Importance of gases in bottling wine. Part 1: The influence of oxygen on bottling

The procedures involved in producing good wine with a proven quality are all enormously important to the final result, but the bottling of the wine is a fundamental step towards success.

The importance of the wine bottling process

It is a definitive aspect since it is the last procedure before the bottle reaches the final consumer. An error in bottling can ruin the entire production process, with the consequent losses that implies in all respects.

But this will not happen if done correctly, eventually achieving the quality south during the storage time in the bottle.

What influence does oxygen have on the bottling process?

O₂ is an indispensable molecule in wine production: It contributes to color stabilisation (polyphenol polymerisation reaction) and favours higher electrochemical potentials that prevent the formation of reductive odorant compounds, the polymerisation of “hard” or “astringent” tannins to “sweeten” the wine, among other aspects. In alcoholic fermentation, it helps in the synthesis of fatty acids and sterols of the cell membrane, facilitating yeast activity.

However, dissolved oxygen is responsible for most oxidation phenomena, which includes both loss and aromatic evolution as well as browning and color loss. These phenomena are more relevant at the time of bottling.

At each stage of the manufacturing process, a certain amount of oxygen is added, reaching saturation levels. Under normal winery conditions, this oxygen is mainly consumed by the SO₂ in the wine when in free form or by the oxidisable components of the wine otherwise. The following table breakdown these contribution values:

Table 1: Oxygen provided during various winery operations.

*Start of the process

A poorly tightened stopper is also a variable source of oxygen.

The solubility of oxygen in wine depends on the pressure, temperature and alcoholic strength.

• At 20ºC and at atmospheric pressure, 8.4 mg/l are required to reach saturation (Moutounet and Mazauric, 2001)
• At 0ºC, saturation is reached with 11.5 mg/l

In the lower part of large tanks, these values increase due to increased pressure.

Another critical moment, as far as dissolved oxygen is concerned, is tartaric cold stabilisation, which includes cooling, stirring and subsequent filtering. When reaching temperatures below 0ºC, the oxygen solubility increases, reaching levels above 12 mg/l after a vigorous stirring.

The dissolved oxygen consumption time in wine also depends on the temperature:

Table 2: Consumption of dissolved oxygen based on temperature

The role of oxygen during bottling

When bottling, wineries attempt to leave sufficient free SO₂ levels to preserve the wine over time. This is a delicate task, as low levels will not protect the wine for as long as necessary and high levels can provide unpleasant odours.

There are three sources of oxygen in the bottle:

1.   The headspace, which cannot be controlled beyond the bottling machine design. This oxygen is consumed in a month and a half, with amounts that can vary from 0.6 to 3 mg/l (Vidal J.C. et al. 2004)

1.   The dissolved oxygen (OD) in bottling is consumed in about 2 weeks. 0.9-6 mg/l. (Vidal J.C. et al. 2004)

1.   The oxygen that enters through the stopper will consume all the free SO₂ of the wine. Depending on the type of stopper, this time may vary, from months to years. This process is also inevitable. OD contributions through the stopper: 0.2-15 µl/day in non-defective stoppers, while in screw caps, much less is transferred.

The OD varies during the bottling process. At first it increases due to the filling of the line and the filters and due to the formation of air pockets in the ducts. At the end of the process, an increase in the OD is also seen due to the rapidity of the tank and subsequent push. The following figure shows this increase:

Figure 1: Evolution of dissolved oxygen content during bottling

Source: Vidal, J.C., Boulet, J.C., Moutonet, M., INRA (2004) 

In white wines, the oxidised character in wines has been shown to occur with free SO₂ levels below 10 mg/l.  In the case of red wines, this behaviour is somewhat different since polyphenols have antioxidant effects.

Figure 2: Evolution of the yellow component (Abs 420nm) of a white wine with different levels of OD at the time of packaging;

Low Concentration < 1 mg/l ; Medium Concentration = 3 mg/l; High Concentration > 5 mg/l

Source: Vidal, J.C.; Boulet, J.C.; Deage, M.; INRA (2004)

It is known that each mg of dissolved O₂ is capable of consuming 4 mg of free SO_2. Thus it is essential to eliminate OD before bottling so that the wine can evolve more slowly, and the aromas and color are respected. This is especially true in white and rosé wines, which the winery can bottle with lower levels of free sulphur. The following graph shows the evolution of free sulphur as a function of different stoppers:

Figure 3: Evolution of free SO₂ as a function of the different stoppers

NB: Natural cork, 5.4 ml of head space by applying vacuum to the capping.

S: Synthetic stopper, 6 ml of head space applying vacuum to the stopper.

SC4, 16 and 64: Screw cap with 4, 16 and 64 ml head space.

Source: “Exposure of red wine to oxygen post-fermentation”. Patrick R. Jones, Mariola J. Kwiatkowski, George K. Skouroumounis, I. Leigh Francis, Kate A. Lattey, Elizabeth J. Waters, Isak S. Pretorius and Peter B. Høj (2004)

The second, less pronounced slope is due to the different permeabilities of the different stopper elements. The Figure shows that the permeability of the screw plugs is lower than that of the synthetic and natural plugs. Figure 3 shows two different slopes for the different stoppers. The first, more pronounced slope is due to the dissolved oxygen content at the time of bottling and to the oxygen contained in the head space. That slope is more pronounced the larger the volume of the head space.

Taking into account the wine’s OD content, the contributions from bottling, the oxygen in the head space and the permeability of the stoppers, we can estimate the useful life of a bottled wine.

Example:

In a white wine bottled with 30 mg/l of free SO₂ and with dissolved oxygen of 3 mg/l, with a headspace of 5 ml and capped with a Nomacorc Classic stopper (permeability of 6 µl/día).

Each mg of oxygen is known to consume 4 mg of SO₂.

•         SO₂ reduction due to OD of wine:

3 mg OD → 12 mg free SO₂

•         Reduction of SO₂ due to headspace:

5 ml of air in the head space is equivalent to 1 ml of oxygen (20% of the atmosphere)

1 ml oxygen = 1.42 mg oxygen (under normal P and T conditions)

1.42 mg will dissolve in a 0.75cl bottle

1.9 mg/l OD            →               7.6 mg/l free SO₂

•         Oxygen inlet through the stopper:

0.006 ml/day = 0.009 mg/day = 0.011 mg/l/day

In one month, 0.34 mg/l oxygen

Monthly consumption of SO₂ 1.36 mg/l/month

After consumption of the free sulphur by the OD and the oxygen that exists in the head space, 10.4 mg/l of free SO₂ will remain, which will be consumed as oxygen enters through the stopper. According to the above calculations, the free sulphur will decrease by 1.36 mg/l/month.

According to these calculations, in eight months, the total value of free SO₂ will have disappeared

Each mg of dissolved oxygen that is removed prior to bottling will ensure an additional 3 months of protection.