wayne wrote:
> Yes I've always wondered at what stage the yeast fermentation changes
> from an aerobic stage ( yeast growth) to an anaerobic ( CO2 productoin)
> stage, and if it was possible to regulate alcohol production from a
> given amount of sugar by sitrring the must and promoting more of an
> aerobic response.
>

Current information indicates that yeast do not really use respiration
(aerobic) when we're talking about wines. The sugar content in the must
is usually high enough that the yeast favor fermentation because of the
speed at which they can obtain the energy. The only time they really
respire is when there is limited sugar, but in a wine fermentation, by
the time the sugar is depleted to this level, the O2 is gone and the
alcohol is high, so the yeast still favor fermentation.

The old, aerobic vs anaerobic "stages" don't really happen. The
introduction of O2 is really to keep the yeast healthy by giving them
some raw material to help maintain the cell wall during growth. Unless
you significantly underpitch or use unhealthy yeast, there should be
enough disolved O2 in the must just from the crush operation.

Andy