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Carl Hudson
January 4, 2020 | Wine "Fun" Facts | Carl Hudson

Acidity in Wine – Part 1 in the Vineyard

First of all, Happy New Year to all of you.  I’ve just been sitting here thinking about topics for 2020, and decided to start off with a post on how important acidity is in wines.  This was a key topic for a presentation and tasting at the Texas Wine and Grape Growers Association Grape Camp last November in Fredericksburg.  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 enhances stability against spoilage mechanisms.  This Part 1 edition of Carl’s Corner will focus on acid compounds as they are created in the vineyard, and migrate to the grapes that will eventually be harvested and become wine.  A Part 2 edition will focus on acidity in the winery as it impacts fermentation, aging, and the finished wine. 

In order to properly discuss acidity, some chemistry issues need to be addressed, and will done so as simply as possible.  Please bear with me.  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.  If you have a swimming pool, and add muriatic acid (actually a form of hydrochloric acid) to adjust acidity, this pH acid strength concept should be familiar to you.  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 grapes to the winery.  Both TA and pH are important and usually trend in the same direction.  However, they do not always directly correlate. 

Please stay with me for just one more chemistry lesson.  Tartaric acid and Malic acid are the two primary acids found in grapes and wine.  The structures of these two acids are similar, but different enough to cause interesting bio-chemical reactions within grapes on the vine.  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.  More on this later. 

It is believed that acids form in grape leaves prior to the fruit ripening stage.  Once the fruit begins to ripen, pretty much all of the acid that will ever be formed migrates from the leaves to the grape clusters.  Tartaric acid concentration remains fairly stable, but malic acid metabolism will reduce acidity.  Why?  Warm night temperatures allow a vine’s metabolic processes to continue.  Cooler temperatures allow a vine to rest and slow or stop these metabolic processes.  At night, without the sun, photosynthesis stops producing sugars that are the vine’s key energy source.  Conversion of sugars is what helps vines to grow and ripen grapes.  Well, here in Texas it can stay fairly warm at night and the vine never really stops working.   In the dark, without sugar production from photosynthesis, the vine keeps looking for energy sources and malic acid gets consumed – thus lowering acidity.  This reduction of acidity in the grapes usually leads to an adjustment (addition) of acidity by the winemaker prior to fermentation. 

It is very common in Texas for grapes to be harvested with low acidity at 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.  Low acidity is usually adjusted by addition of tartaric acid before fermentation.  Because of higher temperatures, not only does grape acid level go down, but sugar level increases at a fairly rapid rate as grapes ripen.  At the same time, flavor compounds, often lumped together and called phenolics, can lag behind in their development.  This can, and often does, result in an acid-sugar-phenolics dilemma for the grape grower and winemaker.  Higher sugar levels mean more alcohol in the finished wine, and sometimes that can just be too much.  Harvest decisions can get very complicated as one tries to balance, as best one can, the rising sugar level, the dropping acidity, and the lagging development of phenolic ripeness in the grapes.  Hey, nobody said grape farming was easy!!

Another way that acidity gets lowered in grapes is by formation of malate and tartrate salts with soluble metal ions of potassium, calcium, and magnesium.  These ions are important soil and plant nutrients, especially potassium (K+).  Potassium is a key component of most fertilizers that contain potassium, nitrogen, and phosphorus.  As these metal ions react with malic and tartaric acid, the level of acidity is reduced, and pH goes up.  These salts can also create a “buffering” effect that limits the effectiveness of any tartaric acid added in the winery to increase acidity in the wine.  Thus, raising acidity during fermentation is not always straightforward. 

A final way that acidity gets reduced in the vineyard is dilution.  As grapes ripen, they grow larger with more liquid inside the fruit.  Since no more acid gets produced as grapes ripen, this means that the overall acidity gets diluted, and pH goes up.  And, if there are rain events near harvest time, vines can absorb extra water and send it to the grapes which swell, further diluting the available acidity. 

As you can see, acidity management is important and can be complicated by several issues in the vineyard and the winery.  Although frustrating, it’s all just part of the “fun” of grape growing and wine making. 


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