Unlike sweetness, all wines have acidity. However, how we perceive that acidity and whether we like it depends on whether it is balanced.
Too much acidity will make a wine taste sour and sharp whereas too little will make it seem flat or flabby.
When the balance is right, the wine feels lively. White wines have crispness and red wines feel more refreshing.
The level of acidity also affects how other aspects of the wine such as tannin, feel to us.
Different sorts of acid in wine
There are various acids found in wine. Grape juice consists mainly of tartaric acid and malic acid.
Tartaric acid is what you get if you dissolve cream of Tartar in water. It has a neutral flavour, more mineral than fruity.
On the other hand malic acid has the taste of green apples. Grapes also contain low levels of citric acid, which we find in lemon juice, and miniscule amounts of ascorbic acid, also known as vitamin C.
Sadly, drinking wine is not going to boost your immune system.
Acidity in sweet wines
The sensation of acidity is decreased by sweetness. That’s why we add sugar to fruits and fruit juices.
Wines containing high levels of acidity can be made a lot more enjoyable by leaving some sugar in the wine.
A wine that would taste mouth-puckeringly sour if dry can be made more balanced by sugar.
A prime example is Sauternes which has very high levels of acidity, balanced by lots of sweetness.
Another is Champagne where sugar is added to the finished wine as a “dosage” to mask the acidity.
Tannins and acidity
Higher acidity tends to make tannins feel coarser and drier and, as both high acidity and tannins cause a drying, puckering sensation in the mouth, it is easy to confuse tannin and acidity in young red wines.
The key is to remember that acidity is mainly a taste whereas tannins is mainly a feeling.
Sugar added to cheap white wines
As grapes ripen on the vine, they lose acidity and gain sugar.
Therefore the obvious way to achieve a balanced level of acidity is to grow grapes in the best climates and pick them at the perfect time.
However, when growers aim for high yields or if grapes need to be picked early to avoid rot, the resulting wine will have too much acidity.
It is common practice for winemakers to add sugar to cheap white wines made from such grapes to create an “off-dry” style that is more drinkable.
It is also possible to remove acidity from the juice or wine by adding an alkali such as potassium or calcium carbonate.
However, that is more work and more costly than adding a bit of sugar.
Adding tartaric acid to red wines
At the other end of the spectrum, grapes left to hang a long time on the vine will develop more interesting flavours, produce stronger wines and give red wines smoother tannins.
However, there is a big risk that those grapes will lack the acidity to make balanced wines.
Therefore it is very common practice, especially in the New World where there are few or no regulations on acidification, to systematically add tartaric acid to the juice.
When done carefully the acid additions are undetectable and generally improve the end-result.
Wineries can control acidity in the fermentation process
As well as the amount, the type of acid is important to the taste.
Malic acid has a zesty, green character which can add a lot of freshness to whites.
However, a bacteria common in wineries can transform malic acid into lactic acid, the acid we find in milk, yoghurt and cheese.
For centuries winemakers didn’t know what caused some wines to come out zesty and others to develop creamier characters.
These days wineries have more control over the process and are able to inoculate their wines with commercially-prepared bacteria cultures or to prevent this malolactic transformation by adding sulphites to the wine before it starts.
Read more: Explained: Sulphites in French wine
Virtually all red wines undergo malolactic fermentation and it is responsible for the buttery character of white Burgundy.
During fermentation a lot of carbon dioxide is produced and some of this remains dissolved in the wine as carbonic acid.
Still wines have their carbon dioxide levels reduced but sparkling and pétillant wines will have a slightly increased acidity.
A balancing act
All wines contain a certain level of acetic acid (vinegar) produced by yeast and bacteria during fermentation and ageing.
At moderate levels, acetic acid is desirable because it increases the aromas of a wine as it evaporates in the glass.
It is thus said to “lift” a wine. It also seems to polish the tannins in red wine.
However, once the concentration of acetic acid becomes noticeable, the hints of vinegar start to overpower the other aromas and the palate takes on a sharp flavour and harsh texture.
Acidity changes as wines age
Acidity acts as a preservative, especially in white wines. Both on its own and in conjunction with sulphites, it helps to prevent the wine from oxidising or being turned to vinegar by bacteria.
High acidity is the reason wines made from Riesling and Chenin blanc can age well for many years, developing rich, complex flavours as the acidity subsides.
Those wines can taste unbalanced when they are young but almost feel as though they have been sweetened over time, even though there is no sugar being produced.
Balance is probably the most important characteristic in achieving a great wine and the level and type of acidity, in relation to sweetness and tannin is the core of that balance.