Dissolved Oxygen and Free SO2

Oxygen contact with wine during processing is nearly always unavoidable. Unfortunately, oxygen exposure is not always desirable. Many clients complain of an SO₂ imbalance i.e. low free SO₂ levels together with high total SO₂ levels. The culprit in these cases is usually too much dissolved oxygen.

SO₂ is added to wine to prevent oxidation reactions. 4mg/L SO₂ will consume1mg/L oxygen. Knowing oxygen levels in wine is especially important at bottling and oxidation is a commonly encountered fault in bottled wines. Historically, oxidation has represented up to 7% of faults/rejections at wine shows and competitions.

Most cellar processes involve oxygen uptake to a greater or lesser degree. Routine cellar operations (pumping, filtration, racking, stabilisation etc) can result in single oxygen additions of between 0.5-8mg/L. Barrel maturation results in oxygen additions of 20-40mg/L/year.

Ideally oxygen in wine for bottling should be measured before bottling begins, and should preferably be less than 0.5mg/L. If pre-bottling oxygen levels are much higher than this the wine should be sparged with nitrogen.

Oxygen pickup at bottling should not exceed 0.8mg/L, and should preferably be less than 0.2mg/L in white wines and less than 0.5mg/L in red wines. If precautionary measures are not taken, total oxygen after bottling can be as much as 4-5mg/L.

Total oxygen or total package oxygen (TPO) includes the oxygen in the headspace. Unless a vacuum filler is used, the headspace may include up to 2.8mg/L oxygen. Knowing that 4mg/L SO₂ reacts with 1mg/L, this headspace oxygen can decrease free SO₂ by 11mg/L. Total oxygen (TPO) levels after bottling should be less than 1-1.25mg/L for red wines and less than 0.5-0.6mg/L for white wines.

Example:

• A wine containing a small amount of dissolved oxygen is bottled without necessary care.
• Each bottle of wine contains 1.7mg/L dissolved oxygen after bottling.
• If the headspace is 5ml, it will contain 1.4mg/L of oxygen.
• The newly bottled wine will have a total oxygen level of 3.1mg/L.
• 4mg/L SO₂ is needed to react with 1 mg/L oxygen, so 12.4mg/L SO₂ is needed to react with the oxygen in the bottle.
• If the wine being bottled contained 30mg/L free SO₂ at bottling, almost half will be consumed by the oxygen, leaving just 17mg/L free SO₂ in the bottle to provide long term protection.

Taking samples for dissolved oxygen during processing:

When taking a sample, the aim is to exclude oxygen from the sample bottle. The easiest way to achieve this is by replacing the oxygen in the bottle with an inert gas. Ideally the sample bottle should then be lowered rapidly into the tank from the top, so that the bottle sinks and the sample is drawn from the middle of the tank. Glass bottles are more efficient for this as they sink faster. If taking a sample through the tank lid is not possible, then the sample will need to be taken from the sample valve – in this case run some sample out first into a separate container and then fill the gassed sample bottle with the sample tap submerged as deep into the inert gas as possible.

Measuring the amount of oxygen that enters wine during processing operations is more easily achieved by measuring the decrease in free SO₂ after each operation. Knowing free SO₂ values before individual cellar operations, and then measuring the free SO₂ again 4-5 days after the operation, can give a good indication of the extent of oxygen pick-up.