The Power of Hydrogen, or pH, is a measure of how acidic (<7) or basic (>7) a solution is on a logarithmic scale. It’s the most fundamental analysis in all of winemaking informing us of microbial inhibition, chemical speciation, and tartrate stability.

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 the risk of food born illness is very low, winemakers have far more leeway with winery sanitation than with other food products. Nevertheless, pH helps winemakers understand the proclivity of wine towards spoilage, thus giving them a framework for sanitary demand. Rather than thinking of sanitation as an absolute, 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:

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) (Mohapatra, 2017).

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. For instance, premium Cabernet Sauvignon producers with high pHs tend to have immaculate cellars with strict sanitation and risk adverse protocols like inoculating with cultivated yeast, whereas small lot Pinot Noir producers tend to take more risks with spontaneous fermentations in porous vessels with less sulfur. How we determine the design and practices for each wine style will be based on sanitary demand, the primary function of pH.

SO2 is a commonly used preservative and antioxidant in winemaking, where it exists in multiple forms, including molecular SO2, bisulfite (HSO3-), and sulfite (SO32-). 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).

While our lab uses the reference methodology for free sulfur determination in wine, aeration-oxidation (A-O), it’s worth noting that A-O doesn’t distinguish between anthocyanin-bound SO2 and truly free SO2.

To learn more about Free SO2 and its importance in winemaking, become a Bound member.

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