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.


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