Acidity in Wine:

The importance of management through measurement

The typical acidity measurements in grape juice and wine are pH and titratable acidity (TA). The
pH measurement is used in the vineyard to assess the ripening pre-harvest, e.g., (Brix * pH2 ), to
calculate sulfur dioxide requirements after fermentation, and to assess oxidation risk because
high pH wines are generally more prone to oxidation. TA is applied to sensory perception of a
wine’s acidity, i.e. its tartness, sourness, crispness. While pH and TA are related, pH is a
measurement of the likelihood and speed of occurrence of pH dependent reactions, while TA is
the best estimate of a wines perceived acidity.

Titratable acidity should not be confused with total acidity: total acidity only quantifies the
molar weights of acids contained in a grape, must or wine; TA is an approximation of total
acidity by titration with a strong base (NAOH) to a pH of 8.2, i.e. TA is the sum of both acid
content and cation content, such as potassium (K+), sodium (NA+), and calcium (Ca++) (1).
While the quantified TA is inflated with cations, measuring total acidity is difficult as it requires
the ability to directly quantify organic acids.

In the winery TA is the best practical expression of the organic acid concentration within must
or wine. The principal organic acids found in grapes are tartaric, malic; to a small extent, citric
and others. Tartaric and malic acid account for over 90% of the total acids present, existing at
roughly a 1:1 to 1:3 ratio of tartaric to malic acid. The actual acid composition and
concentration within the must or wine is influenced by many factors such as variety, climatic
region, and cultural practices; their presence contributes to both a wine’s flavor and to its
stability, color, and pH. By knowing the exact organic acid makeup of a wine or must a
producer can make educated vinification decisions to optimize flavor and stability.

During the berry’s progression to veraison, acids accumulate within the fruit. At veraison, the
total acidity in the fruit decreases, primarily due to the reduction of malic acid; at harvest, the
fruit usually contains more tartaric acid than malic acid, the exact concentrations and ratios to
one another being cultivar specific and harvest date dependent (3). Grapes are one of the rare
fruits that contain tartaric acid. It is present as free acid and in its salt form, e.g. potassium
bitartrate (KC4H5O6), sodium bitartrate (C4H5NaO6), and calcium bitartrate (CaC4H4O6 ).
The presence of the salt form is an important constituent, affecting pH and the cold stability
of the wine (5).

Basic difference between pH and TA

While one may think that TA and pH are directly correlated as acidity indicators, they are not:
The measurement of pH is the number of H+ ions in a solution using a logarithmic scale, with a
lower number denoting a higher concentration of H+ ions. Translation: the difference between
a wine with a pH of 4.0 and with a pH of 3.0 is that the wine with the pH of 3.0 has 10x the
number of hydrogen ions as the pH 4.0 wine (or 1x10-3 versus 1x10-4 H+ ions). The
measurement of acidic content is the acid’s potential to liberate H+ ions as it dissociates. 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 occurs because these compounds exist in
equilibrium between their acid and conjugate base forms; the ratio of the two forms to one
another must be significantly shifted before a noticeable pH change can occur. Just as pH
calibration buffer solutions effectively calibrate pH equipment due to their reliable stability, the
addition of a given amount of acid to a wine may not reduce the pH as expected due to the
wine’s buffering capacity to maintain a stable pH.

In taking pH and TA measurement one is also measuring two different chemical attributes of
the wine or must. With a pH meter one is measuring an electrical gradient created by the
solution inside the cell of the pH probe and the wine. With TA one is measuring the amount of
strong base that it takes to raise the solution to pH 8.2 accounting for both acid content and
buffering capacity. Within the US wine industry, TA is typically quantified in terms of g/L of
tartaric acid, as if it were a quantification of only tartaric acid; in fact, the number represents
the concentration of all titratable acids, e.g. including malic, citric, lactic, succinic acids. (Note-
some industries use Sulfuric acid as the acid of reference, so one may see values given in g/l as
Sulfuric in some European publications).

