pH

What pH actually measures

pH measures the concentration of free hydrogen ions in a solution. The scale is logarithmic, so a wine at pH 3.0 is ten times more acidic than a wine at pH 4.0, and a hundred times more acidic than one at pH 5.0. Most wine falls between 2.9 (very tart) and 4.0 (flat and unstable).

pH is different from total acidity. Total acidity measures the total amount of acid present, including acid that is bound to other molecules. pH measures only the free, active acid, which is what actually affects the palate and the microbes. The two numbers can diverge: a wine with high total acidity but a lot of potassium can have a high pH, and a wine with moderate total acidity but very little potassium can have a low pH.

Why winemakers care about pH

pH affects four things. First, it shapes the perception of acidity: a low pH wine tastes sharp, a high pH wine tastes flat. Second, it affects microbial stability: spoilage bacteria and Brettanomyces prefer higher pH, so a low pH wine is more stable. Third, it affects colour: red wines with low pH have a bright ruby tone, while higher pH wines shift towards brown. Fourth, it affects the effectiveness of sulphur dioxide: free SO2 is more effective at lower pH, so a high pH wine needs more total SO2 to be stable.

pH in UK winemaking

UK winemakers track pH throughout the season. At harvest, the target is usually 3.0 to 3.4 for sparkling base wines and 3.2 to 3.6 for still wines. Above 3.7, the wine becomes unstable and tastes flabby. Below 2.9, the wine can taste aggressively sharp. The art is to keep the pH in the right band for the style.

Cool UK climate keeps pH naturally low, which is one of the great advantages of the region. A warm climate grape can come in with a pH of 3.6 to 3.8 and require acid addition to bring it down. UK grapes typically come in at 3.0 to 3.2 and need little or no adjustment.

Adjusting pH

Three tools are used. The first is acid addition, mainly tartaric acid, which lowers pH. This is permitted within strict UK and EU limits. The second is malolactic conversion, which raises pH slightly because the lactic acid produced is weaker than the malic acid consumed. The third is blending, which can lower the average pH by mixing in a higher-acid component.

De-acidification, lowering the total acidity while raising pH, is also possible using calcium carbonate or other agents. In the UK, this is rarely needed because the natural acidity is already in the right range.

Reading pH on a technical sheet

A UK winemaker's technical sheet will list both total acidity (in g/l tartaric) and pH. A typical English sparkling base wine might be 10 g/l TA and 3.05 pH. A typical English still white might be 7.5 g/l TA and 3.3 pH. A typical English still red might be 6 g/l TA and 3.6 pH. The numbers give the technical buyer a clear picture of the wine's structure before they even open the bottle.