Phenolic Paradox: Antioxidants That Prevent and Accelerate Aging

When we talk about wine aging, especially in white wines, we often think about oxidation as the enemy — the culprit behind browning, faded aromas, and premature aging. But the chemistry behind wine’s evolution is much more nuanced. At the heart of this paradox lies a group of compounds that explains it all: phenolic acids.

Phenolic acids — particularly hydroxycinnamic acids like caftaric acid, p-coumaric acid, and caffeic acid — play a central role in shaping how white wines age. They’re natural components of grape juice and they’re known for their antioxidant properties. But here’s the twist: the very same compounds that help protect wine in its youth can, over time, lay the foundation for premature oxidation.

Phenolic Acids: Wine’s First Line of Defense

In freshly pressed juice, hydroxycinnamic acids act like shields. They mop up oxygen, neutralize free radicals, and even chelate transition metals like iron and copper — both of which are notorious for catalyzing oxidative reactions. This makes them essential in the early stages of a wine’s life, particularly when exposure to air is common. They also react well with other antioxidants like glutathione and sulfur dioxide. For example, caftaric acid can react with glutathione to form a stable, non-browning compound known as GRP (grape reaction product), effectively defusing its oxidative potential. 

When the Phenolics Turn Pro-Oxidant

Things get complicated as wine ages. Hydroxycinnamic acids, especially in the presence of iron, can turn from antioxidants to pro-oxidants. Here’s how: when they scavenge oxygen, they often generate hydrogen peroxide as a byproduct. In the presence of iron (Fe²⁺), this peroxide undergoes Fenton reactions that form hydroxyl radicals — extremely aggressive oxidants that attack aroma compounds, ethanol (producing acetaldehyde), and even tartaric acid. This sets off a chain reaction. The wine begins to lose its crisp, fruity freshness, browns in color, and takes on oxidative notes like bruised apple or sherry. 

Hyperoxidation: A Preemptive Strike

To manage this paradox, winemakers have developed techniques like hyperoxidation. This involves intentionally oxidizing the juice before fermentation, allowing phenolic compounds to brown and precipitate out. The juice transforms from the color of Coca-Cola to a pale white wine resistant to browning and often requiring less sulfur. This technique is often used for non-aromatic whites like Chardonnay that are well suited for aging in barrel and bottle. 

But What About Aromatic Varieties?

Here’s where it gets tricky. Aromatic whites like Sauvignon Blanc or Riesling depend heavily on volatile compounds — esters, thiols, and monoterpenes — to express their character. Hyperoxidation would strip many of those out.

Instead, these wines are made reductively, with strict oxygen control. The phenolics remain intact, but so does the risk. If oxygen sneaks in later (during bottling or through a closure), the wine can oxidize quickly. In Sauvignon Blanc, for example, you might notice fading fruit and a shift to nutty notes — signs that phenolic oxidation and acetaldehyde formation are taking over.

And yet, Riesling can sometimes age for decades. Why?

Exceptions That Prove the Rule

Ageworthy Riesling’s success lies in its low pH. For example, German Rieslings that have exceptionally high acidity and a low pH decrease the amount of phenolics that are in their phenolate form, thus slowing phenolic acid oxidation reactions. This is similar to the technique used in Hunter Valley with Semillon. Rather than relying on hyperoxidation or total reduction, they harvest early (preserving acid), ferment in stainless steel, and avoid oak or skin contact. The resulting wine is taut and neutral in youth but transforms with bottle age into something honey, floral, and toasty. They don’t need hyperoxidation, because preserving Semillon’s acidity sets it up for longevity.

Final Thoughts

Phenolic acids are instrumental to understanding wine's perplexing and often counterintuitive nature. The phenolics narrative as a whole is the best scientific narrative aligning with premium winemaking decisions. Contact us to learn more about how our advising and analytical services can help you integrate phenolics to improve your wines.