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Protecting red wine from oxygen during fermentation

Question:  How important is it to protect a red wine from oxygen during fermentation?

Answer:  Oxygen is a dual edged sword.  It can be both detrimental and beneficial to wine.

As long as the must is actively and aggressively fermenting, the CO2 being produced will keep the yeast in an oxygen deprived state (so it will produce alcohol) and protect the must/wine from oxygen ingress.  Typically punch downs, pump overs, delestage (if they are going to occur) occur with the highest frequency, and without too much worry, during active fermentation.  This is a key bit though …

the wine has to be fully/actively fermenting.  Not just a pocket of it.  This would be checked by progressive changes in density (monitoring the drop of sugar) and temperature.  Once the fermentation begins to slow down – pump overs/punch downs become less frequent.  (Depends on the grape variety and the producer, of course.)

The time to be concerned about oxygen ingress is when fermentation slows down/stops.  This is when you would start topping tanks with an unreactive gas (CO2, argon, nitrogen or a mix) after every winemaking procedure until it is bottled.  Certain wines will continue to be kept in tank, on their skins, after primary fermentation (ie: Cabernet Sauvignon) for an extended maceration to extract more polyphenols (tannins).  3-4 weeks is not uncommon.  A tiny bit of oxygen is ok at this point as it helps to “fix” the color molecules to the tannin molecules (see: Micro-ox) but large amounts will ruin the new wine.

Acetic acid bacteria is a big concern at this stage – it flourishes with oxygen and feeds on alcohol – it can quickly turn a wine into vinegar, if it is not controlled.  Since acetic acid bacteria is widely prevalent in wineries, the easiest method to control it, is to control the wine’s exposure to oxygen.