ACIDS in WINE

An overview of what acids are, how they work and which ones we should care about

By Dale Ims

As home wine-makers, the acids in the fruits or juices that we acquire—and in the wines we make—evoke a special interest. Those of us who have done some reading on the subject of winemaking realize that yeasts require an acidic environment to efficiently do their job, that wines with an insufficient level of acidity are prone to spoilage, and that wines with either too low or too high an acid content suffer from taste defects.

Given the importance of acids in our wines and winemaking, it seems appropriate that we learn something about the specific acids that are likely to be present in our wines, how acid strengths are quantified, and how the small subset of acids of interest to us fit into the general scheme of things. We’ll start by defining an acid, then move to an explanation of pH, then build on that as we move to the concept of pKa, and finally rate and rank our typical wine acids.

What is an acid?

An acid is a chemical entity or molecule with a propensity to eject a hydrogen nucleus. We assume here that all interested readers willingly accept the fact that all matter is comprised of molecules, that molecules are comprised of atoms that may be assembled in myriad, yet specific and ultimately knowable ways, and that atoms are comprised of positively-charged nuclei with negatively-charged electrons orbiting around the nuclei. The atoms of interest to us here will be limited to those of the elements hydrogen, oxygen and carbon, although we may mention others in passing.

Hydrogen is of primary interest here, since it is the propensity of the ejection of a hydrogen nucleus which marks a molecule as an acid. Hydrogen is the smallest and lightest of the elements, with a single proton as its nucleus and a single orbiting electron. In an acid, the hydrogen atom is bonded to some other atom as part of the molecule which comprises the acid; most of the acids we are interested in are called carboxylic acids, and they are characterized by the molecular subgroup [– COOH] where the C is a carbon atom, the O’s are oxygen atoms and the H is a hydrogen atom. At the left end of the acid-forming subgroup, the dash before the carbon atom indicates that the carbon is attached to some other atom as part of the molecule. A given molecule may have more than one acidic subgroup, and we will see later that many of the acids of interest to us are at least bi-functional.

Now, when the hydrogen nucleus is ejected or dissociated from an acid molecule, it leaves its electron behind and goes off by itself, thus creating two electrically-charged ions: a positively-charged hydrogen nucleus and the negatively-charged remainder of the original molecule. The hydrogen nucleus is too small to exist by itself for long, and it quickly attaches to a water molecule to form a positively-charged hydronium ion (H₃O⁺) while the remaining part of the acid molecule carries a negative charge due to the retention of the hydrogen’s electron. These charged particles (ions) are reactive, and it is their presence which accounts for the chemical activity of acids.

We should point out here that the ionization/dissociation process described above is reversible without regard to which specific hydrogen nucleus left a given acid molecule; promiscuity and indecision are the rule here!

What is pH?

The pH of a solution is a measure of the concentration of hydrogen ions in a solution; it is calculated by:

pH = - log₁₀ [H⁺]

which, in words is: pH is equal to the negative logarithm (base 10) of the hydrogen ion concentration. I hope the logarithm doesn’t scare anybody off! Logarithms are probably not used much except in the scientific and technical fields where they are used to deal with really large or really small numbers, and to perform some mathematical manipulations. An inexpensive scientific calculator or the functions built into spreadsheets takes all the trouble out of working with them.

The presence of the logarithm in the definition of pH means that—like the Richter Scale for measuring the size of earthquakes—each unit change in the number corresponds to a factor of ten change in the measured quantity. Thus, a pH 3.0 solution has a hydrogen ion concentration ten times that of a pH 4.0 solution. A pH 7.0 solution is neutral—neither acidic nor basic—and solutions with pH values less than 7 are acidic. For acceptable biological stability, wines should have values no higher than 3.6.

What is pKa?