Due to the presence of various kinds of acids and their salts, the relationship between titratable
acidity and pH is necessarily a complex one. For instance, pH also depends on the ratio between
tartaric and malic acids; so any loss of malic acid, i.e. in the vineyard from respiration during
ripening in warm climates, or in the winery from unintentional MLF, will reduce TA and increase
pH.

Briefly, understanding the role of pH and TA in winemaking is crucial to making good wines.

Overview of the Acids in Wine

Tartaric Acid- A diprotic (two H+ ions) acid - (C4H6O6), tartaric acid is relatively microbial stable
and accounts for a large proportion of a wine’s acidity (along with malic acid) and normally
exists at a concentration between 2.5-5g/l at harvest.

Malic Acid- A diprotic (two H+ ions) acid - (C4H6O5),the levels of malic acid in grape berries are
at their peak just before veraison when they can be found in concentrations as high as 20 g/L.
As the vine progresses through the ripening stage, malic acid is metabolized in the process of
respiration, and by harvest, its concentration falls to around 1-4 g/l.
Lactic Acid- A monoprotic acid (one H+ ion) - (C3H6O3), lactic acid bacteria (LAB) convert sugar
and malic acid into lactic acid, the latter through MLF. This process can be beneficial for some
wines, adding complexity and softening the harshness of malic acidity, but it can generate off
flavors and turbidity in others. Note that lactic acid does not naturally exist within the grape,
but can be produced during vinification.

Citric Acid- A triprotic acid (three H+ ions) - (C6H8O7), citric acid often has a concentration of
less than 1 g/L at harvest; note that citric acid may be converted by LAB to acetic acid and
diacetyl.

Acetic acid- A monoprotic acid (one H+ ion) - (C2H4O2), acetic acid is produced in wine during or
after the fermentation period. It is the most volatile of the primary acids associated with wine
and is responsible for the sour taste of vinegar. During fermentation, activity by yeast cells
naturally produces a small amount of acetic acid if the wine is exposed to oxygen. The U.S.
legal limits of Volatile Acidity are 1.2 g/L in red table wine and 1.1 g/L in white table wine.

Succinic acid- A diprotic (two H+ ions) acid – (C4H6O4), succinic acid is more commonly found in
wine, but can also be present in trace amounts in ripened grapes. While concentration varies
among grape varieties, it is usually found in higher levels with red wine grapes. The acid is
created as a byproduct of the metabolization of nitrogen by yeast cells during fermentation.

The importance of precise measurement

It is recommended that pH for table wines be in the range of 3.1 to 3.3 for musts to ensure
microbial and chemical stability (2); in reality Missouri white wines run close to 3.5 and MO red
wines can exceed 3.6. At these pH values only a tiny proportion of SO2 is in the active,
molecular form; to shift the equilibrium to a point that will allow good stability, one must lower
the pH.

Free sulfur dioxide (SO2) ppm concentration required to maintain concentration of molecular sulfur
dioxide between 0.8 ppm and 0.5 ppm at wine pH range (4)
If the wine’s pH is 3.35, and the intent is to maintain 0.8 ppm of molecular sulfur dioxide to
stabilize the wine, then the winemaker must add the equivalent of 30 ppm free sulfur dioxide
concentration; if the wine’s pH is 3.60, and the intent is to maintain 0.8 ppm of molecular sulfur
dioxide to stabilize the wine, then the winemaker must add the equivalent of 50 ppm free
sulfur dioxide concentration; if pH is 3.75, then add equivalent of 70 ppm…

To achieve the recommended pH, one method to lower the pH of the must is to add an organic
acid. The tricky part is calculating the amount to add to lower the pH without increasing the TA
to an unpleasant level of excessive tartness or sourness in the wine. Given the continual
chemical and microbiological reactions in wine, e.g. pH buffering, MLF, and tartrate
precipitation, it is not always possible to add X amount of an organic acid and achieve Y
reduction in pH.

