Thanks much, Victor, I'll comment in places below. But this is extremely helpful.
originally posted by VS:
The production of acetaldehyde does not occur, as the question seems to infer (sic), by sheer oxidative action, but through the action of metabolism in a reductive environment.
The enzymes make things happen faster or more selectively, but they still go "downhill." If you are turning ethanol into acetic acid, not everything about the environment is reductive.
The wine finds itself in such an environment because the yeast veil needs and consumes both the oxygen that's dissolved in it and the oxygen in the part of the butt with air....
In that metabolism, expressing it in a simple way, the yeast consumes acetic acid and ethanol. There are complex biochemical cycles which explain it more correctly, but I'm not an expert. It seems simple but it's hard to understand, since the natural way of producing acetaldehyde by oxidation consumes ethanol and increases acetic acid."
I think Alvaro means "chemical, not biological" here by "natural," and I agree with him, although the details of 2-carbon metabolism aren't really required to get to our answer. But the next sentence below clarifies, and we are quite in agreement. In any situation, whether you're using a chromium reagent or a living organism, ethanol-->acetate is an oxidation. And the yeast is using oxygen to do it. Other reactions in a complex biochemical situation may not and will not be net oxidations, of course.
Alvaro adds that the question asked seems to leave the biological dimension of the flor action aside. (As asked, yes, although of course the biological dimension is how the selectivity is achieved and why this happens, and was the unstated implication of my question--JD) He has consulted Justo Casas' 2008 book, 'La vinificacin en Jerez en el siglo XX' (Vinification in Jerez in the 20th century) and extracted some quotes:
"Everything seems to indicate that the redox potential of wine in the aging butt depends on the degree of activity of the flor yeast at that time of the year. To us, it's obvious that wine aging under a flor veil is not an oxidative process. In fact, the wine is in a reduced state."
Totally reasonable, and almost had to be true.
"The literature says that the flor veil acts as a screen or a sheet that's impermeable to gases and impedes the physical access of oxygen to the wine. It's hard to agree with this viewpoint, as it's not been demonstrated. I rather think that the yeasts in the veil catch the oxygen un the immediate atmosphere and use it to breathe or to oxidize the ethylic alcohol or acetaldehyde, so that the oxygen molecule in contact to the veil or next to it disappears. This seems to be the most logical reason why the veil is largely impermeable to the passage of oxygen."
Totally agree. The wine is in a reductive state because the yeast eats the oxygen before it can penetrate to the bulk. But as it eats it, it consumes ethanol (made into acetaldehyde, at least first) and acetate (made mostly into CO2).
"Until today ethanol has been quoted as the only component of wine which oxidizes during the aging under flor. It does so through the enzymatic action of the flor yeast in an extremely specific action, with no alteration of the other wine components which might intervene in what we could call the oxidation of wine."
Sure. EXCEPT FOR ACETATE! That was the point of my question, and it seems to be the case.
"We see that, at the same time as the oxidation of alcohol produces acetaldehyde, reductive substances and ions are produced. Part of those will continue their normal metabolic routes until combining with the oxygen in the air that is in contact with the yeast cells in the upper part of the veil, while another part of them carry out their reductive action on the wine components, which explains why the oxidation of ethanol into acetaldehyde simultaneously produces wine in a reduced state, as shown by measures of its redox potential."
OK, sure. Ordinary yeast metabolism produces NADH, glutathione, all those sorts of things. If they are on the top of the veil, they encounter oxygen and reduce it, if they are below the veil, they encounter the wine and render it reductive instead.
"In conclusion, if the intensity of the sharp character of fino sherry depends mainly on its concentration in acetaldehydes, its softness to the nose and in the mouth, which good tasters particularly appreciate, is related, in my opinion, to the level of reduction in the wine, of which we can get an idea through its redox potential, also shown through its naturally paler color."
Alvaro, on his part, concludes: "There's a generalized resistance to consider finos and manzanillas as fundamentally non-oxidative wines. You change your mind quickly the first time you smell a butt in which the flor is working intensely: the reduction stench is not pleasant at all!"
This all seems in line. The yeast eat the oxygen, or most of it, and use it to consume both ethanol and acetic acid in the local biochemically oxidizing layer (for these chemicals). There isn't much left for the wine, which finds itself in a reductive state otherwise. So it is indeed true that the yeast eats acetate, preventing the wine from becoming overwhelmingly volatile, which would be its fate in a simple chemical oxidation that gave a lot of acetaldehyde. This situation leaves quite a bit of acetaldehyde alone, and the bulk wine can continue in a reduced state despite the localized oxidation of one of its components in the surface layer.
Hooray for flor!
