A brief taxonomy of oenological acids

The second of the SWS introductory wine tasting workshops centered on body, acid, alcohol, and sugar. So here's my take on the acidic component of wine.

When tasting wine we often go to lengths to assess the acidity of the wine in front of us, whether we like the acidity, and whether it’s balanced by other elements such as sugar. What you’ll probably never write in your tasting notes was whether you tasted more malic acid, tartaric acid, or citric acid... but isn’t it a stunning topic to contemplate?

Not all acids are created equal. An acid (by the definition most relevant to winemaking) is a compound that raises the concentration of H+ ions in solution. But while the H+ ions are pretty much all equivalent, the compounds that release them are not, and they affect the flavor of wine differently. And they arise from different sources... so understanding how they act is yet another useful building block in the massive palace of wine enjoyment.

Believe it or not, there is a whole Wikipedia article about acids in wine. This post takes some information from that article and also some from Belitz, Grosch, and Schieberle’s Food Chemistry.

For reference: the typical pH of freshly-squeezed grape juice, and also of final wine, is usually somewhere between pH 2.9 and 3.9. This is on par with almost all other fruits, as well as most sodas (which were probably designed to fall within the range of foods that we already consider to taste “refreshing”). On either side of the acidity scale: vinegar and lemon juice are ~ pH 2; coffee ~ pH 5; milk ~ pH 6.3-6.6.

 

Acids you may encounter in wine

Tartaric acid

Tartaric acid has a special place in this lineup: it’s found in very few fruits besides grapes, so there are few other places you’re likely to encounter it in everyday life-- unless you bake frequently, in which case you may use it for stabilizing meringues, or as part of your baking powder. In fact, the crystals of tartaric acid that form on the inside of wine barrels has traditionally been harvested for use; my own little jar of Cream of Tartar, purchased from Whole Foods, claims that it comes from the vineyards of Catalina, Spain. Crystals of potassium tartrate (tartaric acid plus potassium) also form, from time to time, in the bottle itself, resulting in a sparkly-looking cork in some aged wines; winemakers sometimes use a “cold stabilization” process for preventative precipitation of tartrates before bottling.

The flavor of tartaric acid is sometimes described as “sharp” or “hard” (as compared to the juicy acidity of citric or malic acid).

In harvested grapes, tartaric acid usually makes up 65-70% of the total titratable acidity, provided the grapes are ripe.

 

Malic acid

If the grapes are unripe, they’ll have a higher concentration of malic acid. Malic acid is the second major component of acidity native to the grape, though malic is not as unique; it’s a key metabolite that is found in pretty much all living things, and in a wide variety of fruit in high concentrations. The name “malic” is derived from the Latin word for apple (from which it was first chemically isolated, in 1785); its flavor is often associated with green apple or with rhubarb, and described as a “pleasantly sour” fruit acidity. It’s also a common food flavor additive—it makes fruit products taste more refreshing, and, as the white powder that coats Warheads and other “extreme” sour candies, it has burned many a kid’s (including this kid’s) tongue.

Malic acid can be a preoccupation of winemakers, because its concentrations are manipulable—both during grape-growing and during winemaking. Unlike tartaric acid, malic decreases as the grape ripens, so leaving the grapes on the vine longer is one way to reduce acidity. (Too much malic acid contributes to an “unripe” character in some wines.) In the , malic acid concentrations can be decreased during the winemaking process through the use of malolactic fermentation (more below).

 

Citric acid

Citric acid is a third component of acidity naturally occurring in grapes, although it’s present in much lower concentrations—generally < 1/20 that of tartaric acid. Like malic acid, it can be found in a wide variety of other fruits and used as an additive in many foods, and we associate its flavor with “crisp” and “refreshing” fresh fruit flavors.

 

Lactic acid

Lactic acid, notably, does not come from the grape—it comes from microbes. It happens in wine as well as in yogurt, pickles, kimchi, sauerkraut, chocolate, and pretty much any other fermented food you can think of: lactic acid bacteria, in the absence of oxygen, make a living by fermenting sugars or other acids to produce lactic acid. (Aside: your own muscle cells do the same thing when deprived of oxygen, for instance when you’re sprinting.)

Lactic is a “softer” acid, with a flavor sometimes described as “milky.” It’s a monoprotic acid, which means that each molecule has only one H+ to contribute to solution. By contrast, tartaric and malic acids are diprotic, and therefore contribute twice as much acidity per molecule. The pKa of lactic acid is also slightly higher than the first pKa of the other two acids, so solutions of lactic acid will tend toward a slightly higher pH.

Lactic acid bacteria are ubiquitous, so lactic acid fermentation is bound to happen to some extent in almost any wine in the initial stages of fermentation, carried out by whatever bacteria came in on the grapes. However, winemakers will often actively encourage—or even inoculate-- lactic acid bacteria in the later stages of secondary fermentation and aging, as well. At that point, when there’s no sugar left but plenty of malic acid, bacteria such as the aptly named Oenococcus oeni will ferment malic acid into lactic acid, resulting in less-sharp wine. Look for this more often in wines where softness is prized, such as oaked Chardonnay, less often in wines aiming for crispness (especially those with sugar), such as Riesling.

Yeast cannot carry out malolactic fermentation, only bacteria. Usually. There are very few genetically modified wine yeast in existence and none that are commercially used... but one of the few that has been successfully developed is a yeast with the capacity for malolactic fermentation, which supposedly would save winemakers from having to add extra bacteria to get the job done. However, malolactic bacteria do more than just transform acids—they carry out a number of other metabolic processes that contribute to the flavor of the wine (just one example: a buttery flavor from diacetyl), and if you removed them from the equation, you’d miss out on those flavors. There’s always an argument for encouraging biodiversity in wine fermentation.

 

Acetic acid

Acetic acid the other bacterial product you may encounter in wine. However, whereas lactic acid is only produced in the absence of oxygen, acetic acid is usually produced only in the presence of oxygen. Bacteria from the genus Acetobacter use oxygen to oxidize ethanol, through the intermediate acetaldehyde, into acetic acid. This is exactly the process of making vinegar. Acetic acid in wine is therefore a sign that the wine has seen significant oxygen exposure, and it’s often perceived as a fault, though—as with any “fault” in wine—there are some who believe it adds desirable complexity in controlled amounts in some wines.

Acetic acid is also sometimes known as “volatile acidity” in wine, and the VA content is a number you may sometimes find on a wine’s fact sheet. Vinegar is usually 4-18% (~40-180 g/L).  By contrast, the volatile acidity reported for the Albariño we tasted on Thursday of this week was 0.30 g/L. 

 

References

“Acids in Wine.” 2014. Wikipedia, the Free Encyclopedia. http://en.wikipedia.org/w/index.php?title=Acids_in_wine&oldid=591293163. Accessed January 2014.

Belitz, H.-D, W Grosch, and Peter Schieberle. 2009. Food Chemistry. Berlin: Springer.