Acids in Wine

​Acids in Wine
Dr. Beth Chang
September 2020

PH is a major contributor to the following factors in wine: 1) microbial stability, and 2) color stability. Lower pH, preferably < 3.6, is important for decreasing risk of spoilage organisms and increasing efficacy of SO2. Likewise, a lower pH increases color stability, and keeps color molecules, i.e. anthocyanins, in the preferred color range of red-purple, rather than purple-brown. In addition, lower pH can increase rates of reactions for various wine components during storage (1,2).

When we measure pH, we are measuring the strength of the acid (or acids) in a solution, in this case: grape juice or wine. Because of the word “acid”, it is easy to assume that pH is useful in evaluating the acidity, or perception of sourness. And yes, pH does have a loose inverse correlation to sourness. However, a much more predictive metric is TA (titratable acidity), which relates to the concentration of acids in juice and wine. TA has a much stronger (nearly linear) and direct correlation to sourness: rising TA concentrations will result in increasingly sour taste (2,3).
Typically, there is an inverse relationship between pH and TA: lower pHs indicate higher TAs and vice versa. However, the two do not ever track in a perfectly predictable way, and there are times when both high pH and high TA can be present simultaneously in a juice or wine. The two major reasons for this are: 1) different organic acids (which are all weak acids) have different buffering capacities, or to put it another way, some organic acids, e.g. tartaric acid, are stronger weak acids than others. This means that their addition or removal will have a larger impact on pH. Therefore, the relative amounts of organic acids will lead to different pHs at the same TA. And 2) higher concentrations of metals (e.g. K+, Ca+2, Mg+2) in the grape juice and wine matrix can result in increased pHs at a given TA because the metals likewise increase buffering capacity, while masking the total number of acid molecules (i.e. total acidity) through hydrogen displacement.2This second cause is particularly pertinent in the mid-Atlantic region (e.g. Virginia), where excessive potassium absorption frequently occurs (4).

When looking at the pH and TA of a must or wine, it’s helpful to have some reference points against which to compare values. Generally accepted industry ranges for must pH would be 3.0 – 3.4 for whites, and 3.2 – 3.4 for reds (1). The resulting final wine product is broadly between pH 3 – 4, with whites on the lower side and reds on the higher side, e.g. 3.3 – 3.7 (2). Typical values for TA (measured as g tartaric / L) are between 5 – 8 g/L. Red wines tend to skew lower than whites because the potassium extracted from the grape skins will cause increased potassium bitartrate precipitation (1).

Measuring pH and TA is essential prior to fermentation and can also be useful well before harvest to get a better picture of overall fruit quality. As grape berries mature, pH will increase and TA will decrease. Factors related to accelerated berry maturation, e.g. warm temperatures, will result in a faster rise in pH and rate of decline in TA (5).

References
(1) Boulton, R. B.; Singleton, V. L.; Bisson, L. F.; Kunkee, R. E. Principles and Practices of Winemaking; Springer US: Boston, MA, 1999. 
(2) Waterhouse, A. L.; Sacks, G. L.; Jeffery, D. W. Understanding Wine Chemistry; John Wiley & Sons, Ltd: Chichester, UK, 2016. 
(3) Plane, R. A.; Mattick, L. R.; Weirs, L. D. An acidity index for the taste of wines. American Journal of Enology and Viticulture 1980, 31 (3), 4.
(4) Wolf, T. K. Wine Grape Production Guide for Eastern North America; Plant and Life Sciences Publishing: Ithaca, New York, 2008.
(5) Dokoozlian, N. K.; Kliewer, W. M. Influence of Light on Grape Berry Growth and Composition Varies during Fruit Development. 6.