REDOX (Reduction-Oxidation) reactions are at the very core of wine production. For now, we leave behind the connotation between reduction/sulfides and oxidation/aldehydes to focus on the cycle between REDOX states and the reductive strength built into wine’s chemical structure.

What is REDOX?

A REDOX reaction is a type of chemical reaction where electrons are transferred between two molecules or atoms. REDOX is measured as oxidation-reduction potential (ORP) which quantifies the tendency to gain or lose electrons in units of volts (V). REDOX involves two simultaneous processes:

Reduction (-): The gain of electrons. When the ORP is negative, it means that substances have a greater tendency to donate electrons.

Oxidation (+): The loss of electrons. When the ORP is positive, it means that substances have a greater tendency to accept electrons.

REDOX and Wine

Implementing ORP measurement into wineries has yet to be commonly adopted, but advances are continuously being made. One example is at Beringer Vineyards where Bob Coleman worked with Dr. Roger Boulton to implement some of his most innovative research into production practice. The Boulton Model, for example, is being used to predict stuck fermentations using just temperature and change in °Brix (Boulton, 1980). While this may sound simple, it has taken a dedicated team of engineers to achieve the results you see below in Figure 1.

Figure 1. The Boulton Model Prediction using pressure transducers for continuous °Brix in a commercial white wine fermentation. (The Future of Winemaking: Honoring the Vision of Professor Roger Boulton, 2022).

By complementing the Boulton Model ORP probes, pressure transducers, and some clever models, not only are researchers at UC Davis predicting stuck fermentations, but they are also extrapolating other key pieces of information such as yeast viability and nutrient demand. A glimpse into the future of this truly exciting technology can be seen by clicking the image below.

Figure 2. UC Davis’s use of modern data visualization software (The Future of Winemaking: Honoring the Vision of Professor Roger Boulton, 2022).

While these are both truly exciting technologies, it will require a lot of refinement until it finds its way into the average winery. ORP data itself is challenging to extrapolate meaning from, and it doesn't help that wine is a complex system that generates a lot of noise. This is especially true for fermentation which is subject to temperature fluctuation, electrode contamination, chemical interference, and several other interference variables.

So, if it's that much trouble, why do all that work? Well, for Treasury Wine Estates, we can only assume that continuous Brix measurement and ORP technology serve as a crystal ball that will help them predict the future. Timing is everything in winemaking, and this is one way for them to buy time. Keep in mind, that this is a large production facility, so this type of investment is appropriate for their scale. For the rest of us, our palates are our ORP probes, and that might just be enough. We can still take away fundamental lessons from their advancements teach us.

The most important controls all winemakers have to prevent stuck fermentation are temperature and oxidation. As shown in Figure 1, when the temperature was less than 15°C (59°F), the model predicted the fermentation would stick. It wasn't until the temperature was raised that the model predicted a complete fermentation, a perfect fit to the final Brix curve. Therefore, warmer temperatures have a direct relationship with improved fermentation kinetics. In addition, yeast respond more strongly to oxygen than any nutrient. By incorporating oxygen, you help sustain the population of yeast which directly relates to the rate of fermentation.

Vineyard REDOX

Phenolics have a special relationship with REDOX because reductive strength is built into their chemistry, a latent potential we refer to as phenolic reactivity. During the different stages of wine production, we may consider growth phases (anabolism) as being more reductive and decay phases (catabolism) as being more oxidative. Using this as a framework helps us understand how we build flexibility into the wine production process. We begin in the vineyard.

Reduction (-)

Growth, Veraison, & Ripening: Different phenolic families peak in concentration between growth, veraison, and ripening. Generally speaking, iron-reactive phenolics and protein-precipitable tannins peak by the start of veraison whereas anthocyanins peak when sugars stop accumulation by the end of ripening (Kennedy et al., 2000). It is reasonable to assume that peak phenolic reactivity occurs around the start of veraison and decreases slowly through ripening as the seeds lignify. Once ripening finishes, the phenolics are no longer protected by the momentum of growth and phenolic reactivity rapidly begins to decline through field oxidation.

Oxidation (+)

Field Oxidation: The ideal outcome of field oxidation is to modulate astringency and develop flavors. It’s commonly accepted in the industry that if phenolic and flavor balance is achieved in the vineyard, then the wines can practically make themselves. Many revered wines are made this way, but it is not a formula for every site, especially in the context of climate change. If overdone, field oxidation can decrease phenolic antioxidant capacity by 90% in as little as 3 weeks, taking years off a wine's lifetime (Smith, 2013). Extractable Anthocyanins steadily decline from their peak, often losing upwards of 50% of the total color within that same timeframe. Flavors oxidize, and the grapes are more susceptible to quality loss from extreme weather events. There are risks to field oxidation which is only exacerbated by extreme weather events prompting many producers to harvest earlier and control oxidation in the cellar.

Reduction (-)

Fermentation: In a fundamental way, fermentation is a reductive process in which sugars are broken down to pyruvate, decarboxylated to acetaldehyde (and CO2), and finally reduced to ethanol. As yeast biotransform grapes into wine, they consume oxygen so rapidly that it can’t be measured with a dissolved oxygen meter and instead has to be measured with an ORP probe. The ORP can naturally shift from positive to negative, but some grapes are naturally prone to reduction in part due to their higher phenolic reactivity. Once negative, Hydrogen Sulfide (H2S) starts to appear, and adding too much oxygen will only exacerbate this effect because of a phenomenon called regenerative polymerization. In the presence of oxygen, iron-reactive phenolics can actually increase their reductive strength by building larger polymers. You may find this troublesome, but iron-reactive phenolics are likely the most significant anti-oxidant force protecting wine for the rest of its life.

REDOX (+/-)

Regenerative Polymerization is an intriguing phenomenon that beautifully represents the push and pull of REDOX in wine. The reductive strength of wine phenolics will be greatest immediately after fermentation when phenolic reactivity is highest and oxidative aging begins. Counter to what you might think, micro-oxidation during this time actually increases to phenolic reactivity for the life of the wine (Smith, 2013). Under wine’s acidic conditions, phenolics can protect the wine from oxidation for decades.

Summary

If we zoom out to see a wine’s timeline from grape to glass, we can begin to appreciate that wine production is an oscillating series of reductive and oxidative reactions; a REDOX balancing act. This is especially true for grapes and wines with high concentrations of iron-reactive phenolics because phenolic reactivity is built into their chemical structure. Temperature, pH, and light are fundamental variables influencing phenolic reactivity in the vineyard and managing it in the cellar. Incorporating REDOX into our understanding is fundamental for elevating our perspective on the true nature of wine.

References

Boulton, R. (1980). The Prediction of Fermentation Behavior by a Kinetic Model. American Journal of Enology and Viticulture, 31(1), 40–45. https://doi.org/10.5344/ajev.1980.31.1.40

Kennedy, J. A., Matthews, M. A., & Waterhouse, A. L. (2000). Changes in grape seed polyphenols during fruit ripening. Phytochemistry, 55(1), 77–85. https://doi.org/10.1016/S0031-9422(00)00196-5

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

The Future of Winemaking: Honoring the vision of Professor Roger Boulton. (2022, November 7). https://livestream.com/accounts/11451219/rogerboulton