Chemists reserve the term “strength” to refer to the probability that a given acid will ionize by ejecting a hydrogen nucleus in a solution. “Strong” acids will be ionized under essentially all solution conditions, but “weak” acids will ionize (i.e. eject a hydrogen nucleus) only if the hydrogen ion concentration in the solution is not already too high. In a manner that seems to me could be described as “inverse peer pressure”, the hydrogen nuclei of weak acids will stick with the molecules of which they are parts if there are already too many free nuclei in solution, but begin to dissociate as the pH of the solution increases. At a sufficiently high solution pH, all the hydrogen nuclei will have dissociated. The solution pH at which 50% of the hydrogen nuclei of an acid have dissociated from their molecules is called the pKa of the acid and is an indication of its strength. According to Wikipedia, strong acids have pKa values of minus 2 or lower, while the acids of interest to us have pKa values of (plus) 2 or greater.

When an acid’s molecular structure is made up of more than one [– COOH] group, there is a pKa value for the dissociation of the hydrogen nuclei in each [– COOH]  group, and the values are not equal. After a first dissociation from a multi-functional acid, subsequent dissociations occur only at higher pH values as the remaining hydrogen nuclei are bonded more tightly.

The data table

We have finally reached the point where we are going to list the data for the acids of interest to winemakers.

WINE ACIDS TABLE

In the table we list the names of the acids, their chemical formulae, the number of acid groups [– COOH] on the molecules, and the pKa values for each of the acid groups. Tartaric acid is the dominant acid in grapes, but note in the table that malic acid—which is the dominant acid of apples and cherries and is runner-up in grapes—has a chemical formula very similar to that of tartaric, and that it also has two acid groups. However, the pKa values for malic acid are higher than those for tartaric acid and that means that malic is the weaker acid of the two.

Succinic acid is another structure with two acid groups like tartaric and malic acids, but with a slight variation in the internal molecular structure. It is a by-product of the fermentation of sugar and present only in small concentrations in finished wines. It is the weakest of the tartaric/malic/succinic series of acids.

We winemakers try to avoid making the acetic acid (AKA vinegar) that is included in the table, where we see that it is a relatively weak acid (pKa of 4.76) with only a single [– COOH] group. In spite of the fact that acetic is a relatively weak acid, it has a strong taste and is volatile enough that it can be detected in our wines’ aromas if present at excessive levels.

Lactic acid is not normally present in the fruits or flowers we use to make our wines, but it can be produced (intentionally or not) via the bacterially-assisted fermentation of malic acid. As can be seen in the table, the lactic acid molecule has only one acid group whereas malic acid has two. In addition, the pKa value for lactic acid is a bit higher than the lower value for malic acid, so the lactic acid is the weaker of the two.

Oxalic acid is the strongest acid in our table with a pKa of 1.38 for the first hydrogen separation. It is present in small quantities in rhubarb stems and in much higher concentrations in the rhubarb leaves, where it apparently contributes to the poisonous properties of the latter; most of the acid in rhubarb stems is malic. Oxalic is also a bi-functional acid, and from the chemical formula listed in the table, one can see that this acid’s molecular structure is simply that of two back-to-back [– COOH] groups.

Citric acid is the dominant acid in citrus fruits from which it got its name, but is also present in pineapples, strawberries, and other berries. The table shows that citric acid is tri-functional, with three acid groups incorporated into the molecule. The table also shows that with its lowest pKa level of 3.09, it is a relatively strong acid among those listed.

We have also included carbonic acid in the table, where we see that it is the weakest of all those listed. Carbonic acid spontaneously forms in low concentrations from CO₂ dissolved in water, and since the fermentation process involved in winemaking has CO₂ as a by-product, all our wines contain some level of carbonic acid at some point in time. Carbonic acid is the weakest of those listed in our table.

Summary

We have attempted to provide a molecular-level understanding of what acids are, how their levels of acidity are measured (pH), and how the strengths of the acids are specified (pKa). We have then listed the acids of interest to winemakers, provided some information on their structures and relative strengths, and briefly mentioned places where each of those acids might be found. The interested reader might find Wikipedia’s information and structural diagrams for each of these acids to be of value; that source provided most of the information included here.

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