Hyperoxidation Revisited

A few nights ago, I picked up an older issue of Practical Winery & Vineyard (May/June 2010) and read an article called "Sauvignon Blanc: Impact of Fining Treatment on Juice Quality". Reading the article brought to mind the lively discussion we had in the UC Davis Online Course, "Wine Stability" regarding the merit of hyperoxidation based on reading Must Hyperoxidation A Review, written by V. Schneider in 1998.¹
The discussion centered around the introduction of oxygen prior to fermentation of white musts in the absence of sulfur dioxide as a means of precipitating flavonoid and non-flavonoid phenols that are responsible for bitterness, astringency and browning during wine aging. The consensus reached by the class was that "benign neglect oxidation" is good for white wine making.
I've been rereading information regarding hyperoxidation because in making our white wines, we do want to expose our grape must to oxygen in order to precipitate out the phenols that will turn our wine brown, however, I'm learning it's much more complicated than that. According to Ribéreau-Gayon², unsulfited juices exposed to air consume a variable quantity of oxygen that is based on their caftaric acid and flavonoid concentration and is dependent on the grape variety.

All this information was difficult to digest until I found this diagram in the Third Edition of Ronald S. Jackson's book, Wine Science that diagrammed the fate of phenols in the presence of oxygen.⁴ (For a larger version, please click on the diagram below)

The fate of phenolics differs when it comes into contact with oxygen at the must stage and then at the wine stage:

• At the must stage, the phenolics (caftaric acid is by far the most abundant) are converted to quinones by enzymes called polyphenol oxidases (PPO) and in Botrytis infected grapes, by the enzyme laccase. The resultant caftaric acid quinone can go on to catalyze three further nonenzymatic reactions:
• 1) Combine with glutathione to yield the Grape Reaction Product (GRP) 2-S-glutathionyl caftaric acid
• 2) After glutathione depletion, any excess caftaric acid quinone can oxidize other must constituents including GRP and flavanols and regenerate caftaric acid
• 3) Polymerize with its own precursor caftaric acid, regnerating the original reduced phenol form¹
• At the wine stage, the slow oxygen infiltration into barrels and bottles stoppered with cork allows oxidation and polymerization reactions to occur.

The Schneider review goes on to explain the technical application of hyperoxidation of musts and gives specific information on it's results in Chardonnay and Chenin Blanc, two grape varieties that we will be growing.
In addition to exposure to oxygen prior to fermentation, the yeasts also require some oxygen for survival during the declining phases of fermentation when ethanol levels are increasing. The Schneider review says that during this phase, oxygen aids in the removal of toxic medium length (C8-C12) fatty acid chains and accelerates the synthesis of C16-C18 fatty acids and sterols, which contributes to better sugar uptake by the yeasts. Also, and I did not know this, molecular oxygen (how much?) during this time allows for the use of proline as a supplementary nitrogen source for the yeast. So much to think about!
1. V. Schneider, Must Hyperoxidation A Review, Am. J. Enol. Vitic., Vol. 49, No. 1, 1998.
2. P. Ribéreau-Gayon, D. Dubourdieu, B. Donèche, and A. Lonvaud, Handbook of Enology, Volume 1, The Microbiology of Wine and Vinifications, Second Edition, 2006, John Wiley & Sons, Ltd, pg. 418-420.
3. Chemical Structure of Caftaric Acid. All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.
4. Jackson, Ronald S., Wine Science: Principles and Applications, Third Edition, Elsevier, Academic Press, 2008, pg. 298.