Chapter 19

Juice and Wine Acidity

Titratable Acidity in Wine

Titratable acidity (TA) is a measurement of the total concentration of titratable acids and is of primary importance only to the sensory perception of finished wines. Total acidity, the sum of all acids, is commonly but incorrectly assumed to be the same as titratable acidity. While titratable acidity does not measure all acids, it is easier to measure and is considered to be the better way to measure perceivable acidity in wines. Titratable acidity is typically quantified in terms of g/L of tartaric acid equivalents.

Relationship Between pH and TA

The pH of a wine or juice is a measure of the concentration (i.e., the strength) of free hydrogen ions in solution, while the TA is a measure of the total amount of hydrogen ions. Based on these definitions, one might be tempted to think there is a relationship between the pH and the TA. While acid content affects pH, it is not directly predictive of pH (or vice versa). This non-direct correlation is partially due to pH "buffering" caused by a number of compounds in wines, such as sugars, acids, and phenolic compounds.

Buffering Capacity

When making acid additions it should be noted that the effect of acid on pH is dependent on the buffer capacity of the juice or wine. Buffer capacity may be described as representing the concentration of hydrogen ions (H?) required to change the pH by one unit. A wine with a lower buffer capacity means the shift in pH due to a change in acidity (H? concentration) will be greater than that in a wine with a higher buffer capacity.

Adjusting pH or Titratable Acidity

Is it best to adjust acidity to a pH or titratable acidity value? Of course, one would hope that the pH can be adjusted to the desired value and at the same time achieve the desired TA value. However, if the desired values of both parameters cannot be achieved, then preference should be given to the pH, particularly with musts.

Why is pH Important?

The pH influences microbiological stability, affects the equilibrium of the tartrate salts, determines the effectiveness of sulfur dioxide and enzyme additions, influences the solubility of proteins and effectiveness of bentonite and affects red wine color and oxidative and browning reactions (Boulton et al. 1999).

pH and TA for White and Red Wines

The pH value of a juice is dependent on many factors including the degree of maturity at harvest, the variety, the crop level, the season, the soil moisture content during maturation, and the mineral composition available to the vine. For example, soils rich in potassium or grapes harvested during heavy rainfalls and grapes harvested with high sugar content tend to have a higher pH. As a general guide, the preferred TA and pH values of the white must would lie in the range of TA = (6-9g/L), and pH = (3.1-3.4), and TA = (5-7g/L) and pH = (3.3-3.8) for red must.

Analyzing Titratable Acidity

Titratable acidity is measured using a method of analysis called an acid-base titration. In an acid-base titration, the concentration of an unknown acid or base (analyte) is determined by adding a volume of acid or base of known concentration (titrant) until an endpoint is reached.

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