Orange Wines....

While lengthy maceration protects the wine from oxidizing over the long term, the process itself actually increases oxidation during fermentation. Consequently, the wines all have a distinctive oxidative tang that can come across as sherry-like or cider-like.

Dave,
I macerate only until dry, then press to barrel, keep them topped up and add sulphur. Much as one would with a normal red wine ferment.
I have not noticed any sherried or oxidative notes with my whites done this way.

And if I may; why would fermenting whites on the skins be any more oxidative during fermentation than reds?
Best, Jim

No idea. I'm just quoting Saša Radikon, and it is entirely possible that I misunderstood him.

The chemistry involved might just as well be voodoo to most, including most winemakers. Fermentation is an energy pathway requiring absolutely no oxygen, and most fermentative organisms, including most yeasts, can respire as well, which is the preferable method because the organism derives more energy per unit sugar. But fermentation can proceed to completion totally anaerobically.

Wow; I had no idea . . .

Sometimes, winemakers like to get as much oxygen mixed up into the wine as possible, to potentially change the redox state and potentially inhibit production of reduced sulfur compounds. I think that may be what Howie is saying when he says, "Exposure to oxygen during fermentation is necessary for the yeast to ferment properly."

Yes, and for some varieties (eg. syrah) this is imperative.

To the extent that copper is used to control the H2S, it might also serve to oxidize other compounds and add another layer to this discussion.

More food for thought.
Thanks, Craig.
Best, Jim

Regarding the biochemistry involved here:

There are three main biochemical pathways involved here.

Always involved is fermentation. This is the oxidation of a molecule of glucose to two molecules of pyruvic acid,two water molecules, two hydrogen ions, and two high energy electrons. The energy released by the reaction is captured by the formation of five molecules of adenosine triphosphate (ATP) from adenosine diphosphate (ADP). ATP is the main energy storage molecule in living creatures. So the summary reaction is:

glucose + 5 ADP => 2 pyruvate + 5 ATP + 2 H20 + 2H+ + 2e-

The problem for the organism is then what to do about the pyruvate (which is acidic and has to be neutralized somehow), the two hydrogen ions, and the two electrons.

If oxygen is available, the other two pathways come into play.

The first is the Krebs cycle. Pyruvate is first decarboxylated to form carbon dioxide, acetate, and another hydrogen ion/high-energy electron pair:

pyruvate => CO2 + acetate + H+ + e-

Then the acetate is oxidized to CO2, H20, and more hydrogen ion/high-energy electron pairs:

acetate + ADP => 2CO2 + H2O + 8H+ + 8e- + ATP

And the third pathway, called respiration, uses molecular oxygen to oxidize the hydrogen ions and high-energy electrons, capturing the released energy in the form of ATP:

4H+ + 4e- + O2 + 6ADP => 2H2O + 6ATP

This is the essentially the same chemical reaction that caused the explosion of the airship Hindenburg, except that the crew of the Hindenburg didn't capture any energy in the form of ATP. And in the cell the reaction takes place in controlled, measured baby steps, as opposed to releasing all the energy at once.

But what if there isn't oxygen around? The cell has the problem of disposing of the hydrogen ions and electrons, and of neutralizing the pyruvic acid. Organisms such as yeasts solve the problem by using the hydrogen ions and electrons to reduce pyruvate to ethanol and CO2, by way of acetaldehyde as an intermediate:

pyruvate => acetaldehyde + CO2
acetaldehyde + 2H+ + 2e- => ethanol

You'll have noticed that a lot more energy gets captured by fermentation, the Krebs cycle, and respiration; rather than by fermentation to ethanol. So if oxygen is present, yeasts will by choice respire and will convert glucose to water and CO2. This is the process involved in the production of soft drinks such as naturally-fermented root beer. You have to keep the fermentation vessel exposed to oxygen to avoid the production of ethanol.

If you keep oxygen away from the fermentation vessel, the yeasts have no choice other than to do fermentation of glucose to CO2 and ethanol. This is the reaction involved in beer and wine production. They will keep this up until either the glucose supply is exhausted, or the level of the waste product ethanol builds up to toxic levels, in which case the yeast cells form dormant spores and wait for better conditions to occur.

There are also bacteria present in the wine vat. Some of them get their energy by an alternative fermentation pathway that converts the malic acid present in grapes to lactic acid and CO2. Malic acid is what gives green apples their signature tart taste. Lactic acid is a lot less sour-tasting on the human palate. Nearly all red wines, and a large number of white wines, are put through malolactic fermentation to get rid of excessive tartness. I've tasted red Burgundy before it has gone through malolactic, and you'd never want to drink it in that state.

There are also bacteria--and maybe yeasts?--that, under anaerobic conditions, will dispose of those unwanted hydrogen ions and electrons by converting some of the sulfur dioxide (SO2) that's commonly added to fermenting wine must as a disinfectant to hydrogen sulfide (H2S) and water:

6H+ + 6e- + SO2 => H2S + 2H2O

Hydrogen sulfide has the distinctive smell of rotten eggs, and definitely isn't wanted in the wine world. Oxygen is more greedy for hydrogen as a companion than sulfur is, so you can add oxygen to get rid of the H2S:

2H2S + 2O2 => SO2 + 2H2O

A major reason for pumping over the must--to introduce oxygen--is to promote this reaction.

Metallic copper will happily exist in two oxidation states--cuprous (Cu+) and cupric (Cu++), and so it can act as a sink for excess electrons produced by fermentation. This is why adding copper gets rid of the reductive H2S.

As also mentioned, some yeasts (such as the flor yeast involved in the production of Sherry) will liberate free acetaldehyde and related compounds in the presence of sufficient oxygen.

Then there are the bacteria of genus Acetobacter and related genera. They do aerobic metabolism, but they first release free acetic acid, as opposed to passing acetate directly into the Krebs cycle as yeasts so. The resulting acidic environment is hostile to most other organisms. Once they've oxidized all the present glucose to highly acidic acetate, Acetobacter run the Krebs cycle and oxidize the acetate to CO2, H20, and a lot of energy. This is the main reaction involved in vinegar production (stopping before the Acetobacter go all the way to water and CO2, of course).

And then there are the phenolic compounds. These have cyclic aromatic rings (go look up carbon SP2 bonding and quantum mechanics). These can act as antioxidants because they can interact with molecular oxygen (O2) and water--preventing the O2 from attacking other molecules that contribute to the wine's flavor.

Regarding winemaking implications, it's all really, REALLY, complicated, and very poorly understood. But there are general good rules-of-thumb and well known methods for achieving good results.

As regards Orange Wines:

The skin extraction means that you'll be getting more of the phenolic compounds than you would with your conventional white wine. These compounds often exhibit yellow/red/blue/purple color. Hence the orange shade to the resulting wine.

I'd also expect some of the oxidative reactions involving the aromatic (phenolic) compounds that have been extracted, even if they don't exhibit the vibrant blue/purple of the anthocyanins.

I'd expect the result to be similar to a rose, or in the bad cases, a maderized rose. Personally I don't much see the point in intentionally producing wines of that sort.

-Paul W.