Why good wine tastes alive.

Ask people what they like in a wine and they talk about fruit, oak or smoothness. Almost nobody mentions acidity in wine, yet it is the component that decides whether you reach for a second glass or leave the first one unfinished.

Acidity is what makes a wine refreshing, what lets the best bottles age for decades and what makes wine work at the table. Too little and a wine tastes flat and heavy, however ripe its fruit. With enough, it tastes alive. Winemakers argue over it for good reason, since climate, picking dates and cellar choices all move it.

How to recognise acidity in wine

Acidity is the easiest part of a wine's structure to feel once you know what to look for. Take a sip, swallow or spit, then pay attention to your mouth. The more it waters and the longer it keeps watering, the higher the acidity. To calibrate, taste a young Riesling or Sauvignon Blanc beside a ripe red from a hot region; your mouth will tell you which is which within seconds.

Many tasters also feel a tingle along the sides of the tongue. That tingle is real, but the old tongue map that confined sourness to the edges is a myth: taste buds all over the tongue respond to acid. Saliva is the signal to trust.

Do not confuse acidity with the drying grip of tannin. One makes the mouth water and the other dries it. Nor is acidity the opposite of sweetness, since a dry wine can be low in acid and a sweet one very high. It is one of the checks in a four-step tasting method and, in a blind tasting, among the first clues to climate.

The acids in the glass

Two acids, tartaric and malic, make up most of the acidity in grapes. Others play smaller parts in the finished wine.

• Tartaric acid is the backbone, firm and present in every wine. It also forms the harmless crystals sometimes found on a cork or at the bottom of a chilled white. These tartrates can look like splinters of glass but are no fault.
• Malic acid is the sharp acid of green apples and the one that falls most as grapes ripen.
• Citric acid is present in small amounts.
• Lactic acid is softer and milder. Most of it is made in the winery, when bacteria convert malic acid.

That conversion has a name. Malolactic conversion turns sharp malic acid into softer lactic acid, lowering the acidity and rounding the texture. It can also produce diacetyl, a compound that smells of butter, which is why some Chardonnay does too.

Most red wines go through it. For whites it is a choice. Chardonnay often goes through it for a rounder, creamier style, while crisp, aromatic whites such as Riesling are usually kept from it to preserve their bite.

Winemakers measure acidity in two ways. Total acidity is how much acid a wine contains. pH measures how strongly acidic it is, on a scale where lower means more acidic; most wines sit between about 3 and 4. The two do not always move together. A lower pH matters beyond taste, because it helps protect a wine against spoilage, makes each dose of sulphur dioxide more effective and keeps a red wine's colour redder and more stable.

Volatile acidity is a different matter. It is mostly acetic acid, the acid of vinegar, made by bacteria and some yeasts. A trace passes unnoticed; too much is a fault, one of several in the guide to wine faults.

Climate, ripeness and the winemaker's dilemma

As grapes ripen, sugar rises and acidity falls. Heat speeds the fall, because a warm vine uses up its malic acid faster. Cool regions therefore keep more acidity in their grapes. Warm regions prize altitude and cold nights for the same reason, since both slow the loss.

Picking dates are where the argument starts. Pick early and you keep freshness but risk green, unripe flavours; wait for ripeness and the acidity can slip away. In warm regions winemakers often add tartaric acid to make up the shortfall, where the rules allow it. In cool regions a cold year can call for the opposite, taking some acid out.

Climate change has sharpened the argument. In the Languedoc, average figures over 35 years show alcohol rising from 11% to 14% while total acidity fell by a quarter. Researchers report similar shifts in many other regions, which is one reason growers are rethinking picking dates, sites and even grape varieties.

Sweetness and time

Sugar and acidity balance each other. A sweet wine without enough acidity is cloying; with enough, it tastes vivid instead of heavy. Lemonade works the same way: take out the lemon and you are left with syrup. German Riesling is the great example in wine. A Kabinett or Spätlese with noticeable sugar can taste fresher and less sweet than its sugar suggests because its acidity is so high; German Riesling labels explain the categories. Sauternes relies on the same balance, its richness carried on a line of acidity.

Acidity also helps wine last. It acts as a natural preservative, which is why high-acid whites are among the longest-lived wines of all. Riesling, Chenin Blanc and Hunter Valley Sémillon, often made without any oak, can age for decades with acidity as their main support; see white wines that age well. In reds, acidity works alongside tannin.

With time the acidity seems to recede as other flavours develop around it. The wine tastes less sharp, but acidity is still what holds it together.

Acidity at the table

Acidity is the most food-friendly quality a wine can have. It refreshes a palate coated in fat, the way a squeeze of lemon lifts fried fish. It also loves salt, which softens the edge of a high-acid wine and makes it taste rounder and fruitier. Hence oysters with Chablis and salty ham with Fino.

The one rule worth memorising concerns sharp dishes.

A sharp dish needs a wine at least as sharp as it is.

A tomato salad, a dressing made with vinegar or a squeeze of lime makes a soft, low-acid wine taste flat and flabby, while a crisp white holds its own. The same logic explains why high-acid wines work so well with Japanese food, where vinegared sushi rice, soy and pickles all call for freshness. Wine with sushi goes into detail.

High-acid grapes and how to use them

Some grapes are naturally high in acidity:

• Whites. Riesling, Chenin Blanc and Sauvignon Blanc.
• Reds. Nebbiolo, Sangiovese and Barbera. Barbera is the interesting one: plenty of acidity with little tannin, which makes it one of the most versatile reds at the table.

In a red, high acidity reads as brightness: a vivid, juicy quality and a finish that leaves you wanting food. It is one reason Sangiovese, the grape of Chianti, is so hard to beat with a tomato sauce. The grape is only the starting point, though. Climate and picking date move every one of them.

Next time you open a bottle, pay attention to your mouth after the first sip. If a wine seems dull, check the acidity before blaming the fruit. With a sharp dish, choose the crisper wine. If you are buying white wine to keep, look for acidity first: it is one of the best signs that a white has a future.