Tartaric acid is extremely important from a winemaking perspective, due to the key role it plays in maintaining the chemical stability of wine, colour and influencing the finished taste of the wine. Tartaric acid is unique in that it is not found in many fruits or plants, but is the primary acid component in grape vines. It is one of the strongest acids in wine with the ability to resist the impact of other acids, along with malic acid and to a lesser extent citric acid.
Tartaric acid deceases as grapes are allowed to hang on the vine and mature. Very ripe grapes do not have a lot of tartaric acid, as in hotter climates, vines use tartaric acid for respiration, decreasing the total acidity in the grapes. However, the decrease in tartaric acid is not as profound as the decrease in malic acid through the same process.
Acidity is highest in wine grapes just before the start of veraison. Grapes from cooler regions generally have higher levels of acidity due to a slower ripening process. The level of acidity in the grape is an important consideration for winemakers in deciding when to harvest, as this is one of the mechanisms that winemakers use to control the acid content in their wine.
The influence of tartaric acid on the taste and feel of a wine is primarily through its impact on acidity. It contributes to the ‘tartness’ of a wine, but not as much as malic and citric acid. In the winemaking process, acids aid in enhancing the effectiveness of sulphur dioxide to protect the wines from spoilage and can also protect the wine from bacteria due to the inability of most bacteria to survive in low pH solutions.
During the course of winemaking and in the finished wines, acetic, butyric, lactic and succinic acids can play significant role. Most of the acids involved with wine are fixed acids with the notable exception of acetic acid which can be found in vinegar, which is volatile and can contribute to the wine fault known as volatile acidity. Sometimes, additional acids, such as ascorbic, sorbic and sulphurous acids are used in winemaking.
Winemakers will sometimes add acids to wine (acidification), more common in warmer climates where grapes are harvested at advanced ripeness with high levels of sugars, but very low acid levels. The concentration varies depending on the varietal and the soil composition of the vineyard. Some varietals such as Palomino are naturally inclined to having high levels of tartaric acids, while Malbec and Pinot Noir generally have lower levels.
In red wines, acidity helps preserve and stabilize the colour. The ionization of anthocyanins is affected by pH, so wines with lower pH - such as Sangiovese wines have a brighter red and more stable colours. Wines with higher pH - such as Syrah wines have higher levels of less stable blue pigments, eventually taking on a muddy grey hue and also develop a brownish tinge. In white wines, higher pH (lower acidity) causes the phenolics in the wine to darken.
Acids can be added either before or after primary fermentation. They can be added during blending or aging, but the increased acidity will become more noticeable if added at this stage. Tartaric acid crystals are used at the rate of approximately 1 g/L of wine to reduce the pH by 0.1 units. The crystals are dissolved in a small volume of wine and the solution is added to the must before the start of fermentation. If the wine will be cold stabilized, the rate of addition can be increased to 2 g/L.
The pre-fermentation acid addition raises the TA and lowers the pH. During alcoholic fermentation and malolactic fermentation, Tartaric Acid decreases causing the pH to increase. The pre-fermentation addition reduces the risk of the pH rising above the range at which free SO2 loses effectiveness.
The measure of the amount of acidity in wine is known as the ‘titratable acidity’ or ‘total acidity’, which refers to the test that yields the total of all acids present, while strength of acidity is measured according to pH, with most wines having a pH between 2.9 and 3.9. Generally: the lower the pH the higher the acidity in the wine.
Tartaric Acid often crystallizes on the base of the cork or falls to the bottom of the bottle, yielding ‘wine diamonds’ - called tartrates which are harmless. In wine tasting, the term ‘acidity’ refers to the fresh, tart and sour notes of the wine in relation to how well the acidity balances out the sweetness and bitter components of the wine such as tannins. A wine with too much acidity will taste excessively sour and sharp. A wine with too little acidity will taste flabby and flat, with less defined flavours.