Acidity

Acidity is what makes your mouth water. Try it deliberately: take a sip, swallow, and notice whether saliva rushes in. That reaction is acidity.

It is the single most useful thing to learn to notice, because it determines whether a wine feels refreshing or heavy, whether it works with food, and whether it will age.

High acidity is not a fault. It is the backbone of Champagne, Riesling, Chablis, Sancerre and nearly every wine that lasts decades.

Cool climates and early picking give more acidity; heat and ripeness reduce it. If a wine feels flat and soft, low acidity is usually why.

Practical use: if a dish is acidic — anything with tomato, lemon or vinegar — the wine needs acidity of its own or it will taste flat beside it.

Acidity and sourness are not quite the same thing to a drinker. What you perceive depends on what else is in the wine: sugar masks acidity, which is why a good off-dry Riesling with very high acidity does not taste sharp while the same acidity in a bone-dry wine would. Alcohol and glycerol soften the impression too.

The chemistry is mostly tartaric and malic acid, and the difference between them is worth knowing. Tartaric is stable and gives wine its long-term backbone. Malic is the sharp green-apple acid, and malolactic conversion — a bacterial process rather than a fermentation — turns it into softer lactic acid. That conversion is why one Chardonnay tastes of green apple and another of butter, from the same grape in the same year.

Acidity is also the single best predictor of how long a wine will last, which is why the great long-lived whites are Riesling and Chenin rather than anything richer. A wine low in acidity can be delicious and will rarely be old.