In early stages, yeast need O2 to reproduce. When you make wine from fruit,
you keep it in a vat or primary and several times a day you stir the cap
(the fruit pulp that rises to the top) down. This prevents mold from
growing on the pulp and it aerates the must. After about 3 to 7 days the
cap will fall, which means it stops rising back to the top. After this
point it becomes much more important to protect it from O2 so you put it
under a airlock from that point on.

Another way to put it is that the end products of fermentation need to be
protected from O2. Those are not present at the beginning of fermentation.
But after a few days you must start protecting it. With fruit wines, the
indicator people use to determine when you must start protecting it is the
cap falling. With wines made from pure juice, it is up to you as to when
you start protecting it.

Ray

"flowerpot" > wrote in message
oups.com...
> WannabeSomeone wrote:
>> It defeats the purpose if you seal the primary fermentation bucket in a
>> plastic
>> bag.
>>
>> You need air for the primary fermentation so the yeast can multiply
>> quickly.
>>
>>
>> Best Regards,
>> Wannabe
>> =======
>>
>
> But I thought contact with oxygen had to be limited and the must should
> be protected beneath a blanket of carbon dioxide?
>
> flowerpot
>