Oxygen pick-up during packaging – understanding total package oxygen

Oxygen pick-up during packaging – understanding total package oxygen
There are two major contributors to the total amount of oxygen that is present in a bottle of wine at the time of packaging: dissolved oxygen (DO) and headspace oxygen (HSO). Additional oxygen is introduced into the bottle after packaging from oxygen encapsulated in the closure and oxygen that diffuses through the closure during wine ageing.

Dissolved oxygen
Prior to bottling, the gas content of a final wine blend is adjusted by sparging with nitrogen and/or carbon dioxide to ensure dissolved oxygen (DO) and carbon dioxide concentrations reach desired levels. More information on this process is detailed in Gas adjustments at bottling.

Increases in DO during bottling are minimised by sparging empty bottles on the filler with an inert gas and using inert gas cover on the filler tank itself. Nitrogen, argon or carbon dioxide (in the form of dry ice) are used to reduce or keep the DO below 0.5 mg/L as per recommendations in Australian Grape & Wine’s Packaging Guidelines.

Headspace oxygen
Headspace oxygen (HSO) is the gaseous oxygen present in the unfilled headspace (ullage) of a wine bottle. This ullage is required to minimise risks associated with thermal expansion and to meet packaging and regulatory specifications. Headspace oxygen concentrations are generally greater under screw cap closures than cylindrical closures, due to the higher ullage at filling. Australian guidelines state that for standard CE.T.I.E cork mouth bottles, the ullage space should be greater than 12 mm at 20°C. Amon and Simpson (1986) also recommend a minimum ullage distance of at least 13 mm (measured at 20°C) for nominal 750 mL bottles, in order to allow enough volume for expansion of the wine during storage at 20–25ºC. For standard Bague Verre Stelvin (BVS) finish screwcap bottles, however, ullage space should be greater than 30 mm.

Total package oxygen
The total concentration of oxygen in a packaged wine is commonly referred to as the total package oxygen (TPO) and this is quantified immediately after packaging using the sum of DO and HSO. More information about TPO can be found in the AWRI fact sheet Understanding total package oxygen. The AWRI developed a method for calculating the total package oxygen in a bottle of wine and provides access to this as a free online TPO calculator.

TPO specifications for different levels of oxygen management are shown below:

<1.5 mg/L Best practice
1.5-2.5 mg/L Acceptable
>3 mg/L Requires improvement
>4 mg/L Poor practice

AWRI oxygen management audits have shown that typically more than 60% of the oxygen in the bottle is present in the headspace. Many factors can influence the concentration of HSO including fill height, wine temperature, inert gas use and bottle size. HSO can be minimised through appropriate application of inert gases and the use of counter-pressure fillers. Most modern bottling lines are able to achieve displacement of ~60 – 80% of the oxygen from the headspace through vacuum or inert gas application (e.g. nitrogen or argon sparging), the use of liquid nitrogen droplets and inert gas coverage during filling. AWRI case studies have shown that cylindrical closures applied without a vacuum can result in an additional 1 mg/L HSO pickup.

Oxygen encapsulated or entrained in cylindrical closures can have an indirect impact on wine shelf-life. The mechanisms by which oxygen trapped in the lenticels of these closures diffuse into the headspace of bottled wine are not fully understood. The amount of entrained oxygen in a closure can be significant, especially in sparkling wines, which often employ a larger cork closure. While the same mechanism does not apply for screw cap closures, caution is required when applying these closures. A significant proportion of the oxygen residing in the closure can be transferred into the headspace of the bottle if the internal cavity of the closure is not sparged in a similar manner to the empty bottle.

Headspace oxygen and oxygen encapsulated in the closure itself are the main sources of oxygen for bottled wines in the period immediately following packaging (2-4 months). This indirectly results in the loss of free sulfur dioxide (SO2) in packaged wines following packaging (Kwiatkowski et al 2007, Dimkou et al. 2011). Permeation of oxygen through the closure is then responsible for further sulfur dioxide reduction during bottle ageing. Closure companies are continuing to develop closures that can deliver consistent and reliable permeability of oxygen.

References and further reading
Amon, J.M.; Simpson, R.F. 1986. Wine corks: a review of the incidence of cork related problems and the means for their avoidance. Aust. Grapegrower Winemaker 268: 63-80.

Dimkou, E., Ugliano, M., Dieval, J. B., Vidal, S., Aagaard, O., Rauhut, D., Jung, R. 2011. Impact of headspace oxygen and closure on sulfur dioxide, color, and hydrogen sulfide levels in a Riesling wine. Am. J. Enol. Vitic. 62: 261−269.

Hirlam, K.C., Scrimgeour, N., Wilkes, E.N. 2019. Orientation and temperature cycling impacts on the oxygen transmission rate of wine closures. Poster presented at the 17th Australian Wine Industry Technical Conference, July 2019. Available from: https://awitc.com.au/wp-content/uploads/2019/07/3_Hirlam_OTR-Diurnal-Cycling.pdf

Kwiatkowski, M., Skouroumounis, G., Lattey, K., Waters, E. 2007. The impact of closures, including screw cap with three different headspace volumes, on the composition, colour and sensory properties of a Cabernet Sauvignon wine during two years’ storage. Aust. J. Grape Wine Res. 13: 91-94.