Leo Bueno wrote:
> We have all see the pictures (and perhaps the real thing) of a red
> grape cap floating on the surface of an open vat and of winery
workers
> either punching down the cap or pumping-over the must.
>
> I showed a couple of those pictures at a recent wine class and one of
> the students wondered out loud whether the fermentation that was
going
> on was really anaerobic, given the cap's exposure to air. She got me
> wondering too. Obviously the brew is producing CO2 and alcohol, but
> what about the presence of the oxygen?
>
> I understand that yeast may need some oxygen at some point in their
> life cycle and that they can carry out plain old aerobic respiration
> (i.e., can turn sugar and oxygen into CO2 and water instead of CO2
and
> alcohol). Are they both respiring and fermenting simultaneously?
>
> Will greatly appreciate an explanation (or reference to a suitable
> book or article, no matter how technical) about what is going on in
an
> open fermentation vat from the yeast's perspective.
>
Leo, open vat fermentation (or any other kind, for that matter) is
usually intentionally *not* anaerobic. In fact, one of several good
reasons for either pumping over or punching down is to aerate the must.
As has been pointed out by others, between the cap and the blanket of
CO2 that results from fermentation, little oxygen would be present
without some intervention. Anaerobic fermentation can cause yeast to
start consuming sulfur from the must instead, resulting in reductive
byproducts, typically starting with the production of hydrogen sulfide.

Once fermentation is complete, as I'm sure you're aware, winemakers go
to some lengths to exclude oxygen and so prevent oxidation. During
primary fermentation, though, not only is oxygen not excluded but its
presence is actually generally positive for the end product.

- Mark W.