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