5 Ways to Reduce Wine Oxidation
by Brittany Goldhawke R&D Enologist at BarrelWise.
Wine oxidation can occur due to excessive and prolonged contact with an oxygen-containing atmosphere. Air contains about 21% oxygen and, in most circumstances in winemaking, controlling how your wine interacts with air is the key to controlling wine oxidation. After significant exposure, oxygen can wreak havoc on a wine’s color and aroma. As excessive contact with oxygen can occur throughout the entire winemaking process, care must be taken to control and mitigate the negative effects oxygen can have on wine quality.
Of course, not all oxygen contact is detrimental. Controlled oxygen exposure during fermentation can help increase yeast viability and may help to limit reductive off-aromas. After fermentation, wine maturation in barrel also benefits from the limited and slow ingress of oxygen, increasing tannin polymerization and improving mouthfeel. The problems associated with wine oxidation only occur when too much oxygen enters the winemaking process too quickly.
Below are five techniques that winemakers can use to mitigate the negative effects of oxygen on wine:
1. Reduce Botrytis through good canopy management practices
We all know that high-quality wines come from high-quality grapes and this is certainly true when we discuss wine oxidation. In grape growing seasons with low temperatures and high humidity or high rainfall, the fungus Botrytis cinerea has an opportunity to thrive. Importantly, Botrytis releases an enzyme called laccase into the grape berry, which ultimately makes its way into the wine. Laccase survives the entire winemaking process, and in combination with oxygen, will oxidize phenols to quinones, compounds responsible for the destruction of the wine’s desirable aromas.
Laccase is hard to remove once it enters the must, and it survives in higher concentrations of sulfur dioxide. Thus, the best way to prevent oxidation due to Botrytis is to control the infection in the vineyard. Good canopy management is key to achieving this and can greatly reduce the amount and spread of the Botrytis infection. Keeping an open canopy that allows airflow can help to reduce moisture on the berries. If Botrytis has taken hold of your harvest, try to ensure that oxygen exposure is kept to an absolute minimum as laccase requires oxygen for the oxidation reaction to occur. A great source of recommendations for processing infected grapes and treating laccase is the Australian wine research centre’s fact sheet: “Managing Botrytis infected fruit”
2. Inert Gas Blanketing
Important in the wine-making process, is the movement of wine from tank, to barrel, to bottle. This process involves the flow of wine into empty (i.e. air-filled) vessels, where the contact of wine with oxygen is inevitable. Especially in Botrytis infected fruit, oxygen exposure must be reduced to an absolute minimum. So, what can we do to mitigate this?
Using an inert gas cover such as carbon dioxide (CO2), nitrogen, or argon, are great ways to limit oxygen exposure to both must and wine. CO2 is commonly used during the initial grape processing stage, where dry ice (i.e. solid CO2) is placed on the grape must surface where it sublimates and forms a protective blanket over the must before fermentation. Racking must from the tank to tank can also require the use of inert gas. Adding CO2 to the empty tank and on top of the must, prior to the racking procedure, helps control oxygen contact. Post-fermentation, oxygen in the headspace of barrels and tanks is limited by topping the headspace with wine.
3. Sulfite Addition
Sulfur dioxide (SO2) is a common must and wine additive that not only prevents spoilage by unwanted microbes but is also a handy tool for winemakers to reduce the level of oxidation in wine. SO2 in wine is found as both free and bound SO2, and it is the free SO2 (FSO2) that helps to reduce wine spoilage and oxidation. FSO2 is comprised of bi-sulfite ions, sulfite ions, and molecular SO2. Molecular SO2 provides the antimicrobial effect, while bi-sulfite and sulfite help to reduce oxidation.
At a pH between the range of 2 to 7, FS02 in the form of bisulfite dominates in wine. This form of FS02, will be the working force behind reducing chemical oxidation of the wine by free radical compounds like H2O2 that are produced from the oxidation of phenols in wine. Sulfite prevents oxidation by inactivating enzymes like laccase, although higher than normal concentrations of sulfite are required to achieve this. The protective capacity of FS02 is dependant on the wine’s pH and the final molecular SO2 concentration. You can use this handy online calculator, provided by Winebusiness.com, to help you estimate the concentration of FSO2 you need to protect your win
4. Keep a close eye on your barrels
Once fermentation is complete, and the wine has been transferred from tanks to barrels, there are still ways oxygen can sneak in and reduce the quality of your wine. Although the wine maturation process in barrel benefits from the slow introduction of oxygen, it is wise to keep any additional exposure to a minimum.
Wine barrels are active vessels that are consistently interacting with the maturing wine they contain. Being porous in their nature, they tend to soak up wine within the staves, increasing the headspace and volume within the barrel. The ethanol and water components of wine can then evaporate at the surface of the barrel, the rate of evaporation varies according to the humidity, temperature, and environment of the cellar. Due to wine loss by evaporation, the barrel must be continuously topped up with wine in order to prevent oxygen exposure in the newly formed headspace. A good topping cycle every two to four weeks, and controlling the cellar environment, can help keep barrel headspace to a minimum.
5. Choose the right corks
You have done a great job at controlling oxidation, and your wine is ready to be bottled, but you are not out of the red zone yet. The choice of cork will help to determine how your wine ages in bottle, and how much oxygen will enter overtime. Natural corks are made from Quercus Suber, an evergreen oak tree. Due to its wood origins, it is porous and allows in approximately 1 mg of oxygen per year into the wine bottle. After several years of aging, the additive effect of oxygen can cause harm if not initially accounted for.
Other choices available are synthetic corks or screwcaps. Synthetic corks are more consistent in their porosity and are available in different styles that allow different rates of oxygen entry. Screwcaps offer a great option for wines that do not require much bottle aging, as they are the best at limiting oxygen exposure.