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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Leo,

I don't know if yeast carry out respiration & fermentation
simultaneously, but I doubt they do to any significant degree.

As I understand it, yeast ferment preferentially over respiration,
even though fermentation is a far less efficient energy producer per
sugar molecule. The reason seems to be that they can ferment at a much
faster rate than they can respire, and this more than compensates for
the lower ATP yield. I suppose it's a "Fast nickels vs. slow dimes" kind
of thing.

I had always assumed that yeast would switch to respiration if oxygen
were present, because of the higher energy yield per sugar molecule, but
a very knowledgeable researcher type informed me of the above.

Hope this helps.

--


Mike MTM, Cokesbury, New Jersey, USA

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

Mark Lipton > wrote in news:e96dndhsOYjH7mDcRVn-
:

> 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
>

the CO2 tends to "hang over" the tanks as well creating somewhat of a
barrier.or so it was explained to me once upon a time.

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.

Mike Tommasi wrote:
> On 30 Jan 2005 17:24:02 -0800, "markw" > wrote:
>
> if you did not have anerobic conditions you would not get significant
> alcoholic fermentation. You would get mostly CO2 and water, and some
> ethanol. When making bread you want aerobic conditions, not when
> making wine.
>

While fermentation in an anaerobic process, it will occur in the
presence of O2 if there is sufficient sugar present. Wine yeast will
prefer fermentation over respiration due to the speed of fermentation.
Additionally, alcohol will suppress respiration so as the sugar is
depleted the yeast will continue fermenting even in the presence of O2.


> Another nasty effect under the presence of oxygen is that if there is
> any acetobacter you will get acetic fermentation (aerobic by
> definition).

Yes, but the yeast usually overwhelms any other organisms by competing
for the same nutrients and by producing compounds that will suppress
the growth of other organisms. Yeast cells are known to be an oxygen
scavengers even though they aren't using it for respiration. They will
use it to make compounds that maintain cell integrity and will "stock
pile" the O2 even if it is not currently needed. Of course if the yeast
do not reproduce quickly for some reason, there is an increased risk of
spoilage organisms taking hold.

>
> The alluded to aeration is done before the fermentation kicks in, in
> order to provide nourishment to the yeasts and favour their growth,
as
> well as after the fermentation is over, in order to get rid of
odours.
>
> During fermentation the CO2, being heavier than air, "sits" on top of
> the cap and prevents any oxygen from reaching it.

There is a lot of controversy concerning this due to the partial
pressure of gasses. If we're talking about a closed container that
only allows gas to escape, this is probably true, but if O2 has a means
of entering, O2 will dissolve in the wine/must even if it's blanketed
with CO2. The gasses will mix and the O2 will dissolve in the wine
because it has a lower concentration.

Some wine makers are using oxygenation during fermentation in order to
make softer wines. They will pump over the wine and at the same time
remove the seeds which they believe contribute the harsh tannins.
Andy

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