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.
>
>
> --

Isn't punching the chapeau done for red wines to increase the contact
between the solids and the juice leading to colouration? And of course
it will also help Malthusian colony of yeast to get energy to multiply,
is it not?

Once the dissolved oxygen in the must is used up, there frankly there
will be no oxygen available to the poor yeast cells _inside_ the must.
I mean, if I run up 10 flights of stairs in 2 minutes flat, my body is
surrounded by all the oxygen one can imagine, but the muscles have none
and have to resort to anaerobic respiration (a.k.a. fermentation).

The aeration towards the end of the process is perhaps to cleanse the
"wine" of excess carbon dioxide from fermentation. The oxygen will
eventually help "age" the wine by oxidising non-sugar extracts.
Cheers