> How would the yeast work without oxygen?
Yeast have several metabolic pathways that are used to digest grape juice.
The one which liberates the most energy, and is therefore favored by the
yeast, consumes oxygen and sugars to produce carbon dioxide and water. The
one most favored by winemakers is anaerobic and converts sugars to carbon
dioxide and ethanol. In parallel with this, there are pathways that the
yeast uses to make amino acids that are sensitive to both oxygen and
available nitrogen. All these pathways move electrons around at some
point, which according to my crude understanding is more or less where
they can interfere with each other if the yeast is stressed.
Therefore, at the start of fermentation, it's often useful to aerate the
juice slightly so that the yeast get an easy start during their budding
phase. As they take off, the CO2 they produce conveniently blankets the
juice so as to exclude oxygen and start the ***ic fermentation.
Also, by the way, aeration can be a useful treatment for stuck or reductive
fermentations. Basically it gives the yeast an easier time so they can
stop competing for electrons and get back onto the desirable metabolic
Anyway, your question has interesting implications. Now that you mention
it, I suppose it would be harder to start a yeast fermentation under an
inert blanket, and messier to fix it with all those grape bunches sitting
on top of it. Therefore, if you wanted to have a yeast fermentation going
under a carbonic maceration, best to start the juice separately. Then you
are free to handle it as conditions require until fermentation is well
> I am using CO2 from a tank to blanket the berries.
Sounds nice, especially if you have the tank and regulator already.
Lacking these, I'm using dry ice, which has proven really useful for
cooling as well. Although just a few pounds only lasts for a day or
so, it seems to sustain an effective blanket for several days, especially
in deeper tanks. I think both forms of gas should work well.
> I placed/selected the whole grapes carefully, as not to have broken
> berries in the barrel.
For the purposes of carbonic maceration, some broken berries are okay.
They simply won't participate in the carbonic stage, and might start a
yeast ferment on their own, which is not entirely a bad thing, though
see below for some qualifications.
But I think you're right to be selective in doing this. For one thing,
the wine being of a fresh fruity style will probably directly reveal
the quality of berries that went into it.
> I did not wash the grapes.
Nobody seems to have anything to say about this, but I suspect it's one
area that could cause trouble. During the carbonic phase, there is a lot
of time for wild organisms to get established, and even wild yeasts are
not necessarily desirable even if they do add to the gas blanket. On
the other hand, it would be nice to keep processing to a minimum and
leave the grapes dry, so I suspect it's common not to even think about
In my case, I felt there was little choice but to wash the bunches after
selection in 75ppm sulphite. Barbera is a utility grape and seems to
be treated as such at every stage of growing and shipping. The cases
that came in from California were really variable this year, much more
so than the premium varieties from the same region. A lot went straight
into the composter. Even the really nice bunches seemed at risk, so I
washed them just to be safe.
> Do I still need a yeast inoculation?
Under the circumstances, it seems to me you could still go either way.
Taste the grapes and estimate how much more carbonic maceration they
need. If they need more time, you might want to get a cultured yeast
established in there, as much to protect your unwashed grapes as to
produce a gas blanket, though the yeast will save you running as much
bottled gas for sure. But if you are happy with the condition of the
grapes, and you have enough gas to use, maybe you don't need to bother
If you feel the grapes are ready for crush now, then the yeast is due
to go in anyway. But, depending on the condition of the grapes, they
may be at risk a little more from wild organisms. Therefore, if you
plan to sulphite, do that soon afterward, perhaps setting aside a gallon
of unsulphited crush as a yeast starter while the sulphite is binding.
Again, this is all precautionary. If your grapes are in excellent
condition, you should proceed using your normal methods. The only
special treatment would be to try to keep the fermentation cool and
even, so as to preserve the fresh fruit qualities.
. o o . Laboratory for Computational Intelligence
. >v< . University of British Columbia