Oxygen: friend and enemy at every stage

Almost every winemaking decision is in some part an oxygen decision — vessel, closure, racking, topping up, whether to stir the lees, whether to sparge a tank with inert gas.

The two failure modes sit at opposite ends. Too much oxygen gives oxidation; too little can give reduction. Both are real and both are avoidable, and avoiding one increases the risk of the other.

This is one of the areas where a simple rule genuinely does not exist, and pretending otherwise misleads more than it helps.

Yeast require a small amount of oxygen early in fermentation to build sterols for healthy cell membranes. Fermentations run under strict anaerobic conditions from the outset are more prone to sticking.

During maturation, slow ingress through a barrel’s staves and bung allows anthocyanins and tannins to polymerise, which stabilises colour and is generally associated with softer-feeling tannin. This is a large part of what barrel ageing does that has nothing to do with oak flavour.

Oxygen also consumes reductive sulfur compounds, which is why a wine showing struck-match character on opening will often clear with air.

Too much oxygen oxidises phenolics and ethanol. Acetaldehyde accumulates, fresh fruit aromas are lost, and colour changes — browning in white wine, dulling in red.

At bottling, the total package oxygen — headspace plus dissolved — and the closure’s subsequent transmission rate set the wine’s trajectory for years. This is the mechanism behind closure choice being a stylistic decision rather than a cost one.

Sulfur dioxide is the tool that manages all of this, because it scavenges oxygen and its reaction products and slows the whole sequence.

It is the shared explanation behind decanting, barrel ageing, screwcap versus cork, why an opened bottle deteriorates, and why some wines improve on the second day.

It is also why "let it breathe" by pulling the cork an hour early does essentially nothing: the surface area at the neck of a bottle is tiny.