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.
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.