to oxidation, which can cause brownish tones — particularly unwanted in young white wines, where freshness is key.
Oxidation can occur at any pH level, but it progresses faster as pH increases.
On the palate, high-acid wines feel lively and crisp, while low-acid wines feel round and full. When low acidity combines with high pH, however, the wine can taste loose or even soapy. Acid also contributes to a wine’s aging potential — high-acid wines tend to age more gracefully.
Wines with high pH, on the other hand, are more susceptible to microbial spoilage and may turn cloudy. To control oxidation and microbial activity, winemakers add sulfur dioxide (SO₂).
But there’s a trade-off: since the pH scale is logarithmic, a wine at pH 4 requires roughly ten times more sulfite protection than one at pH 3. As sulfite levels increase, preventing “rotten egg” or “cabbage-like” aromas becomes increasingly difficult.
The Source of Acidity — From Vineyard to Glass
A wine’s acidity begins in the vineyard. Potassium in the soil is absorbed by the grapevine and neutralizes some acidity, raising the pH. Unripe grapes are high in acid; as they ripen, acidity decreases. Cool-climate grapes tend to be more acidic because less heat and sunlight slow sugar accumulation. In warmer regions, where grapes ripen quickly, acidity drops and pH rises. To restore balance, winemakers may add tartaric acid before fermentation.
Conversely, if acidity is too high, they may soften it through malolactic fermentation. Think of the taste of a green apple — that tart bite comes from malic acid. Now think of milk — its smoothness comes from lactic acid. Malolactic fermentation transforms the sharp “apple acid” into the softer “milk acid.”
When this happens, total acidity may remain the same, but its character changes. As malic acid becomes lactic acid, pH rises slightly and perceived acidity decreases.
The result is a smoother, creamier wine. Almost all red wines undergo this process, while in white wines it’s a stylistic choice — the hallmark of a “buttery Chardonnay.”
Crystals and Balance
If a wine retains a high level of tartaric acid, it can crystallize under cold conditions, forming tartrate crystals — affectionately known as “wine diamonds.” They are harmless and dissolve again as the wine warms.
Final Word
Whether red or white, the true quality of a wine depends not only on the grape itself but also on how skillfully its acidity is managed.
As Denise M. Gardner, a leading enologist, puts it:
“If pH belongs to chemistry, acidity belongs to the soul of wine. True quality lies in the balance between the two.”
