Getting SO2 Levels Right

September 2014

In our last blog, our winemaking consultant, Justin Knock, looked at the on-going debate about the effect that oxygen (O2) and sulphur dioxide (SO2) can have on the taste and quality of wine. To follow on, I’m looking at how much SO2 should be used to ensure a great tasting wine.

Why is SO2 used in wine?
Many people view the main role of SO2 as an antioxidant, but in reality, it is used principally for its antimicrobial properties. Ideally, the concentration of molecular SO2 needed to provide optimum microbial protection is 0.8ppm for whites and 0.6 for reds. The concentration of free SO2 to reach this level is dependent upon the pH of the wine.

Most wineries look to make wines with a pH below 3.6, because above this level, the amount of free SO2 required to protect the wine starts masking fruit flavours, and can lead to allergic responses in hypersensitive sufferers. In practice, all wines below a pH of 3.5 can be made with levels below 40ppm free SO2 but, to create good products with levels below 25-30ppm, wineries need to consider methods, including low temperature storage and sterile filtration, in combination with SO2, in order to achieve overall wine stability and protection.

The effect of O2 on SO2
In general, 1 milligram per litre (mg/l) of dissolved O2 causes a 4mg/l drop in SO2. This is because SO2 reacts with hydrogen peroxide (H2O2), formed by the reaction of O2 with phenols in the wine. If the H2O2 does not react with the SO2, which is likely as the kinetics of this reaction are very slow, it will act on the alcohol, forming aldehydes that give bruised apple aromas and flavours. SO2 binds to these aldehydes, removing it from the pool that is available to carry out the anti-microbial and anti-oxidative work.

The inclusion of O2 in wine is often seen as a negative, but at the start of the fermentation process it can fix colour, ensure healthy yeast. Post-fermentation, though micro levels of oxygen can modify the tannin profile, high O2 levels however can cause browning, loss of fruit flavours, and accelerated development of the wine, making it drier, flatter and more mature-looking – or in extreme cases vinegar.

Controlling O2 levels
O2 is most likely to be picked up at any transfer but the greatest risk to the final wine is at the bottling stage. This risk can be mitigated by using fillers that double pre-evacuate bottles of any air, before charging them with nitrogen prior to filling. This can ensure that the wine is entering a near 100 per cent nitrogen environment, minimising O2 pickup. Priming the lines with deoxygenated water prior to any transfer will allow a wine movement to occur with no oxygen pick up providing there are good seals on all connecting pipework.

The gas in the headspace contributes much more to the amount of oxygen loading on the wine post-bottling. By reducing the fill height of bottles from 55mm to 30mm, it has been possible to halve the amount of Total Package Oxygen (TPO) – the combined amount of headspace and dissolved oxygen (DO).

Managing SO2 effectively
It is vital to fully understand a bottling plant, so that practices that could require a greater amount of SO2 to be used can be minimised. At the same time, though, it is important to remember that some SO2 in wine is a good thing, as it plays a key role in delivering a good quality and long-lasting product for consumers to enjoy.

– Henry Powles

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