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,
While I have no particular knowledge about yeast metabolism, I doubt
that there's much opportunity for aerobic respiration even in an open
fermenter. The reason I suggest this is that the surface-to-volume
ratio of something the size of a typical fermenter is so low that the
rate of diffusion of oxygen into solution would be too low to have much
effect on the yeast. Analogous situations arise in wine aging (e.g.,
double magnum vs. half-bottle) and in industrial chemistry, where pilot
plant workers can add very oxygen-sensitive pyrophores to 1000 L
reactors in the open air because of the low surface-to-volume ratio.

Mark Lipton