Reductive Regime vs Oxidative Regime for White Wine Making

Reading Clark Smith's article on Making White Wine reminded me of my favorite go to article on hyperoxidation called Must Hyperoxidation: A Review written by V. Schneider and available online. Clark Smith wrote that the reductive treatment of must is the newest methodology in the treatment of white wine making which superseded hyperoxidation. It turns out that hyperoxidation and the use of sulfur dioxide, a totally reductive treatment are two divergent pathways in the treatments of a white must and the article by Schneider explains the chemistry behind this distinction.
Which method is used is also dependent upon the press regime that is employed as well as the desired outcome of the phenolics in the resultant wine. If the press regime includes pressing of the grape skins and seeds, more of the flavonoid phenols will end up in the must.
Schneider defines hyperoxidation as:
          the phenomenon of phenol stabilization via oxygen consumption in the           must by the deliberate use of oxidation prior to fermentation in order to           improve a wines' shelf-life.
Oxidation of must is an enzymatically induced reaction and is different from oxidation of wine. Enzymatic oxidation of must is carried out by tyrosinase, a natural grape enzyme. If the grapes are infected with molds, the enzyme laccase also contributes to the oxidation. Press juice has more tyrosinase activity than free run juice.

The most abundant non-flavonoid phenolic compound in white grapes are the hydroxycinnamic acids and the major derivate is caftaric acid. When white grape juice is processed without sulfur dioxide, enzymatically induced oxidation occurs and leads to a precipitation of phenolic compounds as insoluble brown pigments. For must hyperoxidation to be effective, it has to be carried out directly after pressing and before any further juice processing. Clarification by racking or centrifugation is an important next step and is used to reduce suspended solids to less than 1% by weight in order to eliminate the major part of the phenolic precipitates. Because hyperoxidation occurs prior to the addition of yeast, it has no influence upon fermentation kinetics.
Conversely, when enzymatic activity is inhibited by the use of sulfur dioxide, the phenols are protected against oxidation and stabilized in solution. Therefore, the fate of these phenols is closely related to sulfur dioxide management.
Sulfur dioxide management and the mechanism by which it works is threefold:

• It inhibits and destroys tyrosinase;
• Sulfur dioxide reduces caftaric acid quinone back to its phenolic form, preventing flavonoid oxidation
• Sulfite containing media enhances the solubility of phenolic compounds

Therefore, the early use of sulfur dioxide in the must, preserves the phenolic content.
References:
1. V. Schneider, Must Hyperoxidation A Review, Am. J. Enol. Vitic., Vol. 49, No. 1, 1998.