Before adding organic acids to ultimately lower the pH, it is necessary to have baseline readings
of both TA and pH.

Tartaric acid, in practice, is the preferred acid addition to wine because it is not as easily
degraded microbiologically, as are malic and citric acids, which may lead to unexpected
changes. Because tartaric acid is poorly soluble in ethanol in water solutions, e.g. wine, there is
a limit to the amount of tartaric acid that a winemaker might add without causing cold
instability.

While malic or citric could potentially be used, such practice is a riskier prospect: malic acid can
degrade to lactic acid, and citric acid to diacetyl, the “buttery” aroma found in some wines.
Citric acid may also degrade into acetic acid, which is federally regulated regarding maximum
concentration in wine. There are also commercial acid “blends” that seek to mimic the
proportions naturally found in the grape.

As many winemaking decisions will be affected not only by the total or titratable acidy, it is
necessary to know concentration of the individual acids comprising the wine’s overall acidity.
There are a variety of measurement methods available to the winemaker:

One of the classic methods used by winemakers is paper chromatography, which separates
acids dependent on the speed at which the samples travel up paper moistened by a solvent at
one end. The problem with this method is that it is not quantitative and relies on hazardous
chemicals.

Another method that has gained popularity within wineries is the enzymatic based method.
This method employs an enzyme that selectively uses a given acid to cause a reaction that leads
to a change in absorbance as measured by a spectrophotometer. These tend to be as accurate
as an individual’s ability to pipette. The equipment needed is a spectrophotometer; the
consumable supplies include an enzymatic kit and appropriate micro-pipettes. It should be
noted though that these kits have expiration dates and need to be used soon after opening.

Another method is high performance liquid chromatography (HPLC). This works off the same
basic principle as paper chromatography, but the solvent in this case is forced through a column
at high pressure Unlike paper chromatography this method is both quantitative and very
sensitive; the required equipment is not cost beneficial for the average winery. At the GWI we
have such units in place for research; we plan to soon offer acid quantification and
characterization to the Missouri industry on a trial basis.

GWI will offer a pilot program to gauge interest in this acid measurement service. We would
like interested industry members to contact Michael Leonardelli for information on how to
prepare the sample and where to send it. Our plan is to initially offer this service for free
during our testing period, and then charge a modest fee to cover costs when fully
implemented. For any questions, concerns or suggestions for additional services, please feel
free to contact either Michael Leonardelli (leonardellim@missouri.edu) or Misha Kwasniewski
(kwasniewskim@missouri.edu) at the GWI.

References
1. Boulton, R. 1980. The Relationships between Total Acidity, Titratable Acidity and pH in Wine.
American Journal of Enology and Viticulture 31:76-80.
2. Boulton, R. B., V. L. Singleton, L. F. Bisson, and R. E. Kunkee. 1996. Principles and practices of
winemaking, vol. Chapman and Hall, New York.
3. Kliewer, W. M., L. Howarth, and M. Omori. 1967. Concentrations of Tartaric Acid and Malic
Acids and Their Salts in Vitis Vinifera Grapes. American Journal of Enology and Viticulture 18:42-
54.
4. Margalit, Y. 1996. Winery Technology & Operations: A Handbook for Small Wineries, vol. The
Wine Appreciation Guild, Ltd., San Francisco.
5. Nagel, C. W., and I. W. Herrick. 1989. The Effect of Malate or Lactate Content on the pH-TA
Relationship of Potassium Bitartrate Saturated Alcohol-Water Solutions. American Journal of
Enology and Viticulture 40:81-84.

Michael J. Leonardelli, MS, MBA

Enology Extension Associate
The Grape and Wine Institute,
The University of Missouri
124 Eckles Hall
Columbia MO 65211-5140
Website: http://gwi.missouri.edu
Office: 573-884-2950
Cell: 573-239-6121
Email: leonardellim@missouri.edu