MonthNovember 2020

Dissolved Gases 1

Oxygen

Wine contains several gases in solution, O2, CO2 and N2, each has an impact on wine style or quality. Oxygen is the dissolved gas that can have the most negative impacts on wine via premature aging, browning and oxidized aromatic and flavour impacts.

That said, oxygen is important at various stages on the winemaking process and an important driver of style for some types of wines (sherry, maderia for example). Before and during ferment, oxygen is important for:

-good yeast health during the exponential growth phase

-for sparkling wine juices to bind phenolics reducing colour and phenolics in base wines

-aiding the volatilization of suphides during fermentation.

While CO2 protects the wine from oxidizing during fermentation. Once primary fermentation is complete, winemakers typically want to protect wines from excess O2 via SO2 additions, prevention of O2 pick up during wine movements and during wine aging. For barrel or oak aged wines, the slow pick up via O2 transfer through oak assists tannin polymerization on red wines and the development of texture and mouthfeel in white wines.

The management of oxygen in wine is critical during the bottling process is the final stage of O2 management where winemakers typically want to bottle with very low DO2 levels to prevent the wine from premature aging via in bottle oxidizing.

DO2 levels should be measured at many stages to ensure limited DO2 pickup, identify low SO2 levels and prevent oxidation using luminescence probes either in wine (in tank, barrel etc) or after bottling to determine O2 pick up using luminescence spot probes.

Sulphides in wine

Suphides in wine can bring positive aromatic attributes in the form of thiols (3MH, 3MHA and 4MMP most importantly for Sauvignon Blanc) but more widely can bring negative aromatic attributes in the form of hydrogen sulphide, mercaotans, disulphides and dimethyl sulphides.

The negative sulphides are produced as a result of fermentation, either as a result of elemental sulphur from grapes entering the fermentation or excessive H2S production via yeast nitrogen stress as part of the sulphur reduction sequence.

Each sulphide compound has different aromatic charateristics, H2S being rotten egg, mercaptans being cabbage, onion, garlic and disulphides (DEDS and DMDS) showing cooked cabbage, rubber and garlic aromas.

The commonly used technique to remove undesirable sulphides is the addition of copper sulphate which will cause the S molecules to bind to the Cu molecules and precipitate out of solution. Firstly the type of sulphides must be identified via the copper and cadmium test process which also includes an ascorbic acid treatment to reduce disulphides back to mercaptans (formed from mercaptan oxidation) as disulphides cannot be removed via copper but need reduction before treatment. Alternative treatments such as yeast hull products are being more available and are being accepted as non copper alternative treatments.

Ultimately, reduction in always best treated early, either in the ferment stage with sparging to volatilize, N and amino acid additions to the yeast enough N for the SRS loop, or the additional of copper well before bottling to ensure clean, varietal which will be stable once in the bottle under the reductive environment.

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