Oxidation: who’s responsible?

In the last blog we looked at Oxygen in wine making and the positive and negatives associated with using it in winemaking processes. I thought it would be nice to follow up with a little more attention to detail on the process of oxidation, to look at the different types of oxidation we see and what causes them.  

Oxidation comes in three forms: Enzymatic, Chemical and Microbial, also known as enzymatic and nonenzymatic.  

Enzymatic. This oxidation mainly takes place in juice/must and I wish I could tell you what’s really going on here, but I myself don’t understand all the science behind it so I’ll tell you what I do understand. Basically “The mechanism of enzymatic oxidation is based on the oxidation of phenolic compounds”.  There are two main enzymes that cause oxidation: 

  • Polyphenol Oxidase or PPO (says it all in the name) which breaks down phenols causing browning and production of acetaldehyde/aromas of sherry, and can be treated with SO2 (just think adding PMS into the juice tray of the press) 
  • Laccase aka Botrytis Cinera (this one we all know right!?) which is a little harder to treat as it is more resistant to SO2. This will limit aromatics and cause browning (think wet cardboard) so we might add some carbon at the hopper to try and reduce the effects of Laccase on fresh juice.  And that is enzymatic Oxidation. Moving on… 

Chemical. This oxidation can occur in juice/must but is usually overshadowed by enzymatic, so we see it more commonly in wine. Again, there is a lot of chemistry and reactions that I don’t understand. But basically, “In chemical oxidation, oxygen does not react directly with phenolic compounds.” Instead, we see oxygen reacting with polyphenols and eventually creating Hydrogen Peroxide which coverts ethanol to acetaldehyde resulting in browning, the sherry aromas, and premature aging. Not good stuff.  This is the main form of oxidation we see in wine making.  

Microbial. These are spoilage microorganisms. Commonly seen in both red and white wines during storge. Think Brett (horsey), Acetic acid (nail polish remover) and film yeasts (flor on sherry). These guys are all dependent on Oxygen to survive, so by keeping all ullages gassed, barrels full and free Sulphur’s up you should avoid them.  

Learn about Fortified Wines - Sherry - The Cellar NZ – The Cellar Store NZ
A flor barrel of sherry. NOT what you want in the winery.

As you have read, my chemistry isn’t the best. So, if you would like to fill in the gaps, I think this article would do the trick nicely: The Impact of Oxygen at Various Stages of Vinification on the Chemical Composition and the Antioxidant and Sensory Properties of White and Red Wines 


Tomasz Tarko, Aleksandra Duda-Chodak, Paweł Sroka, and Małgorzata Siuta (March 2020) The Impact of Oxygen at Various Stages of Vinification on the Chemical Composition and the Antioxidant and Sensory Properties of White and Red Wines https://www.hindawi.com/journals/ijfs/2020/7902974/

Ruis A. (n.d) NMIT, CCO410 Wine Finishing and Analysis, S2-20 ‘Winery Oxygen Management’ PowerPoint slide show 

Strategies to manage dissolved oxygen, https://winesvinesanalytics.com/features/article/119752/Strategies-to-Manage-Dissolved-Oxygen 


The cellar store NZ: https://thecellar.store/pages/sherry