Wine consumers hear wine judges and wine makers talking about wine and they hear the term “acid” and “acid balance” frequently. Unfortunately, the term “acid” conjures up the image of the liquid in a car battery or a powerful substance used in trashy murder mysteries to get rid of the body, or the condition of one’s stomach where calming digestion pills are required.

Acid is a natural component of grapes and wine and is in all fruits and vegetables. It is an important part of the taste.

There are lots of acid substances around, but the important acids to wine and food are citrus, malic and tartaric acids. Any refreshing drink has to have acidity – one reason lemon drinks such as lemonade are so good for hot weather, is there tingling sharp taste caused by acidity – in this case the high citrus acid found in lemons. A fresh lemon has a sour bighting taste due to the acid. However, in balance with other components in the drink, acid is a freshening and vital ingredient. Malic acid is the component in green apples – in smaller quantities it is an agreeable part of the wine but at higher levels is harsh and can be softened by a process in winemaking called malo-lactic fermentation.

Wine can have a range of acidities – a younger wine has higher acid than old; whites are higher than reds. A young Riesling can be very acidic which means that when well chilled, it is a great refresher style drink. However, people used to sweeter drinks find a young Riesling too sharp on the taste. More immature tasters prefer sweeter drinks – in wine there have been some quite sweet styles that have been very popular, especially fruity German whites that were available by direct sales and mail order. Very seasoned tasters such as older wine makers and judges are notorious for preferring high acid, dry, subtle wines.

The main counterbalance to acid is sweetness – acid and sweetness cancel each other out. Hence, a sweet drink has to have quite high acid in order to be balanced – soft drinks are high in sweetness and in acidity. Good quality sweet wines have a balance of sweetness and acid. If the acid is not enough, sweet drinks taste sickly sweet and cloying.

Colder climates produce higher acids in the grapes and, hence, cold climates can be “tasted”. Acid is detected as a sharpness towards the front of the tongue. Even on a sweet drink, the acid can be detected.

Acid level is measured in a laboratory – it is described as a number on a scale (called the pH scale) or as an amount in grams per litre. The pH scale is a figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acid and higher values more alkaline. A proud winemaker might even explain on the back label that the grapes were harvested at a certain pH level! The lower the pH measure, the higher the acid so 3.2 is quite high in acid while 3.8 is lower in acid.

The acid level in wine can be manipulated by the grower and wine maker. If a wine is low in acid, the winemaker can add more acid, using tartaric acid – the same kind made by the vine. Grapes picked less ripe have high acid compared to a well ripened crop. There is a relationship between ripeness of grapes, and acid level and flavour. The riper the grapes become on the vine, the bigger the flavour, the sweeter the sugar level, and the lower the natural acidity. All grape growers get to know the best balance point between these components. For example, a Shiraz grower might be looking for full flavours and be quite content with a ripe low acid crop. However, a Hunter Semillon producer knows that high natural acid is the key to freshness in a light wine; it is also the key to cellaring potential. Hence the Semillon producer will harvest the crop at lower ripeness level where there is higher acid. For certain grapes such as Riesling, Chardonnay and Pinot Noir, a cold climate allows the grapes to stay on the vine until late in the season, to become quite ripe, yet retain high acid.

This phenomenon is illustrated well by Champagne. The region is to the north east of Paris and is the wine region furthest from the equator and is the coldest. In many years the grapes struggle to ripen properly, and the highly reflective chalk soil promotes sunlight reflection into the vines that assists ripeness. In good years there is sufficient ripeness while maintaining high acidity. The wine making processes of sparkling wine, particularly Champagne, tend to build up lots of taste complexity. To be in balance, a rich complex sparkling wine needs high acidity in order to give freshness. Lesser sparkling wines tend to have the high acid but without the flavour complexity and hence they can taste thin and sharp. In warmer areas of the world, the method of producing sparkling wine is to harvest the grapes early when they are not fully ripe, but the acid level is still high enough. However, the flavours tend to be greener and have a herbal influence.

If a hot weather vintage occurs, the grapes are naturally low in acid and tartaric acid is added, then there is a limit to the amount of the addition otherwise a spiky spiritous taste can be detected in the wine.

Some proponents for the natural wine movement argue that acid should not be added at all and if acid is regularly required, then there is a problem with the grape growing – maybe the wrong varieties are planted on the location or vineyard practices are wrong.

Tartaric acid in wine tends to join with other components in the wine at cold temperatures and it will form a sediment that looks like a glass crystal – you sometimes see them on the underside of a cork. Most wines are chilled and filtered before bottling in order to prevent too much of these tartrate crystals forming.