The Power of Hydrogen, or pH, is a measure of how acidic or basic a solution is. It’s the most essential analysis in all of winemaking informing us of microbial inhibition and chemical speciation. At wine pH (~pH 3-4), there are no dangerous pathogens to human life, and this has largely shaped the traditions and legislature around winemaking. Since we won’t get sick, we have far more leeway with winery sanitation. Nevertheless, pH helps winemakers understand the proclivity of wine towards spoilage, thus giving us a framework for sanitary demand. Rather than thinking of sanitation as an absolute practice, we can discuss its relevance as a stylistic tool depending on varying pH values. Furthermore, pH also influences the speciation of chemicals in wine, most notably sulfur, phenolics, and metals, and their roles in antimicrobial and antioxidant activity.

Sanitary Demand

The limits for cleaning and sanitation are well-defined (Mohapatra, 2017):

Clean: removal of soil and 90% reduction of colony forming units.

Disinfection: a reduction of 99.9% of colony forming units.

Sanitation: a reduction of 99.999% colony forming units.

Sterilization: a reduction of 99.9999% colony forming units. Chemical sterilants include 0.2% peracetic acid (PAA) and 7.5% hydrogen peroxide (H₂O₂).

Winemaking exists within a grey area of this spectrum. Ultimately, the level of cleanliness in a winery is subject to a winemaker’s microbiome of desire. The preference from native yeasts in the vineyard, to local yeasts in the cellar, or cultivated yeasts from a manufacturer will be determined by design and cleaning and sanitation practices. How we determine the design and practices will be based on our sanitary demand, the primary function of pH.

Sulfur Dioxide (SO₂)

SO₂ is a commonly used preservative and antioxidant in winemaking, where it exists in multiple forms, including molecular SO₂, bisulfite (HSO₃^-), and sulfite (SO₃^2-). The equilibrium between these species is highly pH-dependent. Collectively, we refer to their unbound forms as “free sulfur” and their combined forms as “total sulfur.” The traditional calculation used to determine the speciation of sulfur was the Henderson-Hasselbalch equation, but recent studies have shown that free anthocyanins in red wines complex with free SO₂ converting most of its free form to its inactive bound form. Traditional forms of free sulfur analysis such as Ripper titration and aeration-oxidation (A-O) overestimate truly free sulfur in red wines because they dissociate bound sulfur-anthocyanin complexes (Jenkins et al., 2020). In short, winemakers adding sulfur to red wines are only benefiting from a fraction of free sulfur measured. In addition, the Henderson-Hasselbalch equation is also insufficient for calculating molecular SO₂ in wine because of its matrix. Jenkins et al. published a modified version of the equation in 2020 which includes alcohol and temperature inputs. Click here to download his equation, but note that this will not work for red wines unless you use a different method to quantify free sulfur such as static headspace gas chromatography and sulfur chemiluminescence detection (HS-GC-SCD) (Jenkins et al., 2020).

Iron-Reactive Phenolics

The nature of polyphenols is highly pH-dependent. To begin, protein-precipitable tannin astringency is increased by changing the conformation of saliva and disrupting salivary films (Gawel, 1998; Kallithraka et al., 1997; Sotres et al., 2011). This is why highly tannic red wines tend to benefit from higher pH values (3.6-3.8). It just so happens that higher pH winemaking also accelerates the antioxidant activity of polyphenols in winemaking. This was originally established in the 1970s by Dr. Vernon Singleton who found that vicinal diphenols participate in a cascading antioxidant reaction called regenerative polymerization. We refer to these diphenol-containing compounds as iron-reactive phenolics (IRPs). At wine’s pH, only about 0.00001% of IRPs are in the phenolate form which the reaction necessitates, and thus it proceeds very slowly (Smith, 2013). “At a high pH for wine, 4.0, there would be roughly nine times as many phenolate ions as at pH 3.0, and the autoxidation rate should be nine times as fast” (Singleton, 1987). Nevertheless, these antioxidant reactions can protect the wine for decades in the bottle. During fermentation, IRPs can consume upwards of 50 mL/L/month. After 100 days post-crush, they may consume 4 mL/L/month. After a couple of years, they may consume less than 1 mL/L/month (Smith, 2013).

Conclusion

In conclusion, measuring wine pH is fundamental for understanding wine’s nature. Although a pH <3.6 is ideal for sulfur dioxide speciation, the true levels of molecular sulfur in red wines are overestimated and interrupt essential reactions in winemaking. Higher pH and warmer temperatures play an important role in red winemaking. These conditions are more favorable to spoilage organisms, so maintaining rigorous sanitation and monitoring microbial activity during the first 100 days post-crush is critical while establishing phenolic balance in red wines. Adding SO₂ during this window will bind aldehydes and free anthocyanins, both of which are essential for bound anthocyanin formation. It is ideal to wait until red wines are barreled down for élevage to add SO₂. After the 100 days post-crush window, 60-80% of free anthocyanins have likely depleted thus limiting their interactions with free SO₂ (WINEXRAY).

References

Colantuoni, G., McLeod, S. WINEXRAY LLC. https://www.winexray.com/

Gawel, R. (1998). Red wine astringency: A review. Australian Journal of Grape and Wine Research, 4(2), 74–95. https://doi.org/10.1111/j.1755-0238.1998.tb00137.x

Jenkins, T. W., Howe, P. A., Sacks, G. L., & Waterhouse, A. L. (2020). Determination of Molecular and “Truly” Free Sulfur Dioxide in Wine: A Comparison of Headspace and Conventional Methods. American Journal of Enology and Viticulture, 71(3), 222–230. https://doi.org/10.5344/ajev.2020.19052

Kallithraka, S., Bakker, J., & Clifford, M. N. (1997). Effect of pH on Astringency in Model Solutions and Wines. Journal of Agricultural and Food Chemistry, 45(6), 2211–2216. https://doi.org/10.1021/jf960871l

Mohapatra, S. (2017). Sterilization and Disinfection. Essentials of Neuroanesthesia, 929–944. https://doi.org/10.1016/B978-0-12-805299-0.00059-2

Singleton, V. L. (1987). Oxygen with Phenols and Related Reactions in Musts, Wines, and Model Systems: Observations and Practical Implications. American Journal of Enology and Viticulture, 38(1), 69–77. https://doi.org/10.5344/ajev.1987.38.1.69

Smith, C. (2013). Postmodern Winemaking: Rethinking the Modern Science of an Ancient Craft. University of California Press.

Sotres, J., Lindh, L., & Arnebrant, T. (2011). Friction Force Spectroscopy as a Tool to Study the Strength and Structure of Salivary Films. Langmuir, 27(22), 13692–13700. https://doi.org/10.1021/la202870c