Field Oxidation


Clark- can you elaborate on the following text from your TONG paper? Specifically, the relationship between "active tannins and sulfides"? Also, what do you mean by "field oxidation"?

"Alcohol adjustment enabled California winemakers to achieve full ripeness, but that resulted in new problems. Ripe musts full of well-extracted, active tannins produce stinky sulfides. These unpleasant but transitory compounds are a sign of healthy life energy, but they are disconcerting to the novice winemaker and require a new skill set.

Instead, reductive behavior in highly concentrated wines like Cabernet and Syrah has prompted many winemakers to drive the life energy out of their grapes by excessive hangtime and field oxidation."



Red grapes at peak ripeness have a maximum reductive strength. A typical Napa Cab Sauv will take up 60 – 80 mls of oxygen for a month. If deprived of oxygen, it will become very reductive, often producing H2S as an artifact. These sulfides have nothing to do with dusting sulfur and are an artifact of very low redox potential.

Flavonoid phenols such as catechin exist in grape skins as monomeric, and also as two types of polymers. During early ripening they form enzymatically into non-oxidative polymers with 4-8 bonds and a compact macrostructure. These bonds are acid-labile, so from a functional point of view can be regarded as banked monomer, and become monomeric in must as soon as they are crushed.

The second type of polymeric linkage is oxidative. These are not 4-8 linkages, but instead are initiated by oxidation of the B ring, which contains an ortho-diphenol structure, and result in random linkages which are covalent and permanent, not subject to acidic hydrolysis. For example, the epicatechin gallate polymers in seeds become oxidatively cross linked in seeds during ripening, reducing their solubility and harsh flavor, making berries more palatable to birds.

The field oxidation process takes place in the late stages of ripening, resulting in permanent structures which are oxidatively inactive. Since anthocyanins are compartmentalized in skins, they are not optimally incorporated into these structures, which are prone to over polymerization and precipitation after a few years, leaving a dry, gritty impression on the palate. They are also incapable of forming copigmentation colloids.

Thus field oxidation robs the wine of both anthocyanins and active tannins. A typical Napa Cabernet with three weeks of excessive ripeness will be able to consume only 30 – 40 mls of oxygen for 3 – 5 days, having lost approximately 90% of its reductive strength. Those extra three weeks on the vine steal a decade of cellaring potential.

The technique of field oxidation was perfected by the Australians to soften tannins so no cellar elaboration was necessary, as an easy means to produce bottle-ready fruit bombs today’s marketplace demands. This is fine as long as it is understood that the effect is transitory. It’s all very well for production of flavorful (if shallow) wines intended for current consumption. The problem arises when the technique is used for expensive wines with ageworthy reputation, such as the modern anathemas currently perpetrated in Barolo and in Napa Cabernet.

Postmodern Winemakers do their cooking in the kitchen, not the field. A trained hand will pick ripe but not overripe and utilize oxygen to convert reductive energy into structure, taking advantage of active tannins and monomeric anthocyanins in intimate contact, and producing a rich, light, stable tannin soufflé using oxygen as the wire whisk, building and balancing reductive strength in the process. The result is wines of both youthful finesse and enhanced longevity, to say nothing of the enhancement of profundity of flavor and character.