Acidity in Wine – Part 2 in the Winery
Acidity is very important to the taste, color, and stability of wines. Too little acidity produces flat & uninteresting wines, while too much acidity leads to harsh, tart wines. Acidity not only influences the taste of wine, but also the fermentation process, stability of color and proteins, and resistance to spoilage mechanisms. Part 1 on acidity focused on acid compounds as they are created in the vineyard, migrate to the grapes, and are impacted by various degradation mechanisms that reduce acidity as grapes ripen and are harvested. This Part 2 edition will focus on acidity as it impacts fermentation, aging, and quality in the finished wine.
As previously noted, there are two primary measures of acidity used by growers and winemakers. The first is called pH, and represents the acid strength of the liquid in grapes or wine. Normal tap water is usually close to neutral pH of about 7.0, while wine is acidic with a typical pH range of 3.0-4.0. Since pH is measured on a logarithmic scale (like the Richter scale for earthquakes), pH 3.0 is 10 times more acidic than pH 4.0. pH is the most common measure of acidity used in the vineyard and winery.
Titratable Acidity, or TA, is a different measure of the total acid concentration available in the grape juice or wine. TA is most important to the flavor and mouthfeel of a wine, and mostly represents the organic acid molecules that are created by the vine and carried by the grapes to the winery. Both TA and pH are important and usually trend in the same direction. However, they do not always directly correlate.
Tartaric acid and Malic acid are the two primary acids found in grapes and wine. Tartaric acid is relatively stable and, once formed, pretty much sticks around as the grapes ripen and are harvested. Malic acid, that tart acid found in green apples, for example, is much less stable and can actually get consumed in a metabolic process that significantly reduces acidity of the grape juice or wine. So, most often there will be more tartaric acid than malic acid in grapes delivered to the winery.
It is common in Texas for grapes to be harvested at low acidity in the upper pH range of 3.6-4.2+. A more preferred, higher acidity level would be in the pH range of 3.2-3.6. This means that acidity must be adjusted, or added, to reach a preferred pH level for fermentation. This is usually done by adding tartaric acid (typically 1-4 grams/liter of juice) that will give an optimum pH level for the type of wine being made.
Sometimes raising acidity with tartaric addition is not straightforward because of the “buffering” effect of malic acid and tartaric acid salts that were formed in the grapes with soluble metal ions of potassium, calcium, and magnesium (all important soil and plant nutrients, especially potassium, K+). “Buffering” can limit the effectiveness of tartaric acid added in the winery to increase acidity in the wine. Often adding the right amount of tartaric acid prior to fermentation boils down to the winemaker’s experience with the grape variety, source of fruit, and type of wine being made.
Adjusting acidity before fermentation is by far preferred over adjusting after fermentation. Adding tartaric acid to a finished wine can result in a tell-tale tartness and minerally taste that detracts from the wine’s overall flavor characteristics.
Acid in wine is important to stability issues. Higher acidity thwarts several common spoilage mechanisms in wine. The effectiveness of the key preservative, sulfur dioxide (sulfite), is significantly enhanced at higher acidity (lower pH). The larger amount of sulfite required to protect a lower acidity wine can often lead to an unpleasant burnt match aroma in the wine, typical of higher sulfite concentrations.
The color of a wine is related to relative acidity. Anthocyanin molecules, responsible for color in wines, are dependent on pH or acid level for their tint. Higher acidity leads to more red or burgundy color and a brighter hue in the wine. Lower acidity can lead to more bluish or purple tones and a wine with a duller hue.
Other tools used by winemakers to manage acidity in the winery include blending higher and lower acid wines to reach a more optimum acidity level. Sometimes a portion of fruit is harvested early, before full ripeness, when grape acidity level is higher. Wine from this fruit can then be blended with wine made from fruit harvested later in the season at lower acidity. If left alone in the aging process, wines will typically undergo a malolactic transformation in which bacteria convert tart malic acid into milder lactic acid (think yogurt, butter). This lowers acidity, but can be blocked by the winemaker in order to maintain higher overall acidity. Finally, a process called ion exchange (similar to water softening) can be applied to grape must prior to fermentation to increase acidity by removing metal ions (like potassium or calcium). Ion exchange can be a very effective means of adjusting acidity, but is more costly and labor intensive than just adding tartaric acid.
As you can see, acidity management is important, and can be complicated by several issues in both vineyard and winery. These issues can be frustrating, but, like was said in Part 1, acidity management is all part of the “fun” of grape growing and wine making.