[pensador (OP)]

I made wines from grapes and citrus fruits last year, I had a lot of oranges, so have a lot of orange wine, as well as other types of wines.

All grape wine oxidizes over night if left exposed to the air. My Orange wine does not appear to oxidize if left exposed to the air. Why is this the case?

Has it anything to do with the level of antioxidants in oranges being greater than those in grapes, or is there more too it? My Orange wine is quite strong, could the extra alcohol have something to do with it?

[alancalverd]

The principal constituents, water and ethanol, will not be affected by oxidation, but all the stuff that confers flavor, might. I suspect that years of good living have sensitised your palate to subtle variations in the very complex and balanced flavor of grape wine but the overriding flavor component of orange wine is less familiar, by no means "subtle" (at least in my brews) and possibly less subject to oxidation.

[Bored chemist]

Last time I checked, a significant part of the flavour of sherry was acetaldehyde from oxidation of alcohol. It's a pretty easily spotted trait.

Oranges have a lot of vitamin C which is likely to retard oxidation of other components.

[alancalverd]

I once suggested this (in a New Scientist backpage answer) as the reason for shaking rather than stirring cocktails but my boss (an organic chemist) said the idea was ridiculous. A quick glance at textbooks suggests that ethanol -> acetaldehyde  requires a strong desiccant like H2SO4 to remove the H2O byproduct, or a high temperature and a catalyst with removal of the hydrogen product.

BBC "Bitesize" discusses low temperature oxidation:

Ethanol also undergoes bacterial oxidation to ethanoic acid. This form of oxidation is a problem for wine producers. Air contains a large proportion of bacteria called Acetobacter. Acetobacter bacteria use atmospheric oxygen from air to oxidise ethanol in wine, producing a weak solution of ethanoic acid (vinegar). So once a bottle of wine has been opened it can quickly turn to vinegar due to the large number of bacteria in the air. Wines with a high concentration of alcohol or fortified wines such as sherry and port are very resistant to bacterial oxidation. This is because the ethanol concentration is too high for the bacteria to tolerate[

which explains the need for a fermentation lock during brewing but suggests (a) more complete air oxidation of wine and (b) no bacterial oxidation of sherry.

Obviously I'd rather believe BC than the BBC (which as we all know is run by communist teetotallers) but Madeira does require heat treatment to oxidise, and cold-brewed wines do indeed turn into vinegar. Meanwhile at the back of the bar, we have a gadget that definitely adds £1 worth of character to cheap wine  by spraying it into a carafe with plenty of air - but don't tell the customers.

I'm also baffled by vitamin C, much heralded as an antioxidant but also a known oxidant in high physiological concentrations: its role in the preservation of orange wine is thus, to my mind, moot!

[pensador (OP)]

Thanks for the replies.

I have done a bit of digging and dragged this up, which I dont fully understand

https://www.researchgate.net/publication/229310928_Oxidation_mechanisms_occuring_in_wines
"
Non-enzymatic oxidation, also called chemical oxidation of wine, prevails in fermented wine and begin by the oxidation of polyphenols containing a catechol or a galloyl group.
These phenolic reactions, both enzymatic and non-enzymatic, result in by-products named quinones.
However, in non-enzymatic oxidation, oxygen does not react directly with phenolic compounds. The
limitation on the reactivity of triplet oxygen is overcome by the stepwise addition of a single electron, which
can be provided by reduced transition metal ions, essentially iron(II) and copper(I). The sequential electron
transfer leads to the formation of hydroperoxide radical (HOO•), hydrogen peroxide (H2O2), and hydroxyl radical (HO•). The later radical will oxidize almost any organic molecule found in wine and will react with the
first species it encounters,depending on their concentration. Sulfur dioxide (SO2) and ascorbic acid, when
added to wine, are able to reduce the quinones. Alternative options have been assessed for the prevention of
oxidation during wine storage; nevertheless, these are not fully understood or commonly accepted
"

[alancalverd]

Makes some sense. It's the phenols and other aromatics (hence the name) that give wine its flavor rather than the straight-chain compounds, so you'd notice the change.

[Bored chemist]

A quick glance at textbooks suggests that ethanol -> acetaldehyde  requires a strong desiccant like H2SO4 to remove the H2O byproduct, or a high temperature and a catalyst with removal of the hydrogen product.

And a quick sniff at a glass of Sherry shows that it doesn't.

Which one should you believe (N.B this is a science page) ?

I'm also baffled by vitamin C

Figures.

[Bored chemist]

Non-enzymatic oxidation, also called chemical oxidation of wine, prevails in fermented wine and begin by the oxidation of polyphenols containing a catechol or a galloyl group.

On the whole, the polyphenols are far too big to be volatile so they don't contribute to the aroma. They are often bitter- they are characteristically "tannins" which also add astringency.
Oxidative degeneration of them might give rise to volatiles, but it's more likely to just tinker with the edges of the molecules which won't affect their flavour much.

It's the phenols and other aromatics (hence the name) that give wine its flavour rather than the straight-chain compounds,

Or you could look at the science
For example
https://www.tandfonline.com/doi/full/10.3402/fnr.v59.29290
Where they list rather a lot of compounds that contribute to the flavour of various wines.
28 of them are present in the wine (rather than fruit wine)
2 of those are aromatics.
The others are not- 17 of them are straight chain aliphatics.

[Juan25]

it is not risk-free to leave the wine to itself for months because this can quickly lead to reduce the flavours. but this form of wine making is very attractive for more and more producers because it offers the chance to produce extremely complex and full-bodied wines away from the mainstrea